Georgy V Timofeev

Maksim I Hulnik

Irina V Vasilenko

Francois Ganachaud

Guy Jacob

Sergei V Kostjuk

•OEt2 as Coinitiator: Towards Perfectly Functionalized Poly(epichlorohydrin) Diols

Keywords: Chlorinated Solvent-Free Living Cationic Ring-Opening Polymerization of Epichlorohydrin Using Cationic Ring-Opening Polymerization of Epichlorohydrin Using BF3 Cationic Ring-Opening Polymerization, Living Polymerization, Poly(epichlorohydrin), Functional Oligomers, Activated Monomer Mechanism, Solid Propellants

published or not. The documents may come

INTRODUCTION

Epichlorohydrin (ECH) homopolymers (PECH) and copolymers represent an important class of functional polymeric materials for wide range applications such as in automotive industry, [START_REF] Vandenberg | Address: Reflections on 50 years of polymer chemistry[END_REF] biomedical field, 2 or energy storage. [START_REF] Ikeda | Ionic conductivity of polymer solid electrolyte prepared from poly[epichlorohydrin-co-(ethylene oxide)] of high ethylene oxide content[END_REF][START_REF] Hayano | Characterizations and Functions of Cationic Polyethers with Imidazolium-Based Ionic Liquid Moieties[END_REF] PECH is also the starting polymer to prepare glycidyl azide polymer (GAP), the most important energetic binder for solid propellants. [START_REF] Frankel | Historical Development of Glycidyl Azide Polymer[END_REF][START_REF] Gaur | Azido Polymers -Energetic Binders for Solid Rocket Propellants[END_REF][START_REF] Cheng | Review of Novel Energetic Polymers and Binders -High Energy Propellant Ingredients for the New Space Race[END_REF] This polymer is currently intensively used in place of hydroxyl-terminated polybutadiene, since it provides much higher specific impulse and burn rate. [START_REF] Cheng | Review of Novel Energetic Polymers and Binders -High Energy Propellant Ingredients for the New Space Race[END_REF] For the preparation of such propellants, hydroxylterminated liquid GAP polymer is mixed with inorganic oxidizer and cured by reaction with isocyanates of functionality ≥ 2. As a result, a three-dimensional cross-linked polyurethane network with elastomeric properties is formed. [START_REF] Cheng | Review of Novel Energetic Polymers and Binders -High Energy Propellant Ingredients for the New Space Race[END_REF] To ensure efficient chain extension in parallel to few crosslinking reactions, the functionality in hydroxyl groups of GAP, and thus of synthesized PECH, should be strictly equal to 2. [START_REF] Ivan | Quantitative aspects of chain extension of telechelics[END_REF] Also, because of environmental concerns, all fuels used in aviation and aerospace industries should not contain any banned molecules, in particular chlorinated solvents.

ECH can be polymerized via coordinative, [START_REF] Vandenberg | Organometallic Catalysts for Polymerizing Monosubstituted Epoxides[END_REF][START_REF] Vandenberg | Epoxide Polymers: Synthesis, Stereochemistry, Structure, and Mechanism[END_REF][START_REF] Brochu | Synthesis and Characterization of Glycidyl Azide Polymers Using Isotactic and Chiral Poly(epichlorohydrin)s[END_REF][START_REF] Masuda | Organoaluminum -Cyclic Ether -Cyclic Imide Catalysts for the Polymerization of Epoxides[END_REF][START_REF] Araki | Ethylaluminum Oxide Catalysts from Et 2 AlOLi-Et 2 AlCl Binary System in Relation to Species of AlEt 3 -Water Catalyst[END_REF][START_REF] Xie | Ring-Opening Polymerization of Epichlorohydrin and its Copolymerization with other Alkylene Oxides by Quaternary Catalyst System[END_REF] anionic [START_REF] Hayano | Characterizations and Functions of Cationic Polyethers with Imidazolium-Based Ionic Liquid Moieties[END_REF][START_REF] Carlotti | Living/Controlled Anionic Polymerization and Copolymerization of Epichlorohydrin with Tetraoctylammonium Bromide-Triisobutylaluminum Initiating Systems[END_REF][START_REF] Brocas | Controlled Synthesis of Polyepichlorohydrin with Pendant Cyclic Carbonate Functions for Isocyanate-Free Polyurethane Networks[END_REF][START_REF] Rodriguez | Ring-Opening Polymerization of Epoxides: Facile Pathway to Functional Polyethers via a Versatile Organoaluminum Initiator[END_REF] and cationic [START_REF] Bideron | Polyepichlorohydrin Diols Free of Cycles: Synthesis and Characterization[END_REF]1920,[START_REF] Kim | The Influence of Initiator Diol-BF 3 Complex Solubility and other Factors on the Induction Period in BF 3 Catalyzed Cationic Ring Opening Polymerization of Epichlorohydrin[END_REF]2223,[START_REF] Murali Mohan | Synthesis and Characterization of GAP-PEG Copolymers[END_REF][START_REF] Guanaes | Influence of Polymerization Conditions in the Molecular Weight and Polydispersity of Polyepichlorohydrin[END_REF]2627,[START_REF] Okamoto | Cationic Ring-Opening Polymerization of Epichlorohydrin in the Presence of Ethylene Glycol[END_REF]2930 mechanisms. Catalytic ring-opening polymerization (ROP) of ECH was originally discovered by Vandenberg. [START_REF] Vandenberg | Address: Reflections on 50 years of polymer chemistry[END_REF][START_REF] Vandenberg | Organometallic Catalysts for Polymerizing Monosubstituted Epoxides[END_REF][START_REF] Vandenberg | Epoxide Polymers: Synthesis, Stereochemistry, Structure, and Mechanism[END_REF] Vandenberg's catalyst is prepared by mixing 1 eq. of trialkyl aluminum with 0.5 eq. of H 2 O and 0.5 eq. of acetylacetone to generate, as it was proposed recently, a bis(-oxo)dialkylaluminum active species. [START_REF] Ferrier | Demystifying the Mechanism of Regio-and Isoselective Epoxide Polymerization Using the Vandenberg Catalyst[END_REF] It allows synthesizing high molar mass PECH (M n > 50,000 g mol -1 ) often with isotactic configuration of a polymer chain.

Many other monometallic [START_REF] Masuda | Organoaluminum -Cyclic Ether -Cyclic Imide Catalysts for the Polymerization of Epoxides[END_REF][START_REF] Araki | Ethylaluminum Oxide Catalysts from Et 2 AlOLi-Et 2 AlCl Binary System in Relation to Species of AlEt 3 -Water Catalyst[END_REF][START_REF] Xie | Ring-Opening Polymerization of Epichlorohydrin and its Copolymerization with other Alkylene Oxides by Quaternary Catalyst System[END_REF] and bimetallic [START_REF] Kuntz | Polymerization of Epoxides with Dialkylaluminum Acetylacetonate Catalyst Systems[END_REF] multicomponent catalytic systems for the synthesis of high molar mass PECH were later reported. Despite of the possibility to prepare high molar mass and often stereoregular (co)polymers of ECH, catalytic ROP of ECH suffers from some difficulties in controlling the molar mass and chain-end functionality. In some instances also, incomplete monomer conversion was observed. [START_REF] Vandenberg | Organometallic Catalysts for Polymerizing Monosubstituted Epoxides[END_REF][START_REF] Vandenberg | Epoxide Polymers: Synthesis, Stereochemistry, Structure, and Mechanism[END_REF][START_REF] Brochu | Synthesis and Characterization of Glycidyl Azide Polymers Using Isotactic and Chiral Poly(epichlorohydrin)s[END_REF][START_REF] Masuda | Organoaluminum -Cyclic Ether -Cyclic Imide Catalysts for the Polymerization of Epoxides[END_REF][START_REF] Araki | Ethylaluminum Oxide Catalysts from Et 2 AlOLi-Et 2 AlCl Binary System in Relation to Species of AlEt 3 -Water Catalyst[END_REF][START_REF] Xie | Ring-Opening Polymerization of Epichlorohydrin and its Copolymerization with other Alkylene Oxides by Quaternary Catalyst System[END_REF][START_REF] Ferrier | Demystifying the Mechanism of Regio-and Isoselective Epoxide Polymerization Using the Vandenberg Catalyst[END_REF][START_REF] Kuntz | Polymerization of Epoxides with Dialkylaluminum Acetylacetonate Catalyst Systems[END_REF] It was reported recently about the catalytic regioselective polymerization of ECH using Zn-Co(III) double metal cyanide complex, which favors formation predominantly of PECH diols, albeit of low molar masses (M n <3,500 g mol -1 ) and relatively high dispersity (Đ>1.5) with semi-crystalline properties (solid-liquid transition around 100 °C). [START_REF] Wei | Regio-Selective Synthesis of Polyepichlorohydrin Diol using Zn-Co(III) Double Metal Cyanide Complex[END_REF] Living anionic ROP of ECH was achieved by Carlotti et al. [START_REF] Carlotti | Living/Controlled Anionic Polymerization and Copolymerization of Epichlorohydrin with Tetraoctylammonium Bromide-Triisobutylaluminum Initiating Systems[END_REF][START_REF] Brocas | Controlled Synthesis of Polyepichlorohydrin with Pendant Cyclic Carbonate Functions for Isocyanate-Free Polyurethane Networks[END_REF] using Oct 4 NBr/ i Bu 3 Al initiating system. This initiating system allows preparing well-defined heterofunctional PECH with controlled molar masses (up to M n =80,000 g mol -1 ) and low dispersity (Đ<1.23) as well as random and block copolymers of ECH and propylene oxide. [START_REF] Carlotti | Living/Controlled Anionic Polymerization and Copolymerization of Epichlorohydrin with Tetraoctylammonium Bromide-Triisobutylaluminum Initiating Systems[END_REF][START_REF] Brocas | Controlled Synthesis of Polyepichlorohydrin with Pendant Cyclic Carbonate Functions for Isocyanate-Free Polyurethane Networks[END_REF] Triethylaluminum adduct of (2dibenzylamino)ethoxydiethylaluminum induces slow living anionic ROP of ECH (several days to complete conversion) affording heterofunctional PECHs with low dispersity (Đ<1.3) and molar masses ranging between 14 and 44 kg mol -1 . [START_REF] Rodriguez | Ring-Opening Polymerization of Epoxides: Facile Pathway to Functional Polyethers via a Versatile Organoaluminum Initiator[END_REF] Note that direct living anionic ROP of glycidyl azide was recently successfully performed using triethyl borane in conjunction with Bu 4 NBr as an initiator, to generate heterofunctional GAP. [START_REF] Boopathi | Direct Access to Poly(glycidyl azide) and its Copolymers through Anionic (Co-)Polymerization of Glycidyl Azide[END_REF] Clearly, PECHs synthesized by catalytic or anionic means are not ideal tools for the preparation of GAP, principally because of the lack of desired functionality. In stark contrast, the cationic ROP of ECH in conjunction with a molecular diol as an initiator results systematically in dihydroxyterminated PECH. [START_REF] Bideron | Polyepichlorohydrin Diols Free of Cycles: Synthesis and Characterization[END_REF][START_REF] Panda | Synthesis and Characterization of Hydroxyl Terminated Polyepichlorohydrin and Polyglycidylazide[END_REF][START_REF] Kim | Online Monitoring of Reaction Temperature during Cationic Ring Opening Polymerization of Epichlorohydrin in Presence of BF 3 and 1,4-Butanediol[END_REF][START_REF] Kim | The Influence of Initiator Diol-BF 3 Complex Solubility and other Factors on the Induction Period in BF 3 Catalyzed Cationic Ring Opening Polymerization of Epichlorohydrin[END_REF][START_REF] Hafner | Copolymers based on GAP and 1,2-Epoxyhexane as Promising Prepolymers for Energetic Binder Systems[END_REF][START_REF] Murali Mohan | Spectral and DSC Analysis of Glycidyl Azide Polymers Containing Different Initiating Diol Units[END_REF][START_REF] Murali Mohan | Synthesis and Characterization of GAP-PEG Copolymers[END_REF][START_REF] Guanaes | Influence of Polymerization Conditions in the Molecular Weight and Polydispersity of Polyepichlorohydrin[END_REF][START_REF] Grinevich | Oligo(glycidyl azides): New Approaches to Synthesis and Properties[END_REF][START_REF] Francis | Structural Characterization of Hydroxyl Terminated Polyepichlorohydrin Obtained using Boron Trifluoride Etherate and Stannic Chloride as Initiators[END_REF][START_REF] Okamoto | Cationic Ring-Opening Polymerization of Epichlorohydrin in the Presence of Ethylene Glycol[END_REF][START_REF] Kim | Telechelic Polymerization of Epichlorohydrin using 1,4-Butanediyl Ditriflate as the Initiator[END_REF] Two different mechanisms, i.e. activated chain end (ACE) and activated monomer (AM) mechanisms, simultaneously operate during the ROP of ECH, depending on the reaction conditions (Scheme 1). [START_REF] Bideron | Polyepichlorohydrin Diols Free of Cycles: Synthesis and Characterization[END_REF] In ACE mechanism, the active propagating species are oxonium ions located at the chain ends and propagation occurs through the addition of neutral monomer molecules to these oxonium ions (Scheme 1a). Since the main chain (or terminal) oxygen atoms can also attack the propagating oxonium ion, macrocyclic by-products (Scheme 1a) are generated that finally lead to ill-defined mixture of linear PECH diol and cyclic oligomers.

