Nicolas Bouisset

Andres Carvallo

Perrine Dumur

Sofiane Ramdani

Alexandre Legros
email: alegros@lawsonimaging.ca

Impact of Vestibular Stimulation at Powerline Frequency on Human Pointing Accuracy

Keywords: Arm motor control, electric current stimulation, human vestibular system, Power-line frequency

Background: Electric vestibular stimulations (EVS) up to 300 Hz trigger vestibular myogenic responses. Interestingly, 300 Hz is the upper limit of the so called extremely low-frequency magnetic fields (ELF-MF) range found within the 2010 guidelines written by the International Commission for Non-Ionizing Radiation Protection. Such guidelines are used to protect the workers and the public from neurostimulation triggered by induced electric fields. Since EVS is known to bias reaching and pointing tasks, vestibularspecific electric fields at power-line frequency are likely to impact the safety and performance of workers in high ELF-MF environments. Objectives: This research aimed to investigate the impact of vestibular-specific electric-fields on manual pointing accuracy. Methods: Pointing accuracy of twenty healthy participants was analyzed with both direct current (2 mA) and sinusoidal (peak ± 2 mA at 50 Hz) EVS. Spatial orientation and quantity of movement variables were used to investigate pointing modulations. Results: Despite a pre-trial conclusive positive control effect, no significant effects of both direct current and 50 Hz stimulation exposures were found. Conclusions: Although high vestibular-specific electric fields were used; no pointing accuracy modulation was found. These results suggest that ELF exposure even at high levels are not able to modulate hand pointing performance in humans. Even though this could be explained by context-specific habituation mechanisms rapidly decreasing EVS impact over time, these results represent useful knowledge for the safety and the performance of workers evolving in high ELF-MF environments.

I. INTRODUCTION

In today's world, given the generation, distribution, and use of alternating current (AC) at sources found at 50/60 Hz, depending on geographic location, both the public and the workers are subjected to ubiquitous Extremely Low-Frequency Magnetic Fields (ELF-MF < 300 Hz) [START_REF] Icnirp | Guidelines for limiting exposure to time-varying electric and magnetic fields (1 Hz to 100 kHz)[END_REF]. According to Faraday's law of induction, changing magnetic flux density over time induces Electric Fields (E-Fields) and currents within conductors such as the human body. Incidentally, such E-Fields can modulate human neurophysiology [START_REF] Legros | Neurophysiological and behavioral effects of a 60 Hz, 1,800 lT magnetic field in humans[END_REF]- [START_REF] Saunders | A neurobiological basis for ELF guidelines[END_REF].

Because of the proximity of ELF-MF sources in our daily lives and the constant interaction between the induced E-Fields and the human neurophysiology, answering health and safety concerns to protect workers and the public is of paramount importance. In that regard, international agencies such as the International Commission for Non-Ionizing Radiation Protection (ICNIRP) and the International Committee on Electromagnetic Safety from the Institute of Electrical and Electronics Engineers (IEEE-ICES), review scientific data to establish guidelines and standards enacted at national levels [START_REF] Icnirp | Guidelines for limiting exposure to time-varying electric and magnetic fields (1 Hz to 100 kHz)[END_REF], [START_REF]IEEE, IEEE Standard for Safety Levels with Respect to Human Exposure to Electromagnetic Fields[END_REF], [START_REF]IEEE, IEEE Standard for Safety Levels With Respect to Human Exposure to Electric, Magnetic, and Electromagnetic Fields[END_REF].

