Amy Edo-Osagie
email: edoosagie@unistra.fr

Daniel Sánchez-Resa
email: daniel.resa@isof.cnr.it

Dylan Serillon
email: dylan.serillon@etu.unistra.fr

Elisa Bandini
email: elisa.bandini@isof.cnr.it

Christophe Gourlaouen
email: gourlaouen@unistra.fr

Henri-Pierre Jacquot De Rouville

Barbara Ventura
email: barbara.ventura@isof.cnr.it

Valérie Heitz
email: v.heitz@unistra.fr

Henri-Pierre Jacquot
email: hpjacquot@unistra.fr

Synthesis, electronic and photophysical properties of a bisacridinium-Zn(II) porphyrin conjugate

Keywords: Acridinium, Porphyrin, Electron transfer, Electrochemistry, Transient absorption spectroscopy, Calculated spectra

Introduction

Porphyrins are versatile compounds that have found applications in various domains such as catalysis, Scheme 1. Synthesis of the acridinium benzaldehyde 3• • •PF 6 and the bisacridinium-porphyrin conjugate

1• • •2PF 6 .
perspectives opened by such association. Indeed, Nsubstituted acridiniums are multi-responsive units which modify their chemical and electronic features following chemical or redox stimuli [9]. N-substituted acridiniums and protonated acridines are also πacceptors and as such, were incorporated in the host structure of various systems or used as a guest molecule for molecular recognition. As guest molecules, acridiniums were stabilized through charge transfer interactions in the cavities of bis-porphyrin systems based on clips or macrocyclic structures [10][11][12]. In some host-guest complexes, fast photoinduced electron transfer between the non-covalently linked porphyrin donor and acridinium acceptor units was demonstrated [13][14][15]. Regarding covalent porphyrin-acridinium associations, a free-base porphyrin connected to two acridiniums was reported as a fluorescent detector of superoxide anion. In this system, a fluorescence enhancement of the triad was detected following a two-electron reduction of the acridinium units by the superoxide anion [16].

Herein, the synthesis of a Zn(II) porphyrin transsubstituted with 9-phenyl-N-methyl-acridinium units is reported. The electrochemistry, absorption and emission properties of the conjugate are investigated as well as its photophysics, by means of ultrafast transient absorption spectroscopy. Computational studies have been performed to provide insights into the frontier molecular orbitals involved in the electronic transitions of the conjugate.

Results and discussion

The synthesis of the bisacridinium-Zn(II) porphyrin 1• • •2PF 6 was inspired from Fukuzumi and coworkers [16]. It is based on a 2+2 condensation methodology developed by the Lindsey group to access trans-A2B2-type porphyrins [17]. It started from commercially available 4-bromobenzaldehyde and was performed in four synthetic steps (Scheme 1). The aldehyde functional group was first protected in the presence of pinacol (1.2 equiv.) in toluene [18]. The obtained 2-(4-bromophenyl)-4,4,5,5-tetramethyl-1,3-dioxolane (2) was reacted with nBuLi (0.8 equiv.) in THF at -78 °C followed by addition of N-methylacridin-9(10H)-one (1 equiv.), obtained in one step from commercially available compounds [19]. After acidification of the reaction mixture using HCl (37 wt%), the freshly formed 9-(4-formylphenyl)-10-methylacridin-10-ium chloride (3• • •Cl) was converted to the corresponding hexafluorophosphate salt (3• • •PF 6 ) by anion metathesis and isolated in 78% yield. The key intermediate 3• • •PF 6 was then reacted under Lindsey conditions in the presence of di(1H-pyrrol-2-yl)methane (1 equiv.) [20], and trifluoroacetic acid (TFA, 0.6 equiv.) in CH 2 Cl 2 .

After aromatization of the porphyrinoid using DDQ (3 equiv.), metalation of the free base porphyrin was undertaken using Zn(OAc) O (0.5 equiv.). After purification, the porphyrin-acridinium conjugate 1• • •2PF 6 was isolated in 29% yield.

Electrochemistry

Cyclic voltammograms (CV) were measured at 298 K under argon atmosphere, using tetrabutylammonium hexafluorophosphate (TBAPF 6 ) as the supporting electrolyte. It is worthwhile to note that due to the limited solubility of 1•2PF 6 in CH 3 CN in the presence of supporting electrolyte, DMF was used to perform the experiments (Figure 1). In the cathodic regime, a reversible wave (E red = -0.475 V versus SCE) corresponding to the reduction of the acridinium units followed by two reduction waves of half intensity (E red = -1.34 V versus SCE and E red = -1.43 V versus SCE) were recorded [21]. The observed ratio in intensity between the reduction processes suggests that the first reduction involves the reduction of both acridinium moieties at the same potential since no electronic coupling between these fragments is expected. The first reversible reduction process of the acridinium units allowed for the calculation of the half-wave reduction potential E 1/2 = -0.442 V versus SCE. One of the two last reduction processes is related to the second electron injection to one of the acridinium fragments leading to a carbanionic species. Formation of this species is supported by the reduced intensity of the reoxidation peak of the first reduction wave (-0.409 V versus SCE) leading to an EC mechanism (electron transfer-chemical). This carbanion is known to be highly reactive and capable of reacting with solvent molecules. The obtained CV in the anodic regime (Figure 1) consists of an irreversible anodic wave (E ox = +0.949 V versus SCE) attributed to the oxidation of the porphyrin moiety. Interestingly, this wave has the same intensity (I = 7.3 µA) as the first cathodic process thus indicating that an equal number of electrons is exchanged in both processes. This surprising bi-electronic oxidation of the porphyrin core was further supported by rotating disk electrode (RDE) experiments recorded in benzonitrile (see ESI, Figure S23). These data allow the estimation of the HOMO-LUMO gap to be 1.4 eV.

Photophysical properties

The photophysical properties of the conjugate 1• • •2PF 6 were characterized in CH 3 CN. Model compounds Zn-bisphenyl porphyrin (Zn-bP) and acridinium benzaldehyde (3• • •PF 6 ) (see ESI, Scheme S1) were also studied in the same solvent for comparison purposes.

The absorption spectrum of the conjugate is presented in Figure 2, compared with the spectra of its models Zn-bP and 3• • •PF 6 and their weighted sum. It can be observed that the spectrum of 1• • •2PF 6 correlates reasonably well with the sum in the 280-400 nm region, while the Q-bands of the porphyrin (500-600 nm) are slightly red-shifted with respect to the model porphyrin, as well as the Soret band (400-450 nm) which also shows a decrease in intensity and a broadening. Theoretical analysis demonstrates that the latter absorption features are given by the contribution of three electronic π-π * transitions either centered on the porphyrin or on the acridinium cores (see below).