In contrast to ACE mechanism, in AM mechanism, the chain end is neutral and propagation occurs through the nucleophilic attack of the hydroxyl group at the chain end onto activated (i.e. protonated) monomer. Since cyclization is not possible in the AM mechanism, pure PECH diols form under these conditions. ACE mechanism is prominent in conditions where i) the concentration of co-initiator (Lewis acid or Brönsted acid) is higher than the concentration of initiator (alcohols or other protogenic species) and ii) the concentration of monomer is much higher than the content of initiator. Therefore, to shift the propagation from ACE to AM mechanism and to suppress the formation of cyclic oligomers, the cationic ROP of ECH should proceed at low instantaneous concentration of monomer. [START_REF] Bideron | Polyepichlorohydrin Diols Free of Cycles: Synthesis and Characterization[END_REF] Cationic ROP of ECH was already largely described in the literature, using catalytic systems based on such Lewis acids as BF 3 •Et 2 O, [START_REF] Bideron | Polyepichlorohydrin Diols Free of Cycles: Synthesis and Characterization[END_REF][START_REF] Panda | Synthesis and Characterization of Hydroxyl Terminated Polyepichlorohydrin and Polyglycidylazide[END_REF][START_REF] Kim | Online Monitoring of Reaction Temperature during Cationic Ring Opening Polymerization of Epichlorohydrin in Presence of BF 3 and 1,4-Butanediol[END_REF][START_REF] Kim | The Influence of Initiator Diol-BF 3 Complex Solubility and other Factors on the Induction Period in BF 3 Catalyzed Cationic Ring Opening Polymerization of Epichlorohydrin[END_REF][START_REF] Hafner | Copolymers based on GAP and 1,2-Epoxyhexane as Promising Prepolymers for Energetic Binder Systems[END_REF][START_REF] Murali Mohan | Spectral and DSC Analysis of Glycidyl Azide Polymers Containing Different Initiating Diol Units[END_REF][START_REF] Murali Mohan | Synthesis and Characterization of GAP-PEG Copolymers[END_REF][START_REF] Guanaes | Influence of Polymerization Conditions in the Molecular Weight and Polydispersity of Polyepichlorohydrin[END_REF][START_REF] Grinevich | Oligo(glycidyl azides): New Approaches to Synthesis and Properties[END_REF] SnCl 4 [START_REF] Francis | Structural Characterization of Hydroxyl Terminated Polyepichlorohydrin Obtained using Boron Trifluoride Etherate and Stannic Chloride as Initiators[END_REF] Et 3 O + PF 6 -28 as well as 1,4-butanediyl ditriflate [START_REF] Kim | Telechelic Polymerization of Epichlorohydrin using 1,4-Butanediyl Ditriflate as the Initiator[END_REF] and some acidic clays. [START_REF] Yahiaoui | An Acid Exchanged Montmorillonite Clay-Catalyzed Synthesis of Polyepichlorohydrin[END_REF] The ROP of ECH in the presence of acidic clays was done in bulk, so that the resulting polymer was characterized by low M n , broad multimodal molar mass distribution and contains a large fraction of cyclic by-products. [START_REF] Yahiaoui | An Acid Exchanged Montmorillonite Clay-Catalyzed Synthesis of Polyepichlorohydrin[END_REF] Triethyloxonium hexafluorophosphate (Et 3 O + PF 6 -) in conjunction with ethylene glycol allowed to synthesize well-defined PECH diol (Đ  1.3) of low molar masses (M n < 1,000 g mol -1 ). [START_REF] Okamoto | Cationic Ring-Opening Polymerization of Epichlorohydrin in the Presence of Ethylene Glycol[END_REF] The comparison of efficiency of BF [START_REF] Francis | Structural Characterization of Hydroxyl Terminated Polyepichlorohydrin Obtained using Boron Trifluoride Etherate and Stannic Chloride as Initiators[END_REF] The best control of the synthesis of PECH diols free of cycles with controlled molar mass (M n 2500 g mol -1 ) and low dispersity (Đ<1.20) was published by Penczek et al. in 1991. [START_REF] Bideron | Polyepichlorohydrin Diols Free of Cycles: Synthesis and Characterization[END_REF] They used an ethylene glycol (EG)/BF [START_REF] Murali Mohan | Spectral and DSC Analysis of Glycidyl Azide Polymers Containing Different Initiating Diol Units[END_REF] hexylene diol, [START_REF] Murali Mohan | Spectral and DSC Analysis of Glycidyl Azide Polymers Containing Different Initiating Diol Units[END_REF] 1,4-cyclohexanediol [START_REF] Murali Mohan | Spectral and DSC Analysis of Glycidyl Azide Polymers Containing Different Initiating Diol Units[END_REF] and even polyethylene glycols (M n = 200 -600 g mol -1 ) [START_REF] Murali Mohan | Synthesis and Characterization of GAP-PEG Copolymers[END_REF] were used as initiators of cationic ROP of ECH in conjunction with BF 3 •Et 2 O, mainly at 0 C. However, PECHs obtained with these different initiators are often characterized by relatively high dispersity (Đ = 1.40 -2.15) and a functionality lower than 2.0; moreover, their SEC traces display shoulder or even a separate peak in the low molar mass region, indicating the formation of cyclic byproducts. [START_REF] Kim | Online Monitoring of Reaction Temperature during Cationic Ring Opening Polymerization of Epichlorohydrin in Presence of BF 3 and 1,4-Butanediol[END_REF][START_REF] Kim | The Influence of Initiator Diol-BF 3 Complex Solubility and other Factors on the Induction Period in BF 3 Catalyzed Cationic Ring Opening Polymerization of Epichlorohydrin[END_REF][START_REF] Hafner | Copolymers based on GAP and 1,2-Epoxyhexane as Promising Prepolymers for Energetic Binder Systems[END_REF][START_REF] Murali Mohan | Spectral and DSC Analysis of Glycidyl Azide Polymers Containing Different Initiating Diol Units[END_REF][START_REF] Murali Mohan | Synthesis and Characterization of GAP-PEG Copolymers[END_REF][START_REF] Guanaes | Influence of Polymerization Conditions in the Molecular Weight and Polydispersity of Polyepichlorohydrin[END_REF][START_REF] Grinevich | Oligo(glycidyl azides): New Approaches to Synthesis and Properties[END_REF][START_REF] Francis | Structural Characterization of Hydroxyl Terminated Polyepichlorohydrin Obtained using Boron Trifluoride Etherate and Stannic Chloride as Initiators[END_REF] The main differences between these works and Penczek's one [START_REF] Bideron | Polyepichlorohydrin Diols Free of Cycles: Synthesis and Characterization[END_REF] lie in the nature of initiator, the polymerization temperature as well as the initiator to co-initiator ratio

([EG]/[BF 3 •Et 2 O]  50).
Recently, Kim et al.20 ,[START_REF] Kim | The Influence of Initiator Diol-BF 3 Complex Solubility and other Factors on the Induction Period in BF 3 Catalyzed Cationic Ring Opening Polymerization of Epichlorohydrin[END_REF] observed an induction period during ROP of ECH with BD/BF

O] = 2)
resulted in an increase of exothermicity during the polymerization, after the induction period ceased. [START_REF] Kim | Online Monitoring of Reaction Temperature during Cationic Ring Opening Polymerization of Epichlorohydrin in Presence of BF 3 and 1,4-Butanediol[END_REF][START_REF] Kim | The Influence of Initiator Diol-BF 3 Complex Solubility and other Factors on the Induction Period in BF 3 Catalyzed Cationic Ring Opening Polymerization of Epichlorohydrin[END_REF] The main purpose of this study was originally to better understand the mechanisms of ECH ROP to find conditions avoiding the use of chlorinated solvents. In this contribution, we report detailed mechanistic study of BF 3 •Et 2 O-catalyzed ROP of ECH in toluene solution and bulk conditions. A simple and efficient initiating system was developed for fast synthesis of perfectly-functionalized PECH diols (F n (OH)  2.0) of controlled molar masses (M n up to 4000 g mol -1 ) and low dispersity (Đ < 1.25). Such procedure was easily upscaled to 100 g of polymer. Additionally, a detailed characterization of the synthesized PECHs was performed with the aim to elucidate the reasons leading to the coloration of the polymers during the storage of non-purified samples. Coloration in the course of polymerization was observed before by some authors, [START_REF] Bideron | Polyepichlorohydrin Diols Free of Cycles: Synthesis and Characterization[END_REF][START_REF] Kim | Online Monitoring of Reaction Temperature during Cationic Ring Opening Polymerization of Epichlorohydrin in Presence of BF 3 and 1,4-Butanediol[END_REF][START_REF] Kim | The Influence of Initiator Diol-BF 3 Complex Solubility and other Factors on the Induction Period in BF 3 Catalyzed Cationic Ring Opening Polymerization of Epichlorohydrin[END_REF][START_REF] Hafner | Copolymers based on GAP and 1,2-Epoxyhexane as Promising Prepolymers for Energetic Binder Systems[END_REF] but never explained.