VOLUME XX, 2017 1

To date, the most reliable effect of synaptic polarization is the acute perception of phosphenes, on which both ICNIRP and IEEE-ICES base their in-situ induced E-Fields thresholds [START_REF] Icnirp | Guidelines for limiting exposure to time-varying electric and magnetic fields (1 Hz to 100 kHz)[END_REF], [START_REF]IEEE, IEEE Standard for Safety Levels with Respect to Human Exposure to Electromagnetic Fields[END_REF], [START_REF]IEEE, IEEE Standard for Safety Levels With Respect to Human Exposure to Electric, Magnetic, and Electromagnetic Fields[END_REF]. Phosphenes are flickering visual appearances perceived when exposed to a sufficiently strong ELF-MF [START_REF]Dispositifs pour la mesure des courants alternatifs de toutes fréquences[END_REF]. Nowadays, the main hypothesis regarding phosphenes is that they result from membrane potential modulations of graded potential retinal cells, impacting in cascade the continuous release of neurotransmitters to the downstream retinal cells through their ribbon synapse [START_REF] Attwell | Interaction of low frequency electric fields with the nervous system: the retina as a model system[END_REF]. However, phosphene perception is subjective and both the standards and the guidelines could better profit from an objective outcome measure. Although anatomically different, the vestibular hair cells share, extensive neurophysiological properties with the retinal photoreceptors. Indeed, both types of cells use graded potential for signal processing [START_REF] Juusola | Information processing by graded-potential transmission through tonically active synapses[END_REF], both releasing glutamate gradually from ribbon synapses [START_REF] Lagnado | in the Synaptic Terminal of Retinal Bipolar Cells[END_REF]- [START_REF] Sadeghi | Glutamatergic Signaling at the Vestibular Hair Cell Calyx Synapse[END_REF]. Vestibular hair cells are mechanoreceptors found in both the canals and the otoliths (composed of the utricle and the saccule). Their role is to transduce 1) head movement information and 2) the static head orientation relative to the earth's gravitational pull, into electric signals integrated and treated by the central nervous system (CNS) [START_REF] Eatock | [END_REF]. Compellingly, as for the retinal cells [START_REF] Evans | Frequency-dependent and montage-based differences in phosphene perception thresholds via transcranial alternating current stimulation[END_REF], [START_REF] Laakso | Computational analysis shows why transcranial alternating current stimulation induces retinal phosphenes[END_REF], small intensity E-Fields easily activate the vestibular hair cells [START_REF] Dlugaiczyk | Galvanic vestibular stimulation: from basic concepts to clinical applications[END_REF]- [START_REF] Norris | Mechanisms and effects of transepithelial polarization in the isolated semicircular canal[END_REF]. Moreover, increased activity within pigeons' vestibular nuclei is recorded when they are subjected to the induced currents produced by ELF-MF stimulations [START_REF] Nimpf | Report A Putative Mechanism for Magnetoreception by Electromagnetic Induction in the Pigeon Inner Ear[END_REF]. Furthermore, recorded voltage modulations within a semicircular canal model [START_REF] Nimpf | Report A Putative Mechanism for Magnetoreception by Electromagnetic Induction in the Pigeon Inner Ear[END_REF] also provides evidence for a potential electromagnetic induction impact on the vestibular system [START_REF] Nimpf | Report A Putative Mechanism for Magnetoreception by Electromagnetic Induction in the Pigeon Inner Ear[END_REF]. Thus, given their important sensitivity to E-Fields [START_REF] Dlugaiczyk | Galvanic vestibular stimulation: from basic concepts to clinical applications[END_REF]- [START_REF] Norris | Mechanisms and effects of transepithelial polarization in the isolated semicircular canal[END_REF], vestibular