Luminescence measurements were carried out both at room temperature and at 77 K in CH 3 CN. At room temperature, model 3• • •PF 6 shows a broad emission spectrum peaking at 515 nm, with a quantum yield of 0.045 and an excited state lifetime of 1.73 ns, while Zn-bP shows features typical of Zn-porphyrins, with maxima at 588 and 638 nm, φ fl = 0.035 and τ = 2.30 ns (Figure S24 and Table 1). Conversely, the conjugate 1• • •2PF 6 is weakly emissive, displaying a strong quenching of both the acridinium and the porphyrin units. Selective excitation of the porphyrin component in 1• • •2PF 6 at 543 nm, led to evaluate an emission quantum yield below 10 -4 , i.e. reduced to less than 0.3% the yield of model Zn-bP (Table 1). In order to estimate the quenching of the acridinium unit in 1• • •2PF 6 , excitation of isoabsorbing solutions of the conjugate, the porphyrin model and acridinium benzaldehyde model at 262 nm, wavelength at which prevalent excitation of the acridinium units (ca. 96%) is achieved, was car- ried out and the results are shown in Figure 3. The residual porphyrin fluorescence in 1• • •2PF 6 is far below the 4% of the model emission, percentage which would correspond to the direct excitation of the porphyrin unit in the array, confirming the quenching already observed. Moreover, the weak residual acridinium emission points to a quenching higher than 99% of these units in the array. The observed features indicate that a very fast and efficient photoinduced process depopulates the lowest singlet excited states of both components in the array, likely an electron transfer reaction. This hypothesis is confirmed C. R. Chimie -0000, 1, n 0, 000-000 Scheme 2. Energy level diagram and kinetics of the photoinduced processes occurring in the conjugate 1• • •2PF 6 in CH 3 CN. The singlet and triplet energy levels are taken from data of the present paper and from literature [22]. The energy of the charge separated state (1.39 eV) has been approximated as E ox -E red (with E ox and E red as "redox energy", expressed in eV), by considering the oxidation potential of the porphyrin unit (E ox = +0.949 V versus SCE) and the reduction potential of the acridinium unit (E 1/2 = -0.442 V versus SCE) measured in DMF, whose polarity is similar to that of CH 3 CN.

by transient absorption analysis, as discussed in detail below.

The electron transfer reaction does not exclude a possible energy transfer from the acridinium to the porphyrin singlet excited states, a process thermodynamically allowed (Scheme 2) and supported by a non-zero overlap between the emission spectrum of the acridinium unit and the absorption spectrum of the porphyrin component (Figures 2 andS24). In order to investigate this event, the excitation spectrum of 1• • •2PF 6 was collected at 660 nm, wavelength at which only the emission of the porphyrin component is present and compared to the absorption spectrum (Figure S25). Even if noisy, due to the very weak emission, the excitation spectrum clearly contains the absorption bands of the porphyrin and lacks the characteristic absorption peaks of the acridinium unit at 262 and 362 nm. This outcome leads to conclude that an energy transfer process from the acridinium units to the central porphyrin is not occur-ring in the conjugate, probably due to the competition with the ultrafast electron transfer process.

Emission measurements conducted at 77 K showed fluorescence spectra with higher vibrational resolution for the models 3• • •PF 6 and Zn-bP (Figure S26). In Zn-bP, moreover, phosphorescence emission at 794 nm is clearly observed. The conjugate 1• • •2PF 6 appears to be weakly emissive also at 77 K and its spectrum is hardly detectable due to scattering issues that affect these measurements. This outcome leads to deduce that the decrease in temperature is not affecting the efficiency of the quenching process occurring within the components of the array.

In order to get insights into the fast photoinduced processes occurring within the conjugate, pumpprobe transient absorption measurements with femtosecond resolution were performed on 1• • •2PF 6 and its relevant models.

Two excitation wavelengths were chosen: 560 nm, where a selective population of the porphyrin singlet is achieved and 360 nm, where the acridinium units are prevalently excited (the peak at 262 nm of the acridinium is not experimentally accessible).

The time evolution of the transient spectrum of model Zn-bP upon excitation at 560 nm is reported in Figure 4(a). The initial spectrum with a positive band below 530 nm, ground state bleaching at 542 and 580 nm and stimulated emission at 640 nm evolves into a new spectrum with a positive maximum at 463 nm, with clear isosbestic points. The kinetics reflects the fluorescence lifetime of the molecule (the time profiles are reported in Figure S27), i.e. 2.3 ns; the final spectrum is thus attributed to the triplet state and the process is assigned to S1→T1 intersystem crossing.

The scenario is completely different for the conjugate 1• • •2PF 6 (Figure 4(b)). The end of pulse spectrum shows maxima at 480, 520, 620 and 662 nm, as well as bleaching bands at 545 and 586 nm. In the region 620-660 nm an ultrafast signal rise, of the order of the time resolution of the system, is observed. This spectrum quickly decays, with a lifetime of 0.65 ps (Figure S28a). The observed transient spectrum can be safely ascribed to the charge separated state P + -Ac • • • (P: porphyrin, Ac: acridinium), since the bands between 600 and 700 nm well resembles those reported for a Zn-porphyrin cation, [23] and bands at ca. 480 nm and 520 nm have been reported for the reduced species of N-alkyl substituted acridinium compounds [14,15,24,25]. The singlet excited state of the porphyrin is thus depopulated in ca. 0.3 ps to yield the charge separated state, which in turn lives less than 1 ps. It can be noticed that the decay of the spectrum shows a second component, accounting for a very small fraction of the signal (ca. 2%), with a lifetime of ca. 32 ps (Figure S28b). This second component can be tentatively ascribed to a slower charge recombination occurring in a different conformation of the array, or deriving from another minimum of the first excited singlet potential energy surface. This is consistent with the molecular flexibility in the excited state highlighted by the theoretical results (see below).

Transient absorption analysis with excitation at 360 nm has been performed for 1• • •2PF 6 and models 3• • •PF 6 and Zn-bP. In model acridinium benzaldehyde, 3• • •PF 6 , the initial formation of a signal with a risetime of 1 ps, ascribable to vibrational relaxation, is observed (Figure S29). The formed spectrum displays positive bands in the 450-510 nm and 630-800 nm regions, with maxima at 472 nm and 680 nm, and stimulated emission at 550 nm, red-shifted with respect to that detected from luminescence measurements due to the sum with the positive absorption bands. The signal decays with a lifetime of 1.6 ns, in good agreement with the fluorescence lifetime (Figures 5(a) and S30), and with defined isosbestic points, allowing to ascribe the process to S1→T1 intersystem crossing, even if the spectral features of the triplet are hardly detectable. Zn-bP shows a behaviour similar to that observed upon excitation at 560 nm, with intersystem crossing occurring in 2.3 ns, but preceded by an initial fast evolution of 1 ps, ascribable to internal conversion (Figures S31 andS32) [26].