EXPERIMENTAL SECTION

Materials and methods

Epichlorohydrin (Aldrich, ≥ 99%), 1,4-butanediol (BD, Aldrich, 99%), 3-chloro- differential refractometer thermostated at 35°C. Tetrahydrofuran was eluted at a flow rate of 1.0 mL/min. The calculations of molar mass and dispersity are based on polystyrene standards (Polymer Labs, Germany). [START_REF] Vandenberg | Address: Reflections on 50 years of polymer chemistry[END_REF] H NMR spectra were recorded in CDCl 3 at 25°C on a Bruker AC-500 spectrometer. [START_REF] Araki | Ethylaluminum Oxide Catalysts from Et 2 AlOLi-Et 2 AlCl Binary System in Relation to Species of AlEt 3 -Water Catalyst[END_REF] C and [START_REF] Araki | Ethylaluminum Oxide Catalysts from Et 2 AlOLi-Et 2 AlCl Binary System in Relation to Species of AlEt 3 -Water Catalyst[END_REF] C DEPT-135 NMR spectra were recorded in DMSO-d 6 at 25°C on a Bruker AC-500 spectrometer. A pulse delay of 10 s was given for recording the [START_REF] Araki | Ethylaluminum Oxide Catalysts from Et 2 AlOLi-Et 2 AlCl Binary System in Relation to Species of AlEt 3 -Water Catalyst[END_REF] C NMR spectra and about 15000 scans were accumulated.

Page 12 of 57 ACS Paragon Plus Environment ACS Applied Polymer Materials 13 MALDI-TOF MS was performed on a Voyager mass spectrometer (AB Sciex) equipped with a pulsed N 2 laser (337 nm) and a time-delayed extracted ion source. Spectra were recorded in the positive-ion mode using a reflectron and with an accelerating voltage of 20 kV. Solutions of dithranol (20 mg/mL), polymer sample (10 mg/mL) and sodium iodide (10 mg/mL) were prepared in THF. These solutions were then mixed in the ratio of matrix/sample/cationizing salt = 10:2:1 and 0.5 μL aliquots were applied to a MALDI sample target for analysis. The instrument was calibrated externally using poly(ethylene glycol) standards of appropriate molar masses. All massto-charge ratios (m/z) quoted are monoisotopic, i.e. they include the most abundant isotopes of the elements present in the corresponding ion.

The content of H 2 O in some reactants was determined by a semi-micro method on an automatic titrator Titrino 848/877 using the Karl-Fischer method. Determination of the hydroxyl number of PECH samples was performed via acid-base titration according to ASTM D4274 -16.

Polymerization procedures

Polymerizations were carried out in a 50 mL three-necked flask, connected with a vacuum system, equipped with a magnetic (solution polymerization) or mechanical (bulk polymerization) stirrer, a thermocouple to control the reaction temperature and a rubber septum to introduce the reagents. Before polymerization, the flask was systematically vacuumed and filled with argon three times.

In a typical polymerization experiment in toluene with in situ prepared initiator, the flask was charged with 0.05 mL of H 2 O, 0.034 mL of BF 3 •Et 2 O and 0.43 mL of ECH (ECH/H 2 O = 2/1 mol/mol) and plunged in a water bath at 20 o C. This heterogeneous mixture was stirred for 1 -1.5 h until it became a homogeneous transparent viscous liquid. Then 17.3 mL of toluene/ECH mixture (1/2 v/v) was added by a syringe pump at a rate of 2.6 mL/h (in respect to pure ECH) for 4.5 h. After the completion of monomer addition, the mixture was stirred for an additional 2.5 hours. The reaction was then quenched by addition of distilled water. The reaction mixture was washed with sodium bicarbonate solution (5 wt.%), then several times with distilled water until the washing water comes back to neutral pH. The solvent was evaporated using a rotary evaporator under reduced pressure and the resulting polymer was dried in vacuum at 60 o C. The monomer conversion was determined gravimetrically.

Polymerization of epichlorohydrin in bulk with in situ prepared initiator was carried as followed.

The flask was charged with 0.05 mL of H 2 O, 0.023 mL of BF 3 •Et 2 O and 0.46 mL of ECH (ECH/H 2 O = 2/1 mol/mol) and placed in a water bath at 20 o C. This heterogeneous mixture was stirred for 1 -1.5 h until forming a transparent viscous liquid. Then the water bath was heated to 40 o C and 5.4 mL of ECH was added via a syringe pump at a rate of 1 mL/h during 5.4 h. After monomer feeding, the mixture was stirred for one additional hour. Finally, the polymer was dried in vacuum at 60 o C. The monomer conversion was determined gravimetrically.

Chain-end titration

The number average functionality on hydroxyl groups (F n ) was calculated as the ratio of molar mass determined by size-exclusion chromatography to molar mass calculated from 1 H NMR spectrum based on the content of hydroxyl groups (Figure S1), which is sensitive to the presence of cyclic by-products.

The molar mass as well as the content of secondary and primary hydroxyl groups at the chain ends were determined from 1 H NMR spectra of synthesized PECHs (Figure S1). Because of the overlapping of protons of methylene (-CH 2 -OH) and methine (-CH(CH 2 Cl)-OH) end-groups with the signals of main chain protons, the hydroxyl end-groups were converted into ester end-groups by reaction with excess of trifluoroacetic anhydride. [START_REF] Bideron | Polyepichlorohydrin Diols Free of Cycles: Synthesis and Characterization[END_REF] This reaction was performed directly in the NMR tube containing the solution of PECH in CDCl 3 . The content of secondary hydroxyl groups was calculated from 1 H NMR spectrum (Figure S1) according to the following equation:

% secondary OH=∫(d)/[∫(d)+ ∫(d')/2] ( 1
)
All PECH samples synthesized in this work were found to bear mainly secondary terminal hydroxyl groups ( 96 mol%).

The molar mass of the PECHs (M n,NMR ) was calculated from 1 H NMR spectrum (Figure S1) according to equation presented below:

M n,NMR = [∫(a+b+c)/5]/[∫(d')/4+ ∫(d)/2]92.5 + M I (2)
where M I is the molar mass of the fragment of initiator introduced in the polymer chain, if any (vide infra).

Since M n,SEC is calculated against polystyrene standards, it is not absolute and must be corrected.

To do so, some polymers known to be perfectly difunctional and not containing any macrocycles (confirmed by MALDI TOF MS for certain samples) were titrated to calculate their hydroxyl number and deduce their absolute molar mass (Table S1). The obtained values (M n,titration ) were close to M n determined by 1 H NMR according to chain-end versus main chain proton integration (eq. ( 2)), thus confirming the perfect functionality of analyzed samples. Finally, M n,SEC was plotted against M n,titration and a correction coefficient of 0.93 was deduced from this plot (Table S1, Figure S2). Considering the fact that molar masses determined by 1 H NMR spectroscopy are sensitive to the presence of macrocyclic chains, that have no end-groups, the difference between M n,NMR and corrected M n,SEC values were later used for the calculation of the number average functionality on hydroxyl end groups (F n (OH)) without the need for chemical titration. Then, in the following, the number average functionality of the chains was calculated as F n = [M n (SEC)/M n (NMR)]20.93.

RESULTS AND DISCUSSION

Solution Polymerization

Preliminary experiments with butanediol as an initiator

An initial series of experiments was conducted using 1,4-butanediol/BF for a theoretical M n of ≈ 2500 g mol -1 . [START_REF] Kim | Online Monitoring of Reaction Temperature during Cationic Ring Opening Polymerization of Epichlorohydrin in Presence of BF 3 and 1,4-Butanediol[END_REF][START_REF] Kim | The Influence of Initiator Diol-BF 3 Complex Solubility and other Factors on the Induction Period in BF 3 Catalyzed Cationic Ring Opening Polymerization of Epichlorohydrin[END_REF] Firstly, the effect of solvents of different polarities such as CH 2 Cl 2 , toluene, acetonitrile and nitromethane on the cationic ROP of ECH was briefly investigated. Toluene was selected as the most suited substitute to CH 2 Cl 2 , since this solvent led to polymers with the closest properties from CH 2 Cl 2 -prepared ones (Table S2). Still here, PECHs synthesized in these two solvents showed bimodal molar mass distribution (MWD) (Figure S3)

and the functionality at the chain end was typically much lower than expected for an ideal telechelic hydroxyl-terminated poly(epichlorohydrin) (F n ≈ 1 against an expected value of 2, Table S2). Both the presence of a low molar mass peak in SEC curves and the observed low number average functionality on hydroxyl groups indicate that ACE mechanism competes with AM one, thus leading to the formation of macrocyclic by-products. [START_REF] Bideron | Polyepichlorohydrin Diols Free of Cycles: Synthesis and Characterization[END_REF][START_REF] Panda | Synthesis and Characterization of Hydroxyl Terminated Polyepichlorohydrin and Polyglycidylazide[END_REF][START_REF] Kim | Online Monitoring of Reaction Temperature during Cationic Ring Opening Polymerization of Epichlorohydrin in Presence of BF 3 and 1,4-Butanediol[END_REF][START_REF] Kim | The Influence of Initiator Diol-BF 3 Complex Solubility and other Factors on the Induction Period in BF 3 Catalyzed Cationic Ring Opening Polymerization of Epichlorohydrin[END_REF][START_REF] Hafner | Copolymers based on GAP and 1,2-Epoxyhexane as Promising Prepolymers for Energetic Binder Systems[END_REF][START_REF] Murali Mohan | Spectral and DSC Analysis of Glycidyl Azide Polymers Containing Different Initiating Diol Units[END_REF][START_REF] Murali Mohan | Synthesis and Characterization of GAP-PEG Copolymers[END_REF][START_REF] Guanaes | Influence of Polymerization Conditions in the Molecular Weight and Polydispersity of Polyepichlorohydrin[END_REF] We also looked at the effect of temperature of reaction: raising the temperature from 0 to 30 °C increases the solubility of initiator (BD) as well as its complex with BF 3 •Et 2 O in toluene (Figure S4), as also reported by Kim et al.; this in turn allowed preparing better-defined PECH with improved MWD and functionality (Table S3, Figure S5). [START_REF] Kim | Online Monitoring of Reaction Temperature during Cationic Ring Opening Polymerization of Epichlorohydrin in Presence of BF 3 and 1,4-Butanediol[END_REF][START_REF] Kim | The Influence of Initiator Diol-BF 3 Complex Solubility and other Factors on the Induction Period in BF 3 Catalyzed Cationic Ring Opening Polymerization of Epichlorohydrin[END_REF][START_REF] Hafner | Copolymers based on GAP and 1,2-Epoxyhexane as Promising Prepolymers for Energetic Binder Systems[END_REF] Next, the ratio [BD]/[BF 3 •Et 2 O] was systematically decreased (Table S4). In all cases, an induction period of several hours followed by exotherms of different magnitudes were observed.

The induction period can be ascribed to slow initiation and/or polymerization of ECH due to the poor solubility of initiator (BD) and/or its complex with BF 3 •Et 2 O in toluene (Figure S4). The accumulation of monomer in the reaction mixture then results in a significant runaway of polymerization after BD has been fully consumed, responsible for the exotherm. Again, the ACE mechanism operating here (Scheme 1) decreases the functionality and increases the dispersity of synthesized PECH chains (Table S4). This loss of control is particularly obvious when decreasing S4).

In order to gain deeper insight into the mechanism of ROP of ECH with BD/BF 3 •Et 2 O initiating system, PECH synthesized under the best conditions at this stage (run S17, Table S4) was characterized by MALDI TOF MS. The spectrum shows the presence of several populations of polymer chains of different structures in low and high molar mass regions, respectively (Figure 1, full spectrum in Figure S6). The major population of chains in the low molar mass region corresponds to the diol HO-(ECH) n -OH cationized by Na + (e.g. for n = 15, m/z = 1428.8 g mol -1 ), i.e. which does not embed any fragment of initiator in the polymer chain (Figure 1a). The small population with slightly lower molar mass (-18 m/z) can be assigned to cyclic chains (ECH) n (e.g. for n = 15, m/z = 1410.9 g mol -1 ). The two minor peaks that appear in the low molar mass region at -37 m/z and -69 m/z in Figure 1a correspond to HO-(ECH) n -OH oligomers that lost one and two HCl molecules per chain, respectively (vide infra) (Figure 1a). S4). Full spectrum is given in Figure S6.