hair cells could potentially be predisposed to being modulated by the power-line frequency ELF-MF induced currents and provide objective outcome measures needed for future international guidelines and standards. Given the important role the vestibular system plays in balance, we investigated the impact of powerline frequency E-fields on postural control in the past [START_REF] Bouisset | Human Postural Control Under High Levels of Extremely Low Frequency Magnetic Fields[END_REF]- [START_REF] Bouisset | Human Postural Responses to High Vestibular Specific Extremely Low-Frequency Magnetic Stimulations[END_REF]. However, due to potential biomechanical and neurological low pass filtering mechanisms [START_REF] Forbes | Frequency response of vestibular reflexes in neck, back, and lower limb muscles[END_REF]- [START_REF] Forbes | Electrical Vestibular Stimuli Evoke Robust Muscle Activity in Deep and Superficial Neck Muscles in Humans[END_REF], no postural impact was found. Yet, E-fields at powerline frequencies could be more impactful as the outcome is recorded further up from the feet and closer to head [START_REF] Forbes | Frequency response of vestibular reflexes in neck, back, and lower limb muscles[END_REF]- [START_REF] Forbes | Electrical Vestibular Stimuli Evoke Robust Muscle Activity in Deep and Superficial Neck Muscles in Humans[END_REF]. Indeed, in humans, sinusoidal vestibular-specific electric stimulations (EVS), modulate neck myogenic responses at frequencies ranging up to 300 Hz [START_REF] Forbes | Neural mechanisms underlying high-frequency vestibulocollic reflexes in humans and monkeys[END_REF]. However, to record such modulations, strong isometric neck muscle contractions with the head fixed is needed. This unfortunately does not match daily life or working environments. Furthermore, to obtain neck myogenic results at 300 Hz, electrodes had to be inserted within the muscles under ultrasound guidance which is not convenient for replication studies needed to strongly base the in-situ threshold values. Indeed, to protect and safeguard both the public and the workers in their respective environments, the international Standards and guidelines should profit from more easily recorded behavioral outcomes. Afferent vestibular information is largely used during intentional human motor control tasks. For instance, such sensory information has been illustrated in many pointing or reaching behavioral studies [START_REF] Tunik | Arm-trunk coordination in the absence of proprioception[END_REF]- [START_REF] Bresciani | On the nature of the vestibular control of arm-reaching movements during whole-body rotations[END_REF], and such arm movements are also perturbated when EVS is the source of vestibular modulation [START_REF] Smith | Vestibular feedback maintains reaching accuracy during body movement[END_REF], [START_REF] Guerraz | From head orientation to hand control: evidence of both neck and vestibular involvement in hand drawing[END_REF]. This study investigates the impact of vestibular-specific Efields at power-frequency on arm movement performance during a pointing task. Given that both the retinal photoreceptor and the vestibular hair cells are very similar from a neurophysiological standpoint, this study further investigates an alternative model to the retinal photoreceptors and phosphene perception while appreciating the performance and safety of the employees working in high ELF-MF environments. Given that EVS is known to notably induce myogenic responses above powerline-frequencies [START_REF] Forbes | Electrical Vestibular Stimuli Evoke Robust Muscle Activity in Deep and Superficial Neck Muscles in Humans[END_REF], [START_REF] Forbes | Neural mechanisms underlying high-frequency vestibulocollic reflexes in humans and monkeys[END_REF], we hypothesized it will modulate the pointing task performance by decreasing the performance.