The end of pulse spectrum of 1• • •2PF 6 does not present any of the spectral features of the singlet excited states absorption of the respective components, but recall those observed upon excitation at 560 nm (Figure 5(b)) with clear features of the charge separated state. There is no evidence of signal formation, implying that the process, upon prevalent excitation of the acridinium component, occurs on an ultrafast scale (≤0.3 ps). The spectrum evolves quickly, almost disappearing with τ = 0.70 ps. A minor component, accounting for ca. 5% of the decay and with a lifetime of 33 ps (Figures 5(b) and S33) is detected, in agreement with that observed upon excitation of the porphyrin unit.

The observed photoinduced electron transfer process is, indeed, thermodynamically allowed upon excitation of both the porphyrin (∆G = -0.66 eV) and the acridinium units (∆G = -1.24 eV), as indicated in Scheme 2 and, in the latter case, a higher energy gap accounts for the increased reaction rate, placing the reaction in normal Marcus region [27].

Computational studies

The structure of 1• • •2PF 6 was first optimized without any symmetry and revealed that the phenyl rings are orthogonal to the porphyrin rings (88°) and to the acridinium moieties (88°). Interestingly, the frontier orbitals are mainly composed of two sets of degenerated orbitals localized on the porphyrin fragment for the HOMOs and on the acridinium units for the LU-MOs (Figure S34). This observation unambiguously confirms the interpretation of the electrochemical investigation for the anodic and cathodic process involving the oxidation of the porphyrin and the reduction of the acridinium moieties, respectively. It is worthwhile to note that no contribution from the phenyl rings linking the porphyrin and acridinium units was observed (see ESI). The theoretical absorption spectrum (Figure 6) is composed of two intense absorbing bands at 332 and 406 nm and two weak absorbing bands at 512 and 610 nm, matching the experimental optical transitions observed at 361, 411, 545 and 586 nm (Figure S17).

The nature of the electronic transitions of 1• • •2PF 6 is presented in Figure 7. The main band at 406 nm results from the contribution of three electronic transitions at 397, 406, and 408 nm corresponding to ππ * excitations either centered on the porphyrin or on the acridinium cores (Figure S35). The band at 331 nm is attributed to two π-π * transitions centered on the acridinium moieties exclusively. Finally, the 512 and 510 nm respectively and by a charge transfer transition from the porphyrin to the acridinium core at 610 nm where the electron is delocalized over the two acridinium moieties. For this lowest excited singlet, two calculations were performed imposing a C 2 symmetry in order to preserve the delocalization and a broken symmetry. Already in the symmetric structure, the geometry underwent significant distortions upon relaxation. First, a flattening of the structures was observed resulting in the decrease of the dihedral angles between the fragments (porphyrinphenyl angle = 59.9°and acridinium-phenyl angle = 77.2°) compared to ground state. In this C 2 optimized structure, this singlet emits at 693 nm thus corroborating the experimental data. Finally, breaking the symmetry leads to additional distortions and to localization of the electron in the excited state on one of the two acridiniums (Figure S37). This generates two minima separated by a very low barrier (302 cm -1 ) between which the exciton freely oscillates, thus giving rise to an average symmetrical conformer.

Conclusion

A bisacridinium-Zn(II) porphyrin conjugate has been successfully synthesized in four steps using a novel acridinium benzaldehyde intermediate. Electrochemical characterization via cyclic voltammetry has revealed that the one-electron reduction of the two acridiniums occurs simultaneously, indicating limited communication between the two acridiniums. Furthermore, the observed two-electron oxidation of the porphyrin is supported by computational analysis showing two near degenerate HOMOs for the porphyrin. Comparison of the absorption features of the conjugate with the acridinium benzaldehyde intermediate and the Zn-bisphenyl porphyrin model confirms that the acridiniums are noninnocent moieties, with a complex interplay of porphyrin and acridinium centered transitions accounting for the apparent red-shift of both the Soret and the Q-bands of the porphyrin core, as shown by theoretical calculations. The almost complete quenching of the emission of both components in the conjugate was ascribed to an ultrafast photoinduced electron transfer process. Transient absorption analysis revealed the formation of a charge separated state within 0.3 ps, and its fast decay with a lifetime below 1 ps. In addition, an energy transfer process from the acridinium units to the porphyrin core was excluded. The interesting ultrafast photophysics of the conjugate opens new perspectives for the development of acridinium-porphyrin architectures.

Figure 1 .

 1 Figure 1. Cyclic voltammograms (DMF, WE: Pt, CE: Pt, RE: Hg/Hg 2 Cl 2 /KCl sat., 100 mV•s -1 ) of a solution of 1• • •2PF 6 (c = 1 × 10 -3 mol•L -1 ) in the presence of TBAPF 6 as supporting electrolyte (c = 0.1 mol•L -1 ).

  <1.0 × 10 -4e --a Emission maxima from corrected spectra. b Fluorescence quantum yields, measured with reference to TPP (meso-tetraphenylporphyrin) in aerated toluene as a standard for the porphyrin units and with reference to Coumarin 153 in ethanol for the acridinium units. c Fluorescence lifetimes in the nanosecond range, excitation at 465 nm for Zn-bP and at 368 nm for 3• • •PF 6 . d Energy of the excited state determined as the energy of the 0-0 emission band collected at 77 K. e Upon selective excitation of the Zn-porphyrin at 543 nm, the yield is below the minimum value measurable with steady-state experiments, i.e. 1.0 × 10 -4 .

Figure 2 .

 2 Figure 2. Absorption spectra of bisacridiniumporphyrin conjugate 1• • •2PF 6 (blue), model compounds Zn-bP (red) and 3• • •PF 6 (black) and the sum of the spectrum of Zn-bP with twice the spectrum of 3• • •PF 6 (green dotted) in CH 3 CN.

Figure 3 .

 3 Figure 3. Uncorrected emission spectra of CH 3 CN solutions of 1• • •2PF 6 (blue) and models 3• • •PF 6 (black) and Zn-bP (red), isoabsorbing at 262 nm (A 262 = 0.1). λ exc = 262 nm. The blue spectrum has been multiplied by a factor of 100.

Figure 4 .