In contrast, two major populations of chains of almost equal intensities are present in the high molar mass region of the MALDI TOF MS spectrum (Figure 1b). One family of chains again corresponds to HO-(ECH) n -OH cationized by Na + (for n =25, m/z = 2354.5 g mol -1 ). Another population (m/z = 2334.7 g mol -1 ) could be assigned either to cyclic polymer chains (ECH) n or PECH diol chains containing one fragment of initiator, i.e. HO-(ECH) k -BD-(ECH) m -OH: the peak difference between these two structures is only 2 m/z. The formation of cyclic chains of high molar mass is unlikely, while the presence of 1,4-butanediol fragment in a polymer chain was confirmed by 1 H NMR on the PECH sample reacted with trifluoroacetic anhydride (Figure S7).

Indeed, [START_REF] Vandenberg | Address: Reflections on 50 years of polymer chemistry[END_REF] H NMR spectrum shows a clear signal at 1.67 ppm assigned to the protons of the -CH 2 -group of BD fragment in the polymer chain. Moreover, the molar mass calculated from [START_REF] Vandenberg | Address: Reflections on 50 years of polymer chemistry[END_REF] H NMR spectrum assuming one initiator fragment in the polymer chain (M n =2960 g mol -1 ) is higher than the theoretical one (M n =2470 g mol -1 ), the latter which is higher than the one based on the terminal hydroxyl groups (M n =2370 g mol -1 ). These data confirm the presence of a mix of both types of poly(epichlorohydrin) diols in the sample, i.e. with and without the initiator fragment inside the polymer chain.

One reasonable explanation for the presence of a significant fraction of HO-(ECH) n -OH chains could be the competition brought by adventitious water, as efficient an initiator as BD. Indeed, as determined by Karl Fisher titration, the content of H 2 O in BD (28.2 mM) and in monomer (9.2 mM) is quite high, even after drying with CaH 2 , and anyhow much higher than in toluene (2.9 mM). In addition, the presence of only HO-(ECH) n -OH chains in the low molar mass region indicates that at least part of adventitious water comes from the monomer slowly added into the reactor during the polymerization.

Using toluene as a solvent and applying temperatures at 20 C or above seem to give as good conditions for cationic ROP of ECH with BD as initiator as reported before in CH 2 Cl 2 . However, the use of BD/BF well-defined PECH diols; and finally (iv) the induction period followed by the sharp, uncontrolled increase of the temperature are detrimental to industrial application.

Living polymerization of ECH in toluene

In the following, we decided to use H 2 O as the initiator of BF Then, a precise amount of H 2 O was added to the polymerization system. In this case, the polymerization proceeded in two stages: (i) in situ preparation of initiator in aqueous suspension (ECH/H 2 O from 1.5:1 to 1.05:1 mol/mol) (1.5 -4h) followed by (ii) slow addition of monomer/toluene mixture (rate of addition: 2.5 mL/h,  5 h). The chosen molar ratio of ECH to water is crucial and should be well below 1.5 to control the temperature during in situ initiator preparation (Table S6), especially in a view of a scale up (vide infra). During this stage, a mixture of CLPD, the monoadduct of ECH with water, and ECH oligomers are generated, the content of the former increasing with decreasing ECH/H 2 O ratio (Figure S9). Note that for successful in situ initiator preparation, ECH should be added into the system just after premixing of BF 3 •Et 2 O with water. Indeed, postponing the monomer addition to 4.5 h resulted in significant slowing down of the polymerization (Table S7), most probably, because of partial hydrolysis of BF 3 in aqueous environment, as we showed it in a previous study. [START_REF] Radchenko | Controlled/Living Cationic Polymerization of Styrene with BF 3 OEt 2 as Coinitiator in the Presence of Water: Improvements and Limitations[END_REF] In ideal conditions, H 2 O/BF 3 •Et 2 O initiating system allows synthesizing well-defined PECH (M n ≈ 2500 g mol -1 ) of low dispersity (Đ  1.2)

and perfect chain-end functionality (F n (OH) ≈ 2.0) without any visible change in the reaction temperature (Table 1). Similar results in terms of control of M n , dispersity and functionality were obtained introducing directly commercial CLPD as the initiator whereas, in strong contrast, BD initiator led to an induction period accompanied by a high reaction exotherm in the same reaction conditions (Table 1).

plots (Figure 2a). The molar mass increases in direct proportion to [M]/[I] ratio and a good correlation between theoretical and experimental values of M n (both determined by SEC and 1 H NMR spectroscopy) is observed up to M n ≈ 3500 g mol -1 (Figure 2a). S8 for detailed experimental conditions).

(c) Fragment of MALDI TOF MS spectrum of PECH (M n =2510 g mol -1 ; Đ=1.16) synthesized with H 2 O/BF 3 •Et 2 O initiating system in toluene (run 2, Table 1). Series of peaks are mainly separated either by 92.5 or 94.5 m/z depending on the majority of stable chlorine isotopes Cl [START_REF] Boopathi | Direct Access to Poly(glycidyl azide) and its Copolymers through Anionic (Co-)Polymerization of Glycidyl Azide[END_REF] and Cl [START_REF] Radchenko | Controlled/Living Cationic Polymerization of Styrene with BF 3 OEt 2 as Coinitiator in the Presence of Water: Improvements and Limitations[END_REF] the ECH units contains, respectively. [START_REF] Wei | Regio-Selective Synthesis of Polyepichlorohydrin Diol using Zn-Co(III) Double Metal Cyanide Complex[END_REF] Some deviation from the theoretical line was noted at higher molar masses because of competitive initiation of polymerization by water supplied continuously with the monomer; severe drying of reactant by molecular sieves increased to some extent the molar mass of synthesized polymers (see Table S8). The dispersity of synthesized poly(epichlorohydrin)s only slightly increased with increasing [M]/[I] ratio (Figure 2a, Table S8), while SEC traces shifted towards larger molar masses as [M]/[I] ratio increased (Figure 2b). In addition, the number average functionality of hydroxyl group was measured close to 2 and did not depend on [M]/[I] ratio for all ratios studied (Table S8). These results indicate that cationic ROP of ECH with H 2 O/BF 3 •Et 2 O initiating system proceeds in a living fashion, allowing synthesizing fully functionalized poly(epichlorohydrin) diols without visible traces of macrocyclic side compounds. MALDI TOF MS analysis of PECH samples of different molar masses was performed to confirm the exclusive presence of PECH diols (two full spectra in Figure S10, zoom on one spectrum in Figure 2c). The major population of chains corresponds to HO-(ECH) n -OH cationized by Na + . Similarly to previous MALDI TOF MS spectrum (Figure 1), the two other minor populations were assigned to HO-(ECH) n -OH that lost one or two HCl molecules per chain. No signals of macrocyclic PECH chains (ECH) n were tracked here (Figure 2c and Figure S10).

The preparation of pure PECH diol was finally confirmed by reacting a PECH diol sample (M n = 3160 g mol -1 , Đ = 1.20) with 4,4′-methylenebis(phenyl isocyanate) to readily generate a high molar mass polyurethane (M n = 17,900 g mol -1 ; Đ = 2.48) (Figure S11). In addition, PECH diol distribution almost completely disappeared in the SEC spectrum, thus confirming the perfect chain-end functionality (Figure S11).

To summarize, H 2 O/BF 3 •Et 2 O is a simple but very efficient initiating system for living cationic ROP of ECH, affording macrocyclic-free PECHs with M n between 500 and 4100 g mol -1 , low dispersity (Đ  1.25) and exceptionally high chain-end functionality (F n (OH) ≈ 2.0).

Towards industrially-suitable conditions

Previously-reported monomer-starved conditions to obtain PECH with high chain-end functionality are typically between 20 and 48 h, that is too long for an industrial process. [START_REF] Bideron | Polyepichlorohydrin Diols Free of Cycles: Synthesis and Characterization[END_REF][START_REF] Panda | Synthesis and Characterization of Hydroxyl Terminated Polyepichlorohydrin and Polyglycidylazide[END_REF][START_REF] Kim | Online Monitoring of Reaction Temperature during Cationic Ring Opening Polymerization of Epichlorohydrin in Presence of BF 3 and 1,4-Butanediol[END_REF][START_REF] Kim | The Influence of Initiator Diol-BF 3 Complex Solubility and other Factors on the Induction Period in BF 3 Catalyzed Cationic Ring Opening Polymerization of Epichlorohydrin[END_REF][START_REF] Hafner | Copolymers based on GAP and 1,2-Epoxyhexane as Promising Prepolymers for Energetic Binder Systems[END_REF] We then investigated the influence of the rate of monomer addition on the cationic ROP of ECH so as to optimize the reaction time. As shown in Table 2, the increase of rate of monomer addition from 0.7 to 3.5 mL/h allowed to shorten significantly the polymerization time (from 18 down to 3.5 h), without impairing the monomer conversion and polymer functionality. Dispersity of synthesized polymers only slightly increased when increasing the rate of monomer addition. Faster monomer addition (4.5 mL/h) resulted in incomplete monomer conversion (80%), but led anyway to well-defined macrocyclic-free PECH diols with perfect functionality (Table 2, Figure 3a). Interestingly, when more dilution was applied (toluene/ECH = 2/1 instead of 1/2 vol/vol) together with a higher rate of monomer addition (4.5 mL/h), the polymerization proceeded at lower rate, affording PECH with lower than quantitative functionality (Table S9) and containing macrocyclic by-products (Figure 3b). This result indicates that under "concentrated" conditions, the monomer is consumed faster by propagation, allowing maintaining the instantaneous concentration of ECH at very low level. Logically, reducing the rate of monomer addition under "diluted" conditions resulted in living cationic ROP of ECH to receive macrocycle-free PECH chains (Figure 3b, Table S9). 2, while numbers in Figure 3b correspond to runs in Table S9, respectively.

Then, the effect of temperature was briefly investigated in the H 2 O/BF 3 •Et 2 O initiating system.

In general, similarly to ECH polymerization with BD/BF 3 •Et 2 O initiating system (Table S3, Figure S5), the decrease of reaction temperature below 20 ºC led to a slowing down of the reaction as well as to the generation of a fraction of macrocyclic by-products (Table S10, Figure S12). On the contrary, increasing the temperature from 20 to 40 ºC did not have any negative effects on the reaction rate and preserved the properties of the synthesized PECHs (Table S10).

In all studies related to cationic, anionic or coordinative ROP of ECH, extensive drying and purification of all reagents and monomer are generally done because of the known sensitivity of used co-initiators/catalysts towards even traces of water. On the other hand, our team [START_REF] Radchenko | Controlled/Living Cationic Polymerization of Styrene with BF 3 OEt 2 as Coinitiator in the Presence of Water: Improvements and Limitations[END_REF] and others [START_REF] Satoh | Direct Synthesis of Amphiphilic Random and Block Copolymers of p-Hydroxystyrene and p-Methoxystyrene via Living Cationic Polymerization with BF3OEt 2 /ROH Systems[END_REF] showed that BF 3 •Et 2 O could be considered as a water-tolerant Lewis acid, as it slowly hydrolyzes in aqueous environment. Well-defined PECH diols (M n ≈ 2500 g mol -1 , F n (OH) ≈ 2.0) with quite low dispersity (Đ<1.2) were obtained using non-purified reagents and monomer, as shown in Table 3. Moreover, neither the reaction rate, the molar mass, the dispersity nor the chain-end functionality were affected in these raw conditions (Table S11, Figure S13).