II. MATERIALS AND METHODS

A. PARTICIPANTS

Twenty (20) healthy right-handed participants (10 females-10 males) aged between 19-49 (mean ± SD = 25 ±7) were recruited for the study. All participants were tested within the Euromov-DHM laboratory at the University of Montpellier, France. Written informed consent was obtained from each participant prior to the experiment. The study was conducted in accordance with the Declaration of Helsinki and approved by the ethics committee of the University of Montpellier (IRB # 2001D). Were excluded volunteers with a history of any vestibularrelated pathology or dysfunction, any ophthalmological (including color blindness) and auditory problems, any orthopedic dysfunctions, as well as any chronic illnesses and neurological diseases. We're also excluded participants having permanent metal devices above the neck [START_REF] Behan | Safety of Cochlear Implants in Electrosurgery: A Systematic Review of the Literature[END_REF]. Finally, to avoid any interactions influencing the E-Fields, participants had to refrain from exercise, alcohol, caffeine, nicotine, pharmaceutical and/or drug intake 24 hours before the study [START_REF]Low intensity transcranial electric stimulation: Safety, ethical, legal regulatory and application guidelines[END_REF].

B. EXPERIMENTAL DEVICES

To set up the task, we made our own experimental customdesigned table. To maximally standardize the pointing task, the height of the wooden table could be adjusted from 70 to 98,5 cm to level it with the participants' hips. To provide the experimental visual targets, we embedded four LED lights within the wooden structure (Fig. 1). Each LED was 5 mm in diameter and had identical characteristics. All LED targets produced a green light except the reference LED (LED-R) which, in this case, was red (Fig. 1). All green LED targets were distanced 30 cm away from LED-R (Fig. 1). The first target (LED-1) was placed directly in front of the participants, in-line with LED-R. The second target (LED-2) was located at a 45° angle clockwise from LED-1 (Fig. 1). The last target (LED-3) was set to the right of the participant at a 90° angle clockwise from LED-1. Both the LEDs' ignition and extinction were controlled by a custom MATLAB script (MatLab version 9.3 -The MathWorks Inc., USA). To track the arm movements during the pointing task we used a Liberty motion tracking system (Polhemus Ltd., Colchester, VT, USA). The pointing data was recorded at 240 Hz with the stylus provided with the liberty system. Finally, the liberty's antenna (TX2 model, Polhemus Ltd., Colchester, VT, USA) was set in line with LED-1 at the edge of the experimental table (Fig. 1).

C. ELECTRIC VESTIBULAR STIMULATIONS

We delivered EVS using a transcranial current stimulation device (StarStim, Neuroelectrics, Spain) driven by the NIC software (Neuroelectrics Instrument Controller, version 1.4.1 Rev.2014-12-01) via Bluetooth. To facilitate signal synchrony and data analysis, the NIC software was piloted by the same custom MATLAB script steering the LEDs. To provide proper conduction between the electrodes and the skin, we saturated the circular 25 cm2 Ag/AgCl electrodes (StarStim, Neuroelectrics, Spain) with 8 mL of saline solution. We then secured the electrodes using the StarStim exposure cap and tape. To ensure appropriate stimulations, we maintained electrodes' impedances below 10 kΩ through-out the experiment, as recommended by the manufacturer. We used the same binaural bipolar montage for the Direct (DC -2 mA), the Alternating (AC -peak ± 2 mA at 50 Hz) electric and the SHAM stimulations. The intensity of the current was chosen with the following rational. Vestibular outcomes start being recorded with 0.1 mA [START_REF] Severac Cauquil | Short-latency eye movements evoked by nearthreshold galvanic vestibular stimulation[END_REF] but prickling and burning sensations can be felt above 2 mA [START_REF]Low intensity transcranial electric stimulation: Safety, ethical, legal regulatory and application guidelines[END_REF]. Therefore, we decided not to stimulate above the recommended 2 mA threshold [START_REF]Low intensity transcranial electric stimulation: Safety, ethical, legal regulatory and application guidelines[END_REF]. Here, SHAM is described as a procedure in which the current is ramped up and then turned off at the beginning of the test period that matches the period used with active stimulation. The current is then turned on again at the end of the testing block and ramped down until it is turned off. Each stimulation condition lasted 3.5 min and had the same pattern. The current was initially ramped up over a 15 s period, followed by a 3 min stimulation or SHAM (no current), and ended with an equivalent 15 s ramped down at the end of the condition (Fig. 3). For DC stimulations, we placed the cathode behind the right mastoid process. We first ensured our EVS stimulations were efficient before recording the trials. Thus, before starting the testing, we exposed the participants to 10 seconds 2 mA DC trials while standing feet together, arms by their side and eyes closed, and made sure each participant swayed towards the anodal side in the frontal plane (for review [START_REF] Fitzpatrick | Probing the human vestibular system with galvanic stimulation[END_REF]).

D. BEHAVIORAL TASK

The study employed a double-blind, repeated-measures design. The experiment was carried out over a single 45 min session. It was divided into 3 runs of 3 randomized blocks (one for each stimulation condition: DC, AC, SHAM) (Fig. 3). Each randomized block lasted 3.5 min. The pointing task started once the current or SHAM had reached its plateau (or no current) and ended before the current was ramped down. During this 3 min block period the participants had to point 30 times to the randomly lit LEDs. Thus, each pointing maneuver lasted 6 seconds. All 3 experimental runs were carried out in a completely darkened room. Only the lighted LEDs provided visual information to offer guidance for spatial orientation. At the beginning of each trial (pointing action), we asked the participants to set the stylus pen on the red LED-R which was considered as the starting point. After 1.5 seconds the LED-R was switched off, and the participants heard a beep indicating to start pointing. Simultaneously, one of the green LED was randomly turned on during 1 second. To avoid any rhythmic habituation, the participants ended the pointing maneuver by coming back to the lit LED-R once they heard a final beep which was randomly set in time after the green LED was turned off. Within a given run (Fig. 3), the 3 blocks were performed consecutively, with each block starting 30s after the previous one, This resting period was done to dissipate the stimulation effects and allow the vestibular system to reach its normal resting firing rate between blocks [START_REF] Bresciani | Vestibular signals contribute to the online control of goal-directed arm movements[END_REF]. To avoid fatigue and boredom, the lights were switched back on during 3 min, and the participants could relax and rest between each run (Fig. 3).