 4 Figure 4. Transient absorbance of (a) Zn-bP and (b) 1• • •2PF 6 in CH 3 CN at different delays. Excitation at 560 nm (A 560 = 0.1, 0.2 cm optical path, 2 µJ/pulse).

Figure 5 .

 5 Figure 5. Transient absorbance of (a) • •PF 6 and (b) 1• • •2PF 6 in CH 3 CN at different delays. Excitation at 360 nm (A 360 = 0.2, 0.2 cm optical path, 2 µJ/pulse).

Figure 6 .

 6 Figure 6. Theoretical absorption spectrum of 1• • •2PF 6 in CH 3 CN.

Figure 7 .

 7 Figure 7. Character of the singlet states generating the lowest energy absorption band in the absorption spectra of 1• • •2PF 6 .

  2

  2

Table 1 .

 1 Emission data for models and conjugate in CH 3 CN

				RT				77 K
			λ max (nm) a	φ em	b	τ (ns) c λ max (nm) a	τ (ns) c	E (eV) d
	3• • •PF 6	3•PF 1 6	515	0.045	1.73	471, 500	2.70 (20%); 16.6 (80%)	2.63
	Zn-bP Zn-bP 1	588, 638	0.035	2.30	605, 648	2.31	2.05
		Zn-bP 3					794	-	1.56
	1• • •2PF 6		590, 642 e

C. R. Chimie -0000, 1, n 0, 000-000

Acknowledgements

We thank the European Union (H2020-MSCA-ITN grant "NOAH", project ref. 765297) for financial support. Italian CNR (Project PHEEL) is also acknowledged.

Supplementary data

Supporting information for this article is available on the journal's website under https://doi.org/10.5802/ crchim.100 or from the author.


Christopher M Plummer

Noémie Gil

Pierre-Emmanuel Dufils

D James Wilson

Catherine Lefay

Didier Gigmes

Yohann Guillaneuf
email: yohann.guillaneuf@univ-amu.fr

James Wilson

Radical Polymerization: An Interesting Tool to Insert Weak Bonds into Poly(vinyl esters)

Keywords: thionolactone, radical ring-opening polymerization, degradable (co)polymers, polythioester, poly(vinyl acetate)

come   L'archive ouverte pluridisciplinaire

INTRODUCTION

The versatility and durability of plastics, coupled with their low cost of production, has resulted in an annual worldwide production exceeding 335 million tonnes. While some polymer waste is incinerated for energy production, most is either discarded into landfill or directly into the natural environment. [START_REF] Miller | Sustainable Polymers: Opportunities for the Next Decade[END_REF][START_REF] Napper | Environmental Deterioration of Biodegradable, Oxo-biodegradable, Compostable, and Conventional Plastic Carrier Bags in the Sea, Soil, and Open-Air Over a 3-Year Period[END_REF] Microplastic particles are currently also becoming pervasive throughout the natural environment, with unknown consequences. [START_REF] De Souza Machado | Microplastics as an emerging threat to terrestrial ecosystems[END_REF] There is therefore a necessity for polymeric materials intended to undergo programmed (bio)degradation, thereby returning the polymer chains to oligomers that can be readily consumed by microbial action.