Intermediate conclusion on ECH polymerization in toluene solution

H 2 O/BF 3 OEt 2 initiating system where the initiator is prepared in situ by reaction of ECH with water allowed synthesizing well-defined PECH diols at much faster rate that BD/BF 3 •Et 2 O 20,21 and ethylene glycol/BF 3 •Et 2 O 18 initiating systems. This achievement is indebted to the much better solubility of both CLPD and low molar mass oligomers formed in the course of in situ initiator preparation in the reaction media, as compared to BD and ethylene glycol (Figure S4, Figure S14, see also section "Scale-up of ECH polymerization" for more details). Polymerizations performed under "concentrated" conditions are more efficient than in "diluted" conditions (compare Figure 3a and Figure 3b) thanks again to the better solubility of CLPD or ECH oligomers in monomer than in toluene (Figure S14). An increase of polymerization temperature also results in enhanced solubility of initiator/low molar mass oligomers and, in turn, facilitates the preparation of welldefined PECH diols via a fast living ROP of ECH using H 2 O/BF 3 •Et 2 O initiating system. In optimized conditions (temperature  25 ºC; toluene/ECH 1:2 vol/vol), H 2 O/BF 3 •Et 2 O initiating system allows synthesizing well-defined macrocycle-free PECH diols (F n (OH) ≈ 2.0) with M n up to 4000 g mol -1 and low dispersity (Đ  1.25) at relatively high rate of monomer addition and a total polymerization time of 3 -4 h. The robustness and high potential for industrial use of this initiating system is also strengthened by the possibility to conduct the polymerization using unpurified reagents and monomer.

Bulk Polymerization

Page 32 of 57 ACS Paragon Plus Environment ACS Applied Polymer Materials

33

Motivated by the much better solubility of ECH oligomers in monomer than in toluene (Figure S14), the cationic ROP of ECH in bulk using H 2 O/BF 3 •Et 2 O initiating system was then investigated. To account for the relatively high viscosity of PECH with M n ≈ 2000 g mol -1 at 20 ºC, [START_REF] Bideron | Polyepichlorohydrin Diols Free of Cycles: Synthesis and Characterization[END_REF] the ROP of ECH at [ECH]/[H 2 O] > 15 was performed at 40 ºC under mechanical stirring (all previous experiments in toluene were performed using a magnetic stirrer) (Table S12). The bulk cationic ROP of ECH using H 2 O/BF 3 •Et 2 O initiating system proceeds in a living fashion: the experimental values determined by both SEC and 1 H NMR correlate well with the theoretical line up to M n ≈ 2500 g mol -1 , while dispersity remains low (Đ  1.23) (Figure 4a). SEC traces of PECH samples are symmetrical at all [M]/[I] ratios studied here and shift towards higher molar masses with increasing [M]/[I] ratio (Figure 4b). S12) synthesized with

H 2 O/BF 3 •Et 2 O initiating system in bulk.
Importantly, the functionality of the chain-ends does not depend on [M]/[I] ratio and is close to quantitative for all [M]/[I] ratios studied (Table S12). In order to confirm the absence of macrocyclic PECH chains and other side products in polymers obtained via bulk polymerization, MALDI TOF MS was performed (Figure 4c,d, full spectrum in Figure S15). The major population of chains in both low and high molar mass regions corresponds to polyepichlorydrin diol HO-(ECH) n -OH cationized by Na + . Minor peaks (-36 and -68 m/z) can be assigned to PECH diol chains that lost one and two HCl molecules, respectively. Finally, a small massif of peaks corresponding to cyclic PECH (-18 m/z) is present only in the low molar mass part of MALDI TOF MS spectra. Thus, bulk cationic ROP of ECH with H 2 O/BF 3 •Et 2 O initiating system produces predominantly PECH diol and a negligible fraction of cyclic by-products.

It should be noted that the synthesis of PECH of similar molar mass in bulk and using ethylene glycol/BF 3 •Et 2 O initiating system, was reported previously by Penczek et al. [START_REF] Bideron | Polyepichlorohydrin Diols Free of Cycles: Synthesis and Characterization[END_REF] The main improvement achieved in the present work using H 2 O/BF 3 •Et 2 O initiating system is the successful reduction of the polymerization time (from 48 to 5 -6 h) without any deleterious effect on the dispersity and functionality at the chain end of synthesized PECHs. This is ascribed to the much better solubility of in situ prepared initiator in reaction medium as compared to ethylene glycol.

Reasons for polymer coloring

PECHs synthesized both in solution and in bulk are transparent viscous liquids just after reaction, but then become slightly yellow (especially those obtained in bulk) upon storage for several months. The conditions of storage influence strongly the color intensity: a sample stored at -20 ºC remains transparent, while the strongest coloration is observed for the same sample stored at 45 ºC (FigureS16). The extension of the storage time also increases the intensity of the color, especially for the sample stored at 45 ºC (Figure S17). The coloration of PECHs prepared in bulk is tentatively associated with the presence of traces of co-initiator in the samples that were not purified (Figure S16). Indeed, a PECH sample obtained in bulk and washed with Na 2 CO 3 and water is still transparent even after 2 months of storage (compare samples 2 and 5, Figure S16).

The coloration of the sample upon storage does not affect the molar mass and the dispersity of the synthesized polymers (Figure S18). Moreover, PECHs samples stored at -20 ºC (sample 1, Figure S16) and 45 ºC (sample 4, Figure S16) show almost identical MALDI TOF MS spectra (Figure S19), i.e. the functionality at the chain end is not affected with time.

Based on these observations, we can conclude that changes inside the main chain are most probably responsible for the observed coloration of PECH samples. To check so, the structure of the PECH stored at 45 ºC was investigated by [START_REF] Araki | Ethylaluminum Oxide Catalysts from Et 2 AlOLi-Et 2 AlCl Binary System in Relation to Species of AlEt 3 -Water Catalyst[END_REF] C and [START_REF] Araki | Ethylaluminum Oxide Catalysts from Et 2 AlOLi-Et 2 AlCl Binary System in Relation to Species of AlEt 3 -Water Catalyst[END_REF] C-DEPT NMR spectroscopy (Figure 5). The signals at 44.2 and 78.1 ppm were assigned to the main chain carbon atoms of chloromethyl (c) and methine (b) groups in [START_REF] Araki | Ethylaluminum Oxide Catalysts from Et 2 AlOLi-Et 2 AlCl Binary System in Relation to Species of AlEt 3 -Water Catalyst[END_REF] C NMR spectrum, respectively, while the carbon signal of the methylene groups of the main chain (a) is splitting into three signals corresponding to regular head-to-tail (68.7 ppm, H-T), inverse tail-to-tail (70.2 ppm, T-T) and head-to-head (70.6 ppm, H-H) units. The carbon signals of terminal epichlorohydrin unit connected to hydroxyl group were identified with the help of [START_REF] Araki | Ethylaluminum Oxide Catalysts from Et 2 AlOLi-Et 2 AlCl Binary System in Relation to Species of AlEt 3 -Water Catalyst[END_REF] C DEPT-135 NMR spectroscopy (Figure 5b) at the following chemical shifts: 47.1 (c'), 69.3 (b') and 70.8 ppm (a'). In addition to these signals, a resonance at 15.2 ppm (d) in the aliphatic part of [START_REF] Araki | Ethylaluminum Oxide Catalysts from Et 2 AlOLi-Et 2 AlCl Binary System in Relation to Species of AlEt 3 -Water Catalyst[END_REF] C NMR spectrum was assigned to the carbon of the methyl group close to an internal main chain double bond (Figure 5). The signals at ca. 101.5 ppm (e) on both [START_REF] Araki | Ethylaluminum Oxide Catalysts from Et 2 AlOLi-Et 2 AlCl Binary System in Relation to Species of AlEt 3 -Water Catalyst[END_REF] C NMR and [START_REF] Araki | Ethylaluminum Oxide Catalysts from Et 2 AlOLi-Et 2 AlCl Binary System in Relation to Species of AlEt 3 -Water Catalyst[END_REF] C DEPT-135 NMR spectra can be assigned to carbon atoms of internal olefinic methine group in the main chain (Figure 5b). Another signal in the olefinic part of [START_REF] Araki | Ethylaluminum Oxide Catalysts from Et 2 AlOLi-Et 2 AlCl Binary System in Relation to Species of AlEt 3 -Water Catalyst[END_REF] C NMR spectrum at 162.0 ppm (f) corresponds to the signal of the tertiary carbon atom connected to an external main chain double bond, as confirmed by [START_REF] Araki | Ethylaluminum Oxide Catalysts from Et 2 AlOLi-Et 2 AlCl Binary System in Relation to Species of AlEt 3 -Water Catalyst[END_REF] C DEPT-135 NMR (Figure 5b). Note that the signal of the carbon atom of olefinic methylene group (CH 2 =) is theoretically assigned at ca. 77 ppm and, therefore, surely overlaps with the resonances of main chain carbon atoms. Based on these data, the following mechanism for the sample yellowing is proposed (Scheme 2).

HCl abstraction by a trace of Lewis acid, together with temperature and/or sunlight irradiation, can occur during the polymerization or storage of the bulk sample, leading to the formation of a side olefinic group in a main chain. Then, this double bond is protonated by water with the formation of a stable tertiary cation. The elimination of proton from the neighboring methylene group leads to the formation of a thermodynamically stable internal double bond. Similar repetition of reactions with neighboring chloromethyl group results in formation of conjugated double bonds (through the oxygen), which, most probably, are responsible for the observed coloration of PECH samples under long storage. Note that for GAP application, the loss of HCl groups in the course of polymerization or during the storage of PECH decreases the degree of functionalization by azide, which means undesirable loss of energetic properties of solid propellants. Therefore, for the preparation of high quality PECH diol, not only the polymerization process should be mastered, but also the conditions of storage and/or transportation of polymer need controls.

Scale-up of ECH polymerization

The solution process in toluene was first successfully upscaled to 200 g of PECH using a low

[BF 3 •Et 2 O]/[H 2 O
] ratio (1:26 mol/mol) and a slow monomer addition (Table S13). The in situ initiator preparation as well as polymerization proceeded smoothly under such conditions since reaction temperature did not exceed 35ºC. Final results are a conversion above 98% after 23 h of polymerization, PECH chains of M n of 2300 g mol -1 , low dispersity (Ð < 1.15) and quantitative functionality (F n (OH) ≈ 2.0) (Table S13).

An attempt to decrease the polymerization time, using the best conditions found in small-scale batch experiments (10 g ECH, run 9, Table 2) led to the following difficulties: i) an uncontrolled reaction with a high temperature jump during in situ initiator preparation when

[BF 3 •Et 2 O]/[H 2 O]
was increased from 1:26 to 1:15; and ii) a significant temperature increase (up to 70ºC) rapidly after the beginning of the polymerization of ECH (typically in less than 1 h). This uncontrolled rise of temperature was attributed to the much faster ROP of ECH than the initiator formation under these homogeneous conditions. To solve this problem, we added some solvent (toluene) into the reaction mixture to maintain the heterogeneity of the system during the in situ initiator preparation. Doing so, the reaction mixture remained heterogeneous all along this first step, as shown in Figure S20, and no temperature change was observed, even when preparing about 50 g of initiator solution (for about 1 kg of final PECH content). Increasing the rate of addition of ECH also resulted in an increase of the reaction temperature up to 75ºC approximately after 30 min since the beginning of the polymerization (Table S14). This temperature jump coincided with the change of the state of reaction mixture from heterogeneous to homogeneous. Still, apart from this exotherm, the characteristics of the final polymers were basically the same.

To better understand this solubility issue, oligomers of ECH with degree of polymerization (DP n ) from 1 to 5 were synthesized (Table S15) and introduced in toluene or ECH. Oligo(ECH)s with DP n  3 are poorly soluble in toluene, while perfect solubility was observed for oligoECHs with DP n  4 (Figure S21a). All oligomers were fully soluble in ECH (Figure S21b).

In order to avoid the temperature rise during ROP of ECH, it was proposed to conduct a twostage polymerization consisting first in slow monomer addition in heterogeneous conditions (in solution), followed by fast monomer addition in homogeneous conditions (in bulk). This approach allowed controlling the exothermicity of polymerization with a maximal temperature not exceeding 35ºC (Table 3). The total reaction time was reduced from about 20 h (Table S14) to 8 -9 h (Table 3), while well-defined PECHs with controlled molar mass (M n  2600 g.mol -1 ), low dispersity (Ð < 1.2) and ideal functionality were obtained (Table 3).

Another strategy to prepare well-defined PECH at large scale under fast polymerization rate without any temperature jump was tested: it consisted in using toluene-soluble oligoepichlorohydrin (DP n ≥ 4) as macroinitiator of ROP of ECH in conjunction with BF 3 OEt 2 as co-initiator.

The desired PECH chains with controlled molar mass and low dispersity were obtained in less than 6 h (Table 3). Implementing the same protocol as in solution (conducting the polymerization in two stages) allowed controlling the reaction temperature during the polymerization and synthesizing well-defined PECH of M n  2500 g mol -1 , low dispersity and expected functionality (Table 3). It should be noted that thanks to the excellent solubility of both CLPD and low molar mass PECH in ECH (Figure S21b), the polymerization in bulk could be easily upscaled to 100 g of polymer and did not require a two-stage monomer addition and/or using ECH oligomers as initiator (Table 3).

CONCLUSIONS

In this work, a detailed fundamental mechanistic study of BF 3 •Et 2 O-catalyzed cationic ringopening polymerization of epichlorohydrin was performed to reach industrially-relevant polymerization conditions. First of all, the possibility to conduct the living cationic ROP of ECH in non-chlorinated solvents (typically toluene) or in bulk was demonstrated. The solubility of initiating complex in the reaction medium is a key issue for providing fast and living ROP of ECH.

It was unambiguously showed that the poor solubility in toluene of both molecular initiators (BD, CLPD) and low molar mass oligomers (up to DP n = 3) resulted in retardation of the initiating process and accumulation of monomer in the reaction mixture at the beginning of the polymerization. This monomer accumulation gave rise to an induction period, followed by a fast polymerization accompanied by a significant increase of reaction temperature (up to boiling of reaction mixture).

To solve this problem, we proposed a simple and efficient initiation approach consisting in first forming the initiator through reaction of ECH with water (ECH/H 2 O=1:1.05 or 1.5:1 mol/mol) in the presence of the water-tolerant BF 3 •Et 2 O co-initiator. Doing so, CLPD and longer oligomers displaying perfect solubility in reaction media were prepared in situ. As a result, the induction period was reduced and, thereof, ROP of ECH could proceed at much faster rate (4 -8 h) as compared to previously described processes (20 -48 h). The proposed initiating system (H 2 O/BF 3 •Et 2 O) could be efficiently used in a non-chlorinated solvent such as toluene as well as in bulk conditions. In best conditions, fast synthesis of pure PECH diols (F n (OH) ≈ 2.0) with controlled molar mass (M n up to 4000 g mol -1 ), low dispersity (Đ < 1.25) was achieved. The developed procedure for fast synthesis of PECH diol was successfully scaled-up to 100 g of polymer, either in solution or in bulk, eventually using prepared oligoECHs as initiators. The developed procedure was successfully upscaled to 100 g of polymer as an incentive to industrial production of PECH as well as its derivative glycidyl azide polymer (GAP).

Finally, detailed characterization of the synthesized PECH was performed with the aim to elucidate the reasons leading to the coloration of the polymers in the course of polymerization observed by some authors [START_REF] Bideron | Polyepichlorohydrin Diols Free of Cycles: Synthesis and Characterization[END_REF][START_REF] Kim | Online Monitoring of Reaction Temperature during Cationic Ring Opening Polymerization of Epichlorohydrin in Presence of BF 3 and 1,4-Butanediol[END_REF][START_REF] Kim | The Influence of Initiator Diol-BF 3 Complex Solubility and other Factors on the Induction Period in BF 3 Catalyzed Cationic Ring Opening Polymerization of Epichlorohydrin[END_REF][START_REF] Hafner | Copolymers based on GAP and 1,2-Epoxyhexane as Promising Prepolymers for Energetic Binder Systems[END_REF] or during the storage of non-purified samples (this work). Such process occurs via the abstraction of НCl by Lewis acid with formation of side double bond followed by its protonation/proton abstraction with the formation of internal conjugated double bonds.

ASSOCIATED CONTENT

[BF 3 •

 3 Et 2 O]/[BD] down to 1/25 or less. In addition, the slower polymerization at low BF 3 •Et 2 O concentration led to incomplete monomer conversion at [BF 3 •Et 2 O]/[BD]<1/25 (Table

Figure 1 .

 1 Figure 1. Zooms of (a) low molar mass and (b) high molar mass parts of MALDI TOF MS

Figure 2 .

 2 Figure 2. (a) M n and Đ vs. [M]/[I] plot, (b) SEC traces at different [M]/[I] ratios for cationic ROP

Figure 3 .

 3 Figure 3. SEC traces of poly(epichlorohydrin) synthesized using the H 2 O/BF 3 •Et 2 O initiating

Figure 4 .

 4 Figure 4. (a) M n and Đ vs. [M]/[I] plot, (b) SEC traces at different [M]/[I] ratios for cationic ROP

Figure 5 .

 5 Figure 5. (a) 13 C and (b) 13 C DEPT-135 NMR spectra of PECH (M n = 2230 g mol -1 ; Đ = 1.22)

F

  n =[M n (SEC)/M n (NMR)]20.93 was 1.99±0.2 for all experiments. No temperature jump was observed, the temperature of reaction continuously increased from 25 to 31 ºC during the polymerization. M n,theo = 2500 g mol -1 . b Total reaction time for 2 stages. c Oligoepichlorohydrin (DP n = 4) was used as an initiator. d CLPD was used as an initiator.

  3

  •Et 2 O and SnCl 4 in the cationic ROP of ECH revealed that BF 3 •Et 2 O afforded PECH with higher functionality in chain-end hydroxyl groups and lower dispersity, even if the corresponding polymerizations were performed at slightly different

ECH/1,4-butanediol (BD) ratios.

  3

  3

  3 •Et 2 O initiating system still shows several limitations: (i) a high amount of BF 3 •Et 2 O is required to moderate the polymerization exotherm (BD/BF 3 •Et 2 O =1/6); (ii) synthesized PECHs exhibit relatively high dispersity (Đ=1.3 -1.4) and too low a functionality (F n (OH)=1.80-1.90); (iii) competitive initiation via adventitious H 2 O prevents the synthesis of

Table 1 .

 1 3 •Et 2 O-catalyzed cationic ROP of ECH. A first test in presence of adventitious H 2 O as initiator has shown that the polymerization is slow because of the relatively low concentration of initiator. This led to an accumulation of monomer in the medium during the starved addition and promoted the ACE mechanism, generating predominantly low molar mass macrocyclic poly(epichlorohydrin)s (Table1, TableS5, FigureS8). Although it is out of the scope of the present study, this method could be considered as a cheaper and more efficient alternative to B(C 6 F 5 ) 3 -catalyzed synthesis of macrocyclic polyethers.[START_REF] Haque | MALDI-TOF MS Study of Macrocyclic Polyethers Generated by Electrophilic Zwitterionic Ring Expansion Polymerization of Monosubstituted Epoxides with B(C 6 F 5 ) 3[END_REF] Effect of the Type of Initiator (I) on the BF 3 •Et 2 O-Catalyzed Cationic ROP of

	Epichlorohydrin. a
	run	initiator b T max	conv.	M n,theo c	M n,NMR	M n,SEC	Đ	F n (OH) d
			(˚C)	(%)	(g mol -1 )	(g mol -1 )	(g mol -1 )
	1 e	none	25	19	-	2790	630	1.51	0.43
	2	H 2 O	25	99	2475	2380	2510	1.16	2.00
	3	CLPD	25	94	2350	2340	2370	1.20	1.98
	4	BD	74	92	2300	1980	1940	1.32	1.86
	butanediol;	CLPD:	3-chloro-1,2-propanediol;	c	Theoretical	M n	calculated	as
	M n = [ECH]/[initiator]92.5conv.; d Number average functionality at the chain end determined as
	F n = ([M n (SEC)/M n (NMR)]20.93); e Reaction time: 4.5 h; V(ECH)=3.95 mL; ECH/toluene = 2/1 v/v;
	[ECH]/[BF 3 •Et

a Conditions: [ECH]/[I] = 26; [BF 3 •Et 2 O]/[I] = 1/15; T = 25 ºC; ECH/toluene=1/1 v/v; V(ECH)= 12.7 mL; time = 7 h. Rate of monomer addition (in respect to pure monomer): 2.5 mL/h; b Abbreviations: BD: 2 O] = 623. Rate of monomer addition (with respect to pure monomer): 1.5 mL/h.

Table 2 .

 2 Effect of the Rate of Monomer Addition on the Cationic ROP of Epichlorohydrin. a

	run ECH addition rate	Time	conv.	M n,theo c	M n,NMR	M n, SEC	Đ	F n (OH) d
		(mL/h) b	(h)	(%)	(g mol -1 )	(g mol -1 )	(g mol -1 )
	5	0.7	18.0	96	3370	2890	3060	1.21	2.01
	6	1.5	8.0	94	3300	3190	3130	1.20	1.96
	7	2.5	3.3	88	3110	2650	2790	1.25	2.01

Table 3 .

 3 Scale-up of Cationic ROP of ECH with H 2 O/BF 3 OEt 2 Initiating System in Toluene and

	in Bulk. a
	run process 1 st stage	Time	2 nd stage	Time	conv.	M n,NMR	M n,SEC	Ð
			ECH rate		ECH rate
			addition		addition
			(mL/h)	(h)	(mL/h)	(h)	(%)	(g mol -1 )	(g mol -1 )
	14 solution	1.5	3.6	13.6	9	97	2510	2580	1.18
	15 solution	3.2	2.8	13.6	8	97	2430	2480	1.18
	16 c solution	-	-	20.4	6	99	2560	2710	1.19
	17 d	bulk	-	-	13.6	8	99	2590	2610	1.21

a Conditions: [BF 3 •Et 2 O]/[H 2 O] = 1/15; [ECH]/[H 2 O] = 28; ECH total : 100 g; ECH/toluene = 2:1 v/v for polymerization in solution; T = 25 C; V(ECH) total = 85 mL. F n at chain-ends determined as

Table of Contents

 of GraphicChlorinated Solvent-Free Living Cationic Ring-Opening Polymerization of Epichlorohydrin Using BF 3 •OEt 2 as Co-initiator: Towards Perfectly Functionalized Polyepichlorohydrin Diols

	Page 57 of 57	ACS Applied Polymer Materials
		ACS Paragon Plus Environment

Georgy V. Timofeev, Maksim I. Hulnik, Irina V. Vasilenko, Francois Ganachaud, Guy Jacob and Sergei V. Kostjuk*

ACS Paragon Plus EnvironmentACS Applied Polymer Materials

The authors declare no competing financial interest.