An emerging technique to produce (bio)degradable polymers from vinyl-based materials is the copolymerization of specifically designed monomers with conventional monomer feedstock. This can be readily accomplished by the polymerization technology known as radical ring-opening polymerization (rROP) which allows the ability to install ester-linkages into the backbone of polymers produced by free-radical polymerization. [START_REF] Agarwal | Chemistry, and limitations of the radical ring-opening polymerization of cyclic ketene acetals for the synthesis of degradable polyesters[END_REF][START_REF] Tardy | Radical Ring-Opening Polymerization: Scope, Limitations, and Application to (Bio)Degradable Materials[END_REF][START_REF] Pesenti | 100th Anniversary of Macromolecular Science Viewpoint: Degradable Polymers from Radical Ring-Opening Polymerization: Latest Advances, New Directions, and Ongoing Challenges[END_REF] Such ester-linkages impart (bio)degradability, with only a minor quantity being required to be introduced into the polymer backbone for its complete degradation, pending homogeneous incorporation. In detail, the technique of rROP involves the (co)polymerization of cyclic monomers bearing either a vinyl or exomethylene group (or an analogous sp2 system) which can undergo polymerization by a free-radical pathway via a ring-opening mechanism. Radical ring-opening polymerization combines the advantages of both ring-opening polymerization (ROP) and radical polymerization in one polymerization system i.e., the production of polymers having heteroatom and/or functional groups within the backbone chain, combined with the robustness and mild conditions of a free-radical polymerization process. [START_REF] Agarwal | Chemistry, and limitations of the radical ring-opening polymerization of cyclic ketene acetals for the synthesis of degradable polyesters[END_REF][START_REF] Tardy | Radical Ring-Opening Polymerization: Scope, Limitations, and Application to (Bio)Degradable Materials[END_REF][START_REF] Pesenti | 100th Anniversary of Macromolecular Science Viewpoint: Degradable Polymers from Radical Ring-Opening Polymerization: Latest Advances, New Directions, and Ongoing Challenges[END_REF] A complication encountered with (bio)degradable copolymers produced via rROP is that the incorporated monomers are required to be situated in a homogenous distribution to impart substantial hydrolysable functionality into the parent polymer. [START_REF] Gigmes | Simulation of the Degradation of Cyclic Ketene Acetal and Vinyl-Based Copolymers Synthesized via a Radical Process: Influence of the Reactivity Ratios on the Degradability Properties[END_REF] The control of such phenomena relates to the reactivity ratios of the two monomer compounds, as well as variables such as temperature, monomer concentration, and polymerization time, etc. The most well-studied and understood rROP monomer family can be considered the cyclic ketene acetals (CKAs) (Figure 1a, example 2-methylene-1,3-dioxepane, MDO) which produce polyesters. Various CKA molecules with diverse ringsizes and substituents have been synthesized and evaluated for their rROP ability. [START_REF] Agarwal | Chemistry, and limitations of the radical ring-opening polymerization of cyclic ketene acetals for the synthesis of degradable polyesters[END_REF][START_REF] Tardy | Radical Ring-Opening Polymerization: Scope, Limitations, and Application to (Bio)Degradable Materials[END_REF] Even if the use of CKAs has led to the preparation of a wide variety of (bio)degradable materials, such monomer families are sensitive to protic sources and also have a rather low reactivity compared to many vinyl-based monomers. [START_REF] Tardy | DFT-calculation-assisted prediction of the copolymerization between cyclic ketene acetals and traditional vinyl monomers[END_REF] Thus, there remains room for innovation to prepare other rROP monomers. Other structures such as cyclic allyl sulfide or allyl sulfone have been designed but require long synthetic procedures. 9, 10 A relatively new class of rROP monomers are the thionolactones, which were simultaneously discovered by Roth [START_REF] Bingham | Degradable vinyl copolymers through thiocarbonyl addition-ring-opening (TARO) polymerization[END_REF] and Gutekunst 12 (Figure 1b). In particular, Roth [START_REF] Bingham | Degradable vinyl copolymers through thiocarbonyl addition-ring-opening (TARO) polymerization[END_REF] showed that among the various thionolactone compounds, only dibenzo[c,e]oxepine-5(7H)-thione (Figure 1b, DOT) is reported to solely undergo ring-opening copolymerization, leading to polymers with thioester functionality within the polymer backbone. Its copolymerization with acrylonitrile, acrylates, acrylamides and maleimides derivatives were reported to be efficient whereas copolymerizations with styrene, methacrylate derivatives and vinyl acetate were not successful. [START_REF] Bingham | Degradable vinyl copolymers through thiocarbonyl addition-ring-opening (TARO) polymerization[END_REF][START_REF] Spick | Fully Degradable Thioester-Functional Homo-and Alternating Copolymers Prepared through Thiocarbonyl Addition-Ring-Opening RAFT Radical Polymerization[END_REF] The homopolymerization was also not efficient. [START_REF] Spick | Fully Degradable Thioester-Functional Homo-and Alternating Copolymers Prepared through Thiocarbonyl Addition-Ring-Opening RAFT Radical Polymerization[END_REF] Since only a single thionolactone monomer (DOT, 2) has been reported to be able to undergo rROP, we sought to evaluate other potential analogues. The addition-fragmentation ability of the C=S bond has been widely investigated during the development of the RAFT process. [START_REF] Moad | Living radical polymerization by the RAFT process -A first update[END_REF][START_REF] Moad | Living Radical Polymerization by the RAFT Process -A Second Update[END_REF] In particular, it was demonstrated that dithiobenzoate derivatives were efficient for stabilized vinyl monomers such as acrylate derivatives but unreactive with low-stabilized monomers. [START_REF] Chiefari | Thiocarbonylthio compounds (S=C(Z)S-R) in free radical polymerization with reversible addition-fragmentation chain transfer (RAFT polymerization). Effect of the activating group Z[END_REF][START_REF] Coote | Effect of substituents on radical stability in reversible addition fragmentation chain transfer polymerization: An ab initio study[END_REF] Changing aryl to alkyl groups is known to decrease the transfer constant of the RAFT agent and thus allow for the use of less activated monomers. [START_REF] Chiefari | Thiocarbonylthio compounds (S=C(Z)S-R) in free radical polymerization with reversible addition-fragmentation chain transfer (RAFT polymerization). Effect of the activating group Z[END_REF] By analogy, we could expect that preparing a thionolactone without the presence of the phenyl group could lead to an efficient rROP monomer that is compatible with less activated monomers, such as vinyl ester derivatives. Indeed, previous research into the CKA monomers has revealed that 7membered rings most readily undergo rROP and therefore the removal the substituted benzene rings of the DOT structure to give the 7-membered core was envisioned. During the writing of this manuscript, Destarac and coworkers [START_REF] Ivanchenko | epsilon-Thionocaprolactone: an accessible monomer for preparation of degradable poly(vinyl esters) by radical ring-opening polymerization[END_REF] presented the εthionocaprolactone as a new thionolactone that could be used as a comonomer with vinyl acetate, but that could not homopolymerize.

RESULTS AND DISCUSSION

Radical reactivity. To access the targeted molecules, we applied a procedure for ε-caprolactone thiation involving phosphorus sulfide (P 4 S 10 ) and hexamethyldisiloxane (HMDSO). [START_REF] Curphey | Thionation with the reagent combination of phosphorus pentasulfide and hexamethyldisiloxane[END_REF] The target thiono-caprolactone (thCL) and thiono-decalactone (thDL) were readily isolated as yellow oils in 56-64% yield after flash chromatography (Scheme 1). It was found that the ε-thionocaprolactone (thCL) was unstable in the presence of water, particularly in the presence of alcohols which appeared to catalyse the degradation. Degradation reactions include an oxidation to ε-caprolactone, as well as hydrolysis to give the ring-opened product. The compound is however stable stored neat in a refrigerator. Attempts to initiate the homopolymerization of thCL using AIBN, BPO and ammonium persulfate under various conditions all failed, as also reported. 18

Scheme 1. Preparation of the seven-membered ring alkyl thionolactones.

One hypothesis to explain such results is related to the initiation of the polymerization. Indeed, the initiating radical should first add efficiently to the C=S bond. Secondly the intermediate radical should fragmentate to release the thioester bond and a new primary (for thCL) alkyl radical, without reversal to give back the thCL monomer and the initiating radical. In this case, the produced radical is a non-stabilized radical and thus the reversal could be the main reaction event, inhibiting the polymerization. To test this hypothesis, we thought to copolymerize the thCL monomer with the CKA monomer 2methylene-1,3-dioxepane (MDO). In the case of CKA, it is known that AIBN-derived radicals add efficiently onto the exomethylene double bond and thus irreversible β-fragmentation occurs leading to a primary non-stabilized radical. [START_REF] Tardy | A comprehensive kinetic study of the conventional free-radical polymerization of seven-membered cyclic ketene acetals[END_REF] After addition to this non-stabilized radical, the ringopening of the thCL will be favored. Interestingly, such a copolymerization system initiated by AIBN readily provided copolymers poly(MDO-co-thCL) and even the homopolymer of thCL, depending on the feed ratio (Scheme 2) (See Figure S5 and S6 for NMR spectra). This result showed that contrary to the hypothesis of Destarac, [START_REF] Ivanchenko | epsilon-Thionocaprolactone: an accessible monomer for preparation of degradable poly(vinyl esters) by radical ring-opening polymerization[END_REF] the difficulty in homopolymerizing thCL is not due to a slow propagation profile, but to the initiation system. Indeed, common initiating systems such as AIBN lead to stabilized radicals that add very efficiently to the C=S bond but could also undergo reversible fragmentation to release the radical and the starting thionolactone. When MDO is present in the medium in a sufficient quantity, the irreversible addition of initiating radicals onto MDO occurs allowing for the production of nonstabilized alkyl radicals that initiate the efficient homopolymerization of the thCL, or the copolymerization, depending on the monomer ratio. When the amount of MDO is too low (MDO only as a co-initiator at 3 mol%) no polymer was obtained, showing that most of the AIBN-type initiating radicals are sequestered by the thCL. When the amount of MDO is moderate, only the polythioester poly(thiocaprolactone) or poly(thCL) is obtained, demonstrating that the rate of thCL polymerization is higher than that of MDO. Only when a high ratio of MDO is used that a copolymer is obtained (Scheme 2). We thus attempted to use other systems to generate non-stabilized alkyl radicals to initiate the polymerization: via iodine-transfer polymerization (ITP) using perfluorinated 1-iodohexane as the alkyl iodide, [START_REF] Wolpers | Iodine-Transfer Polymerization (ITP) of Ethylene and Copolymerization with Vinyl Acetate[END_REF] and using Fenton chemistry involving FeSO 4 and H 2 O 2 to generate methyl radicals. [START_REF] Moad | THE CHEMISTRY OF RADICAL POLYMERIZATION[END_REF] These system did not allow access to the polythioester, demonstrating that a specific initiation process has still to be determined. Preparation of P(VAc-co-thCL/thDL). The successful polymerization conditions to produce the homopolymer of thCL were performed in THF, but when analogous copolymerizations were attempted with vinyl acetate (VAc) they returned only minor quantities of polymer. Taking the minor quantities of polymer as a promising sign, a solvent change to ethyl acetate (EtOAc) was then performed due to its structural similarity with vinyl acetate. [START_REF] Kermagoret | Improved photo-induced cobaltmediated radical polymerization in continuous flow photoreactors[END_REF] Such polymerizations readily produced copolymers of poly(thCL-co-VAc) and therefore a variety of polymerizations with differing feed ratios were then performed (Table 1). We showed that at higher feed ratios of the thCL monomer (≥ 10 %) the amount of thCL linkages within the polymer became enriched. Feed ratios above 20 % thCL monomer failed to provide polymer, presumably because the thCL monomer was sequestering the AIBN derived radicals and thereby inhibiting the polymerization.