ACKNOWLEDGMENT

The authors thank EURENCO for financial support of this research. S.K. and F.G. thank Dr. Geneviève Eck and Dr. Thibaud Alaime for useful discussions of the obtained results. This work is also partially supported by State Program for Scientific Research of Belarus "Chemical processes, reagents and technologies, bioregulators and bioorganic chemistry" (project 2.1.01.03).

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Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes


Dr Drishya Elizebath

Dr Jia Hui Lim

Dr Yoshiharu Nishiyama

Dr Balaraman Vedhanarayanan

Dr Akinori Saeki

Yu Ogawa
email: yu.ogawa@cermav.cnrs.fr

Vakayil K Praveen
email: vkpraveen@niist.res.in

] Jia

Hui Lim

Dr ] D Elizebath

Nonclassical Crystal Growth of Supramolecular Polymers in Aqueous Medium

Keywords: self-assembly, supramolecular polymer, order, nonclassical crystallization, 3D electron diffraction

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

One of the predominant leitmotifs of the hierarchically organized biopolymers is the "pre-programmed" chemical entities that could forecast their precise structure and functions. [1] A great deal of effort in mimicking natural systems led to the development of supramolecular polymers. [2] Akin to biopolymers, the molecular building blocks of supramolecular polymers can predict the outcome of competing intermolecular interactions and provide the possibility of exploring new coordinates of the free energy landscape. [3,4] Studies in this direction not only unraveled mechanistic insights but also provided methods for creating supramolecular structures under kinetic control. [5,6] However, leveraging the pathway complexity and integrating the supramolecular polymers into the mesoscale requires adequate building strategies. [7] For instance, nonclassical crystallization that utilizes various intermediate structures other than atoms, ions, and molecules [8,9] provides a feasible approach to material fabrication based on supramolecular polymers.

The classical crystallization theory considers the crystalline nucleation and growth process by simple monomer-by-monomer addition without many structural changes in the bulk or at the surface.

However, numerous examples of crystallization in biological [10] and geological [11] systems challenge the classical model and are readily explained with the nonclassical nucleation and growth models. The nonclassical, multistep crystallization model involves either a prenucleation cluster formation or follows a particle assembly pathway, which highlights the diversity and complexity of the process and has been widely studied in recent years. [8][9][10][11][12] During the advanced stages of crystallization, the nonclassical crystal growth via oriented particle attachment and mesocrystal formation is frequently observed. [x] Although particle-based crystal growth is common in inorganic and protein crystallization, only a few reports on organic dye-based systems are known. [12] Recently, Rybtchinski and co-workers have demonstrated the supramolecular polymers of an organic molecule could act as potential building blocks in the nonclassical crystallization pathway. [12b,c] However, a molecular-level understanding that could forecast the crystal growth pathway is currently lacking in the supramolecular systems.

Extrapolating the general features of nonclassical crystallization mechanisms to supramolecular polymers could aid the formation of nanostructures and their spontaneous long-range alignment towards the construction of materials in mesoscale. It is envisaged that supramolecular polymers with favorable surface topologies could promote interpolymer interactions and the gradual evolution of order across different length scales. [14] Herein, we demonstrate the nonclassical crystal growth pathway adopted by the supramolecular polymer fibrils of superbenzene (coronene)-dipeptide conjugate via flat ribbons to crystals. The molecular packing in the crystal originating from the specific molecular interactions was unraveled by 3D ED, [15] a method relatively underexplored in the field of supramolecular chemistry. [16] Interestingly, varying the hydrophilic/hydrophobic balance of the molecular structure led to the formation of helical and twisted structures that remained in the fibrillar morphology.

Results and Discussion

Self-assembly behavior of Cr-FFOEt and Py-FFOEt

The molecules Cr-FFOEt and Py-FFOEt consist of a coronene and pyrene moiety, respectively conjugated to the N-terminal of a C-protected diphenylalanine (FFOEt) via a butyl chain linker (Figure 1). In contrast, Cr-FF contains a C-terminal deprotected diphenylalanine while maintaining the rest of the molecular structure of Cr-FFOEt (Figure 1). These molecules were synthesized following the multistep synthesis strategy outlined in Scheme S1 and S2 and were characterized by FT-IR, 1 H, and 13 C NMR spectroscopy and high-resolution mass spectrometry (see Supporting Information). To explore the self-assembling behavior of Cr-FFOEt and Py-FFOEt, they were first dissolved in a good solvent (THF) and then diluted with water (Figure 2a). The aggregation process was then monitored using spectroscopy techniques. Cr-FFOEt in THF (cT = 5 ´ 10 -5 M) showed UV-vis spectral features signature to that of the monomerically dissolved state with an absorption maximum at 304 nm (Figure 2b). Furthermore, Cr-FFOEt in THF displayed a photoluminescence spectrum with well-resolved vibronic features (lmax = 429 nm) and a monoexponential lifetime decay, t = 28.63 ns (Figure S1). When diluting the Cr-FFOEt solution in THF with water at a concentration of cT = 5 ´ 10 -5 M of Cr-FFOEt in water/THF (6:4 v/v), a gradual hypochromic effect occurred in the UV-vis absorption, indicating the aggregation induced by the solvent exchange (Figure S2). Compared to the UV-vis spectral features of the monomerically dissolved Cr-FFOEt, solvent replacement into water (6:4 v/v) leads to a significant broadening of absorption over a period of 1 h (Figure 2b and Figure S3). The aggregation processes led to the appearance of bands at the longer wavelength region (357, 371, and 411 nm). The photoluminescence spectrum of the corresponding aggregates was broad with a significant red-shift (lmax = 491 nm), and the average lifetime (tav) value was significantly enhanced to 58.19 ns (Figure S4). [17] Interestingly, Py-FFOEt, which contains pyrene with a lower carbon-to-hydrogen ratio in comparison to coronene, aggregated at a high-water content (water/THF; 7:3 v/v cT = 5 ×10 -5 M) (Figure 2b and Figure S5). Upon aggregation, the vibronically well-resolved absorption and photoluminescence spectra of Py-FFOEt in THF underwent a bathochromic shift with a spectral broadening. Furthermore, the lifetime value was significantly reduced from t = 21.17 ns (monomer) to tav = 9.04 ns (aggregate) (Figure S6). Atomic handed helical fibers having heights ranging from 80-300 nm and lengths of several micrometers (Figure 2f and Figure S8). These fibers showed a well-defined twist with a pitch of ca. 230 nm (Figure 2g,h)

During the assembly process of Cr-FFOEt, the appearance of the solution significantly changed over the course of 12 h after the addition of water, where the turbid solution transformed into suspensions of visible needle-like particles (Figure S9). These micrometer-long needle-like aggregates showed strong birefringence, indicating the crystalline nature and strong orientation of the crystal domains of Cr-FFOEt aggregates (Figure S10). [18] AFM and transmission electron microscopy (TEM) studies revealed that the width and height of the ribbons increased continuously over the same time period (Figure S11). In contrast, the suspension of Py-FFOEt showed no such change over time and remained in their helical morphology (Figure S12a,b). Similarly, the bright field image and the corresponding images of Py-FFOEt under crossed polarizers showed negligible birefringence compared to that of Cr-FFOEt microcrystals (Figure S12c,d).

The evolution of the assembly process of Cr-FFOEt was further monitored using TEM and electron diffraction (ED). The specimen, after 5 min of aging, shows the formation of thin hairy 1D fibrillar structures having a width of 10-30 nm (Figure 3a). These thin fibers provided poorly resolved ED patterns and thus possessed small crystalline domains with disordered molecular packing. These fibers tend to laterally assemble into loose bundles of a width of a few hundred nanometers (Figure 3b). The fibrillar structures grew into ribbon-like crystallites with a broad width distribution ranging from 30 to 200 nm in the next few hours. These ribbons yielded well-defined spot ED patterns as shown in the 1 and 2 h samples and are thus single crystalline. (Figure 3c-e). Longer crystallization, 4 h time, resulted in thicker ribbons with diameters often larger than a few hundred nanometers (Figure 3f). These structures are not single crystalline as their ED patterns were often contained reflections from multiple and slightly misaligned crystal domains in single ED diagrams (red arrows in Figure 3f). This observation indicates that these larger fibers formed via the oriented attachment of the smaller crystalline fibers. The single-crystalline thin ribbons were likely laterally stacked to form these thicker ribbons. After 4 h, larger aggregates with diameters of micrometers became predominant, which were too large for TEM ED studies.

To evaluate the topological features of the microcrystals obtained after 12 h of aging, the samples were subjected to field emission scanning electron microscopy (FE-SEM) and AFM. The FE-SEM images showed microcrystals with a length of 300 ± 60 µm and a width of 2 ± 1 µm (Figure 4a and Figure S13).

The magnified FE-SEM images of the microcrystal (Figure 4b,c) and its fractured surface (Figure 4c

3D ED analysis of Cr-FFOEt microcrystals

We employed the 3D ED technique to get an insight into the molecular interactions leading to the observed nonclassical crystallization of Cr-FFOEt . A nanoribbon with a width of ca. 50 nm yielded highresolution spot ED patterns with a resolution close to 1 Å (Figure 5a directions. Thus, the space group is likely P21. This non-orthorhombic, low-symmetry space group implies that the unit cell is indeed pseudo-orthorhombic, and the crystal structure is originally monoclinic with a unique axis being the b-axis. The transition from monoclinic to orthorhombic systems is likely due to the assembly process of smaller imperfect crystallites assembled into larger single crystalline ribbons. The FT-IR spectrum of Cr-FFOEt in CDCl3 exhibited peaks at 3424 cm -1 and 1668 cm -1 , corresponding to the symmetric N-H stretching vibration (amide A) and C=O stretching vibration (amide I). The nonclassical crystallization process induced shifts of the amide A band to 3282 cm -1 and the amide I band (uC=O) to 1638 cm -1 (Figure S17). [19] These spectral changes corroborate the observed intermolecular H-bonds between the amide moieties in the 3D ED-based molecular model.

Assembly mechanism governed by intermolecular interactions

The ED analysis shows that the c-axis is along the fiber axis of the crystallites. The crystallization is, therefore, faster along the c-axis than in other crystallographic directions. This observation suggests that the two strong interactions, the H-bonds and the p-stacking interactions drive the initial stage of the crystallization of Cr-FFOEt. Indeed, the poorly resolved ED pattern after 5 min of aging (Figure 3a) already exhibited the smeared reflections with a periodicity of 4.9 Å, corresponding to the spacing between H-bonded diphenylalanine residues. [20] However, the pronounced impact of the hydrophobic interaction of coronene moieties towards the later stages of the assembly process resulted in the formation of flat ribbons. Even though a similar formation of H-bonded structures occurred in the Py-FFOEt system, the H-bond-driven helical organization [21] of the pyrene moieties was prominent in Py-FFOEt compared to that of the Cr-FFOEt (Figure S18,19). Circular dichroism (CD) studies provided further insight into the molecular packing and the possible induced chiral imprint of the dipeptide unit into the chromophore (Figure 6a). [22] The molecules were CD silent in the monomerically dissolved state. However, upon aggregation Cr-FFOEt, and Py-FFOEt showed CD patterns with varying intensity and pattern. The intensity of the CD signal is often found to be dependent on the rotational degree of chromophores, the reduced CD intensity of Cr-FFOEt compared to that of Py-FFOEt aggregates, shows the restricted impact of the peptide H-bonding network, thereby asymmetry transfer over the packing of coronene.