We thus tried to replace thiono-caprolactone (thCL) with thiono-decalactone (thDL) which would produce a secondary non-stabilized alkyl radical after ring-opening. It was hypothesized that this could enhance the ring-opening and also be more compatible with the low-stabilized secondary PVAc macroradical. The polymerization of a thDL:VAc = 10:90 ratio led to poly(thDL-co-VAc 4 ), showing a similar amount of thioester units (20-22%) in the polymer backbone and thus still a higher amount of thioester in the polymer than in the feed. The 1 H NMR spectrum of the copolymerization between thCL and VAc and thDL and VAc is presented in Figure 2. Destarac and coworkers [START_REF] Ivanchenko | epsilon-Thionocaprolactone: an accessible monomer for preparation of degradable poly(vinyl esters) by radical ring-opening polymerization[END_REF] performed NMR analyses of the thCL/VAc copolymerization system and concluded that a large amount of the thCL is inserted as the cyclic ("closed") structure, analogous to the vinyl propagation of CKA monomers. Nevertheless, they specified that the cyclic structure was not concentrated enough to give definitive attributions. The 1 H NMR of the copolymer poly(thCL-co-VAc) samples displayed signals relating to the protons adjacent to the thioester, and from these protons the ratio of the monomers can be calculated (see ESI for details). The average of these two signals was used within calculations as they are not equal due to minor diad phenomena within the polymer chain. Interestingly, we did not observe large amounts of signals occurring between 3.2 -4.15 ppm that were ascribed to the "closed" structure. [START_REF] Ivanchenko | epsilon-Thionocaprolactone: an accessible monomer for preparation of degradable poly(vinyl esters) by radical ring-opening polymerization[END_REF] The 1 H NMR analysis of PVAc obtained using similar polymerization conditions presented a peak at 4.1 ppm that could arise from chain branching or transfer to solvent, which is known to occur during the radical polymerization of VAc. [START_REF] Britton | Chain transfer to polymer in free-radical bulk and emulsion polymerization of vinyl acetate studied by NMR spectroscopy[END_REF][START_REF] De Rybel | Interplay of Head, Tail, and Mid-Chain Radicals in Bulk Free-Radical and Reversible Degenerative Addition Fragmentation Chain Transfer Polymerizations of Vinyl Acetate[END_REF] Moreover, the non-stabilized alkyl macroradical that is produced after the fragmentation of the thCL is prone to intra/intermolecular H-transfer. Such a feature has been widely documented in the case of MDO polymerization where 1,4 and 1,7 H-intramolecular transfers and intermolecular H-transfer were demonstrated, [START_REF] Jin | A Study of the Mechanism of the Free-Radical Ring-Opening Polymerization of 2-Methylene-1,3dioxepane[END_REF] and whose amount increased exponentially with monomer conversion. [START_REF] Tardy | A comprehensive kinetic study of the conventional free-radical polymerization of seven-membered cyclic ketene acetals[END_REF] The branching occurring via these processes could also be responsible for the signals at 3.2 -3.8 ppm. In our case and contrary to the experiments done elsewhere, [START_REF] Ivanchenko | epsilon-Thionocaprolactone: an accessible monomer for preparation of degradable poly(vinyl esters) by radical ring-opening polymerization[END_REF] the copolymerization were performed in solution and thus the occurrence of H-transfer to the polymer was reduced. This could explain the very low amount of residual signal at 3.15 -4.2 ppm. The copolymerization of thDL with VAc was also analyzed by [START_REF] Miller | Sustainable Polymers: Opportunities for the Next Decade[END_REF] H NMR and presented a signal at 3.5 ppm that is related to the methine group bearing the pendant butyl alkyl group. This NMR shift is in agreement with the hypothesis of H-transfer occurring in the case of thCL. The presence of the n-butyl group could impede some intra/intermolecular H-transfers and lower the total amount of H-transfer. A small signal (3-5 %) is still present at 3.2 ppm that could be ascribed to 1,4 H-transfer. Degradation studies. The degradation was then studied by aminolysis such as previously reported. [START_REF] Bingham | Degradable vinyl copolymers through thiocarbonyl addition-ring-opening (TARO) polymerization[END_REF][START_REF] Smith | Radical Approach to Thioester-Containing Polymers[END_REF] Poly(thCL-co-VAc 26.7 ) (Table 1, Entry 2) was thereby treated in THF by the addition of N-isopropylamine. It should be noted that the SEC trace before degradation is not completely symmetrical owing to chain-transfer during the polymerization of VAc. [START_REF] Grcev | Determination of molecular weight and size distribution and branching characteristics of PVAc by means of size exclusion chromatography/multi-angle laser light scattering (SEC/MALLS)[END_REF] After degradation, the molecular weight (M n ) of the polymer was observed to change from 17,000 to 6,800 g/mol (Figure 3). The decrease in the M n (three-fold) is lower than expected (6-7-fold according to 3.6 mol% of thioester linkages) since the higher reactivity of thCL compared to VAc lead to diads or longer sequences of thioester units in the backbone. This result thereby confirmed only a relative homogeneous incorporation of the thCL linkages within the PVAc backbone, even if the whole distribution is shifted towards lower masses. As expected, the increase of the thCL ratio into the PVAc backbone led to an enhanced degradation (Figure 3, M n decrease from 15,400 to 1,700 g.mol -1 ). Compared to the radical copolymerization of CKA and vinyl acetate which also leads to degradable materials via hydrolysis, [START_REF] Agarwal | Synthesis of Degradable Materials Based on Caprolactone and Vinyl Acetate Units Using Radical Chemistry[END_REF][START_REF] Ayala | RAFT/MADIX Copolymerization of Vinyl Acetate and 5,6-Benzo-2methylene-1,3-dioxepane[END_REF] thCL allows the use of aminolysis as demonstrated in this work, while both transthioesterification [START_REF] Bingham | Thioester-Functional Polyacrylamides: Rapid Selective Backbone Degradation Triggers Solubility Switch Based on Aqueous Lower Critical Solution Temperature/Upper Critical Solution Temperature[END_REF] and oxidative hydrolysis [START_REF] Bingham | Thioester-Functional Polyacrylamides: Rapid Selective Backbone Degradation Triggers Solubility Switch Based on Aqueous Lower Critical Solution Temperature/Upper Critical Solution Temperature[END_REF] should also be able to be applied as stimuli for degradation.

Destarac and coworkers [START_REF] Ivanchenko | epsilon-Thionocaprolactone: an accessible monomer for preparation of degradable poly(vinyl esters) by radical ring-opening polymerization[END_REF] showed that vinyl pivalate could be efficiently copolymerized with thCL. In a similar way, we were interested to extend the scope of vinyl acetate derivatives that could be copolymerized with thCL or thDL. Polyvinyl acetate is widely used as a precursor for the preparation of polyvinyl alcohol. [START_REF] Fabini | AS THE DISPERSANT FOR SUSPENSION VINYL-CHLORIDE POLYMERIZATIONS[END_REF] Dove and Gibson 32 reported the preparation of degradable poly(vinyl alcohol) via the radical copolymerization of MDO and vinyl chloroacetate (VAcCl). 1, Entry 2) and poly(thCL-co-VAc 3.5 ) (Table 1, Entry 3) before and after degradation by aminolysis using N-isopropylamine.

Indeed, it is reported that chloroacetate esters have a hydrolysis rate 700 times higher than unmodified acetate esters, allowing the hydrolysis of the pendant group without the degradation of the ester in the main chain. [START_REF] Olijve | Influence of Polymer Chain Architecture of Poly(vinyl alcohol) on the Inhibition of Ice Recrystallization[END_REF] We thus try to copolymerize vinyl chloroacetate (VAcCl) and thCL to prepare (bio)degradable polyvinyl alcohol.

Unfortunately, copolymerizations of thCL and VAcCl were found to be unsuccessful. Tercopolymerizations between thCL, VAc and VAcCl (10:45:45) were also found to be unsuccessful. Such results are rather surprising since the macroradical that is produced after the ring-opening of the thCL is similar to the one that is produced after the ring-opening of the MDO. One explanation to the absence of polymer could be that reversible addition on the C=S bond led to an increase in the lifetime of the growing poly(thCL) macroradical which is prone to abstract the chlorine atom of the VAcCl. This was indeed another indirect proof that extensive H-transfer occurred when thCL was polymerized. To decrease the ability to undergo transfer reactions, we also replaced the thCL with thDL to obtain a secondary alkyl radical instead of a primary radical. The copolymerization between thDL and VAcCl was then performed but was also found to not lead to the desired copolymer. This result showed that the ester moiety of the vinyl ester derivatives has to be carefully chosen when copolymerizing with thionolactone derivatives.

It has been reported that polyvinyl acetate can be stoichiometrically hydrolyzed to polyvinyl alcohol (PVOH) when the hydrolysis is performed in a mixture of acetone/H 2 O using calculated quantities of NaOH. [START_REF] Chana | The Synthesis of High Molecular Weight Partially Hydrolysed Poly(vinyl alcohol) Grades Suitable for Nanoparticle Fabrication[END_REF] Using this protocol, attempts were therefore made at the selective hydrolysis of the acetate groups within the poly(thCL-co-VAc) copolymer to provide (bio)degradable copolymers of polyvinyl alcohol, i.e. poly(thCL-co-VOH). Unfortunately, it was found that the hydrolysis reaction affected both esters and thioesters at equal rates. A variety of conditions were attempted, including targeting different quantities of hydrolysis (i.e., 20%, 88%) but without success.

Block copolymerizations using thCL. To extend the scope of the copolymerization between thCL and VAc, we then explored their RAFT polymerization in order to control the length and the dispersity of the copolymer. We performed the RAFT copolymerization using xanthates [START_REF] Stenzel | Xanthate mediated living polymerization of vinyl acetate: A systematic variation in MADIX/RAFT agent structure[END_REF] and dithiocarbamates [START_REF] Moad | A Critical Survey of Dithiocarbamate Reversible Addition-Fragmentation Chain Transfer (RAFT) Agents in Radical Polymerization[END_REF] as RAFT agents. Rhodixan A1 (Figure 4) has been previously shown to be very efficient in controlling the polymerization of less activated monomers, especially vinyl acetate. [START_REF] Harrisson | RAFT Polymerization of Vinyl Esters: Synthesis and Applications[END_REF] Interestingly it was found that RAFT copolymerizations employing Rhodixan A1 failed when using a ratio [RAFT Agent] 0 /[AIBN] 0 = 10, but gave a relatively well-controlled polymerization when a ratio of 3 was used. This is presumably due to the reversible addition phenomenon described above, thereby sequestering the free-radical initiator species and preventing propagation. The polymerizations were performed using a thCL:VAc ratio of 10:90 and thus led to a relatively well-defined polyvinyl acetate (M n = 4,200 g.mol -1 , Ð = 1.5) containing 26.7 % thioester units. These results are in good agreement with recent study of Destarac and coworkers. [START_REF] Ivanchenko | epsilon-Thionocaprolactone: an accessible monomer for preparation of degradable poly(vinyl esters) by radical ring-opening polymerization[END_REF] At such ratios, non-stabilized alkyl moieties could be the dominant RAFT end-group, explaining the only moderate control of the polymerization. D'Agosto and coworkers [START_REF] Wolpers | Aromatic Xanthates and Dithiocarbamates for the Polymerization of Ethylene through Reversible Addition-Fragmentation Chain Transfer (RAFT)[END_REF] recently reported that dithiocarbamate RAFT agents were particularly efficient for the control of polymerizations involving ethylene, whose end-group is similar to the thioester moiety. We thus tried a dithiocarbamate RAFT agent, (i.e. the cyanomethyl methyl(phenyl)carbamodithioate) with a [RAFT Agent] 0 /[AIBN] 0 = 3 but the polymerization was not successful.