To further evaluate the required hydrophobic/hydrophilic balance of the molecule in predicting the surface topology, we thought to hydrolyze the terminal ester group of Cr-FFOEt. Interestingly Cr-FF in water/ DMSO (8:2 v/v, cT = 5 ×10 -5 M) showed a well-defined exciton-coupled CD with an increased intensity compared to that of Cr-FFOEt aggregates (Figure 6a and Figure S20). To corroborate the observed CD spectral changes the aggregate showed right-handed twisted nanoribbons with a width of 60 ± 20 nm and lengths of several micrometers (Figure 6b,c). This observation of the Cr-FF aggregation behavior highlights the delicate nature of the nonclassical crystallization of Cr-FFOEt.

Mechanical and optoelectronic properties of mesocrystals

In accordance with the observed layered structural features, the microcrystal showed elastic, flexible behavior. Along with the one-dimensional (1D) array of strong p-stacking and the H-bonding interactions, a multitude of weak, dispersive interactions in the other two directions might be acting as the structural buffer for the deformation (Figure S21). [23] The optoelectronic properties of the Cr-FFOEt microcrystals were studied by flash-photolysis time-resolved microwave conductivity (FP-TRMC). [24] Upon excitation of the microcrystals of Cr-FFOEt with a laser pulse of 355 nm produced transient conductivity, given by φΣµ (φ, photocarrier generation yield; Σµ sum of the mobilities of photogenerated charge carriers) was observed. The maximum transient conductivity was estimated to be 1.40 ×10 -4 cm 2 V -1 s -1 which is a 3.5 times enhancement in the conductivity compared to Cr-FFOEt in its powdered form obtained by drying THF solution. Similarly, we have measured the transient conductivity for Py-FFOEt. The fibrous assembly of Py-FFOEt showed a 1.4 times lesser conductivity compared to that of the powdered form obtained by drying THF solution, due to the peptide-driven helical stacking of the pyrene moieties (Figure 7). The pronounced p-orbital interactions, as observed from the increased photoluminescence lifetime of the Cr-FFOEt microcrystals, further support the increased conductivity. The results emphasize the significance of the fundamental particulate character of microcrystals, as well as the importance of shaping the energy landscape to obtain optimal material properties.

Conclusion

In summary, we have demonstrated the nonclassical crystal growth of supramolecular polymers of a coronene-dipeptide conjugate (Cr-FFOEt). The detailed spectroscopic, microscopic, and crystallographic characterization revealed the structural and morphological evolutions of Cr-FFOEt from hairy imperfect 1D nanofibers to micron-sized crystalline ribbons. The lateral interactions among these 1D structures resulted in the spontaneous growth across multiple length scales leading to mesoscale crystals. 3D ED analysis showed that peptide H-bonding and the p-stacking of coronene units drove the formation of the fibrillar assemblies. The observed intermediates of the nonclassical crystallization pathway were found to be quite flexible in optimizing the intermolecular interactions to accommodate the impending structural changes. The modifications in the molecular structure of Cr-FFOEt resulted in distinct supramolecular assembly behaviors, as Py-FFOEt and Cr-FF formed helical and twisted fibers, respectively. The difference in the assembly manners of the three molecules highlights the delicate balance of intermolecular interactions governing the nonclassical crystal growth process of Cr-FFOEt. A thorough understanding of the system is pivotal in balancing the H-bonding and hydrophobic interactions for creating supramolecular structures with appropriate surface topology in aqueous media. Further precise internal ordering of supramolecular polymers makes them potential building blocks (supramolecular elements) [7,25] for the next step in the hierarchy of organizations, a formidable challenge in synthetic supramolecular materials.

Figure 1 .

 1 Figure 1. Chemical structure of coronene dipeptide (Cr-FFOEt and Cr-FF) and pyrene dipeptide (Py-FFOEt) conjugates.

Figure 3 .

 3 Figure 3. Evolution of fibrillar assemblies of Cr-FFOEt in water/THF (6:4 v/v, cT = 5 × 10 -5 M) at 25 °C. TEM images (top) and ED patterns obtained from the circled area (bottom) of specimens prepared (a) 5 min, (b) 10 min, (c) 30 min, (d) 1 h, (e) 2 h, and (f) 4 h after the solvent switch. Scale bar = 500 nm.

Figure 2 .

 2 Figure 2. (a) Sample preparation procedure followed for the self-assembly of Cr-FFOEt and Py-FFOEt. (b) UV-vis absorption spectra of Cr-FFOEt (cT = 5 × 10 -5 M) in THF (blue) and aggregates in water/THF (6:4 v/v, red) at 25 °C; Py-FFOEt (cT = 5 × 10 -5 M) in THF (blue) and aggregates in water/THF (7:3 v/v, red) at 25 °C. (c) AFM height image of Cr-FFOEt aggregates in water/THF (6:4 v/v, cT = 5 × 10 -5 M) at 25 °C and inset shows the magnified AFM image of the greenish-blue box. (d) Magnified AFM height images of the yellow box in Figure 2c. (e) Cross-sectional analysis along the white dotted line shown in Figure 2d. (f) AFM height image of Py-FFOEt helical nanofibers in water/THF (7:3 v/v, cT = 5 × 10 -5 M) at 25 °C. (g) AFM height images of a single helical fiber. (h) Cross-sectional analysis along the white dotted line shown in Figure 2g depicts the pitch of helical fibers.

Figure 2 .

 2 Figure 2. (a) Sample preparation procedure followed for the self-assembly of Cr-FFOEt and Py-FFOEt. (b) UV-vis absorption spectrum of Cr-FFOEt in THF (blue), aggregates in water: THF (red, 6:4 v/v, cT = 5 × 10 -5 M at 25 °C (upper panel), Py-FFOEt in THF (blue), aggregates in water: THF (red, 7:3 v/v, cT = 5 × 10 -5 M at

Figure 3 .

 3 Figure 3. Evolution of fibrillar assemblies of Cr-FFOEt in water/THF (6:4 v/v, cT = 5 × 10 -5 M) at 25 °C. TEM images (top) and ED patterns obtained from the circled area (bottom) of specimens prepared (a) 5 min, (b) 10 min, (c) 30 min, (d) 1 h, (e) 2 h, and (f) 4 h after the solvent switch. The asymmetry of the ED patterns is due to the slight tilt of the crystallites. Scale bar = 500 nm.

, d )

 d revealed the laterally stacked microribbons (2D layers) aligned parallel to the longitudinal axis of microcrystals. AFM height and phase images also showed the multilamellar features of the microcrystals (Figure4e-g). The fluorescence microscopic image of the microcrystals under excitation at 330-380 nm exhibited bright green fluorescence (Figure4h). The fusion of the ribbon structures is visible on the tip of the microcrystal (inset of Figure4h). The combination of the morphological and ED-based analyses indicates that the micron-sized crystalline fibers of Cr-FFOEt formed through a nonclassical crystal growth process, as schematically represented in Figure4i. The poorly crystalline hairy ribbons act as basic building blocks and fuse into single crystalline ribbons. These single-crystalline flat ribbons further assemble into larger micron-sized fibrous aggregates.

Figure 4 .

 4 Figure 4. FE-SEM images of (a) microcrystals of Cr-FFOEt, (b) magnified image of the microcrystal showing the longitudinal attachment of microribbons, (c) magnified image of a fractured microcrystal (side-to-side attachment of microribbons is indicated by blue arrows), and (d) magnified image of the white dotted box of Figure 4c, showing the parallel stacking of 2D layers (lamellation-indicated by white arrows). AFM images of microcrystal (e) AFM height image of a microcrystal (f) corresponding phase image, (g) magnified phase image showing the lamellation of 2D layers (indicated by white arrows). (h) Fluorescence microscopy image of the microcrystals excited at 330-380 nm: inset shows a magnified image showing the side-to-side attachment of flat microribbons. (i) Schematic representation of the nonclassical crystallization pathway adopted by Cr-FFOEt.

  ,b). Representative 2D ED patterns at different tilt angles are shown in Figure S14. The reconstruction of ED intensity data in 3D space allowed us to determine the unit cell (Figure S15,16). The unit cell is orthorhombic with parameters a = 18.1 Å, b = 24.5 Å, and c = 4.95 Å, with c-axis corresponding to the fiber and supramolecular polymer chain axis. The systematic absence indicates the two-fold symmetry along the b-axis but not in the other

Figure 5 .

 5 Figure 5. 3D ED analysis of Cr-FFOEt crystal structure. (a) Diffraction contrast image (left) and a representative base-plane ED diagram corresponding to [1 0 0] zone axis pattern. Scale bar = 500 nm. A comparison between (b) experimental and (c) simulated patterns based on the packing model. (d) Packing model of Cr-FFOEt crystal.

Figure 6 .

 6 Figure 6. (a) CD spectrum of Cr-FFOEt in THF, aggregates in water/THF (6:4 v/v, cT = 5 × 10 -5 M) at 25 °C (upper panel), Py-FFOEt in THF, aggregates in water/THF (7:3 v/v, cT = 5 × 10 -5 M) at 25 °C (middle panel) and Cr-FF in DMSO, aggregates in water/DMSO (8:2 v/v, cT = 5 × 10 -5 M) at 25 °C (lower panel). The blue and red spectral lines indicate the monomerically dissolved and the aggregated states, respectively. (b) AFM height image of Cr-FF helical nanoribbons in water/DMSO (8:2 v/v, cT = 5 × 10 -5 M) at 25 °C. inset shows the height profile corresponding to the white dotted line. and (c) the corresponding phase image.

Figure 7 .

 7 Figure 7. (a) Transient photoconductivity (FP-TRMC) profiles of Cr-FFOEt (top panel), Py-FFOEt (bottom panel), self-assembled state (red), powder form dried from THF (blue). (b) Plot showing the variation of fSµ maxima.

Acknowledgments D.E. acknowledges CSIR, Government of India, for a research fellowship. J.H.L., Y.N., and Y.O. acknowledge Agence Nationale de la Recherche (ANR grant number: ANR-21-CE29-0016-1) for the financial support and the NanoBio-ICMG platform (FR 2607) for granting access to the electron microscopy facility. V.K.P. is grateful to the Council of Scientific and Industrial Research (CSIR), Government of India and Department of Science and Technology (DST), Nano Mission, Government of India (DST/NM/TUE/EE-02/2019), for the financial support. D.E. and V.K.P. thank Ms. Soumini Mathew, Ms. Viji S., and Mr. Kiran Mohan (CSIR-NIIST) for their experimental support.

Supporting Information

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The concept of nonclassical crystallization is applied to the fibril-to-crystal conversation of a supramolecular polymer. Gradual optimization of the intermolecular interactions and the favorable surface topology of the intermediates were found to be critical in the facile creation of a complex form of hierarchy.

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