To further showcase the usefulness of this thionolactone monomer, block (co)polymerizations were then also investigated. We focused on the preparation of an amphiphilic block copolymer consisting on a hydrophilic block of N,Ndimethylacrylamide (DMAA) and a hydrophobic block of vinyl acetate. With this combination of monomer, it is possible to insert thioester linkages via the DOT monomer into the DMAA backbone, [START_REF] Bingham | Thioester-Functional Polyacrylamides: Rapid Selective Backbone Degradation Triggers Solubility Switch Based on Aqueous Lower Critical Solution Temperature/Upper Critical Solution Temperature[END_REF] and thCL into the vinyl acetate backbone to degrade selectively one or two blocks of the copolymer.

To do this, a homopolymer of N,N-dimethylacrylamide was prepared using Rhodixan A1 as a RAFT agent. [START_REF] Vakili | Polymerization-Induced Thermal Self-Assembly of Functional and Thermo-Responsive Diblock Copolymer Nano-Objects via RAFT Aqueous Polymerization[END_REF] A copolymer of DMAA and DOT was then also prepared in an analogous manner (Figure 4). Using the isolated polymer samples as RAFT macroinitiators, a second (co)polymerization was then performed using VAc, or a mixture of VAc and thCL, to give three dissimilar block copolymers (Figure 4). These block copolymers were then degraded by aminolysis and their molecular weights examined by SEC. As expected, the first copolymer without any thioester units poly(DMAA)-b-(VAc) did not undergo any degradation via aminolysis. The incorporation of thioester linkages via the copolymerization of VAc and thCL was then performed using a VAc:thCL 95:5 ratio. The degradation via aminolysis using N-isopropylamine in THF showed the degradation of the second block even if there was some residual VAc units that could remain linked to the PDMMA end-group. The last copolymer was designed to insert thioester units both into the hydrophilic and hydrophobic cores via the use of DOT and thCL, respectively. The degradation was then performed via aminolysis using ammonia in methanol since this method was reported to be more effective for the degradation of DMAA-DOT copolymers by Roth and coworkers. [START_REF] Bingham | Thioester-Functional Polyacrylamides: Rapid Selective Backbone Degradation Triggers Solubility Switch Based on Aqueous Lower Critical Solution Temperature/Upper Critical Solution Temperature[END_REF] The analysis by SEC of the degradation confirmed the degradation of both the hydrophilic and hydrophobic segments thereby showing the effectiveness of thionolactone technology to prepare degradable polymers (Figure 4).

CONCLUSION

In this article we reported the synthesis and rROP polymerization of two thionolactone monomers, i.e oxepane-2-thione (thiono-caprolactone, thCL) and 7-butyloxepane-2-thione (thiono-decalactone, thDL).Samples of homopolymer, as well as copolymers of thCL/thDL and vinyl acetate were prepared by free-radical polymerization and controlled RAFT polymerization. It was demonstrated that these monomers polymerize to provide thioester linkages within the polymer backbone which are readily degradable by aminolysis, thereby allowing the preparation of (bio)degradable poly(vinyl acetate) copolymers. Additionally, the preparation of a variety of block copolymers consisting of DMAA and vinyl acetate monomers with thioester units inside either one or two blocks is reported, their subsequent degradation being performed and examined by SEC to further demonstrate the usefulness of the reported monomer.

ASSOCIATED CONTENT Supporting Information

The Supporting Information is available free of charge on the ACS Publications website.

Synthesis of the thionolactone monomers; 1 H NMR spectra and SEC chromatograms of the (co)polymers (PDF).

Figure 1 .

 1 Figure 1. a) Preparation of polyesters via the radical ring-opening polymerization of cyclic ketene acetals. b) Radical ring-opening copolymerization of a thionolactone to introduce thioester linkages into the polymer backbone.

Scheme 2 .

 2 Scheme 2. Free-radical polymerization of thiono-caprolactone (thCL) and 2-methylene-1,3-dioxepane (MDO) performed in THF with AIBN initiation (1 mol%) and held at 70 °C. a) Reversible trapping of AIBN in the presence of thCL thereby impeding the homopolymerization of thCL. b) Irreversible radical initiation during the copolymerization of thCL and MDO. c) (Co)polymers obtained depending of the initial feed ratio [MDO] 0 :[thCL] 0 .

Table 1 .

 1 Free-radical polymerization of ε-thionocaprolactone (thCL)/ε-thionodecalactone (thDL) and vinyl acetate (VAc) performed in EtOAc with AIBN initiation (1 mol%) and held at 70 °C. a Calculated using 1 H NMR; b Data measured using SEC (THF) with PS standards for calibration.

Figure 2 . 1 H

 21 Figure 2. 1 H NMR analysis of the reference polyVAc (Table1, Entry 1), poly(thCL-co-VAc 26.7 ) (Table 1, Entry 2), poly(thCL-

Figure 3 .

 3 Figure 3. SEC traces of poly(thCL-co-VAc 26.7 ) (Table1, Entry 2) and poly(thCL-co-VAc 3.5 ) (Table1, Entry 3) before and after degradation by aminolysis using N-isopropylamine.

Figure 4 .

 4 Figure 4. Preparation of block copolymers containing both N,N-dimethylacrylamide and vinyl acetate using both DOT and thCL to insert thioester units.

Table

Table 1 ,

 1 Entry 3) and the poly(thDL-co-VAC 4 ) (

1, Entry 1), poly(thCL-co-VAc 26.7 ) (Table 1, Entry 2), poly(thCL-co-VAc 3.5 ) (

Table 1 ,

 1 Entry 4).

Funding Sources

This work was supported by Solvay Novecare. We thank the French National Research Agency (ANR-18-CE08-0019) for PhD funding (NG).

AUTHOR INFORMATION

Corresponding Author * Dr. Yohann Guillaneuf, Email: yohann.guillaneuf@univ-amu.fr

Author Contributions

The manuscript was written through contributions of all authors.

ACKNOWLDGMENT

We thank the CNRS and Aix-Marseille University for support.