E. DATA ANALYSIS -Pointing errors

The arm movement time series were filtered with a low pass bidirectional 4th order Butterworth zero-phase digital filter with a cutoff frequency of 20 Hz. To have a broad view of the pointing errors, we decided to analyze them both quantitatively and qualitatively. The quantitative approach aimed at looking at the amount of error while the qualitative looked more specifically at the spatial distribution. Thus, for the three specific targets, for each type of stimulation (DC, AC, SHAM), we analyzed the pointing errors using five different variables: 1) the global pointing errors, 2) the Antero-Posterior (AP) pointing errors, 3) the Medio-Lateral (ML) pointing errors, 4) the mean direction of the pointing errors and 5) the error variability in space. First, quantitatively, for the first variable, we processed the global pointing errors as the Euclidean distance between the Cartesian coordinates of the mean pointing score and a given LED (Fig. 4A). Then, for the second and third variables, we computed the pointing errors more specifically as the distance between the mean pointing score and the LED along both the AP and ML axes (Fig. 4A). Qualitatively, during each trial, the participants targeted one of the enlightened LED 10 times (30 pointing maneuvers for 3 specific LEDs). To analyze the spatial dispersion of these pointing errors around a specific LED, we computed the ellipse encompassing them. The ellipse was computed using a Principal Component Analysis (PCA) method [START_REF] Oliveira | Calculation of area of stabilometric signals using principal component analysis[END_REF]. Two variables were extracted from this analysis. First, the mean direction (θ) of the pointing errors in space (Fig. 4B). The main direction of the pointing errors is described by the first principal component (PC1) which accounts for the largest part of the variance. θ, the angle between the ML axis and the PC1 axis was computed to describe the main direction of the pointing errors. θ was always presented within 0° and 180°: 0° being aligned with the ML axis (Fig. 4B). Finally, we used the ellipse area as a measure of error variability (Fig. 4C). -Statistical analysis A level of significance of α = 0.05 was adopted throughout data analysis. We performed all linear statistical analyses using the open source JASP software (University of Amsterdam, Netherlands, version 0.15). Two-way ANOVAs (3 stimulation modalities (DC/AC/SHAM) × 3 LEDs) for repeated measures were used to test the effect of the stimulation exposure types on the Global, AP and ML pointing errors, as well as for the ellipse area. For θ analyses, circular statistics were used using the circular library in R. Using Rayleigh's test for spacing test for circular uniformity of the distributions, we first ensured that θ data samples were not distributed uniformly. Mean θ and Angular Deviation (±AD) were used to describe the main direction of sway. A Watson-Williams multi-sample test was used per LED to investigate the effect of the different stimulations on the main error direction.

III. RESULTS

-Quantitative differences in pointing errors Two-way ANOVAs for repeated measure did not provide evidence of interaction effects (LED positioning * Stimulations) for Global (F (4,184) = 0.942; p = 0.441), AP (F (4,184) = 0.872; p = 0.482) and ML (F (4,184) = 0.193; p = 0.942) pointing errors respectively. Likewise, no stimulation main effect was found for Global ((F (2,92) = 0.281; p = 0.756) (Fig. 5), AP (F (2,92) = 2.306. p= 0.106) and ML (F (2,92) = 0.943; p = 0.394) pointing errors. However, a main effect of LED positioning was found for Global ((F (2,92) = 27; 374; p < 0.001) (Fig. 5), AP (F (2,92) = 8.474; p < 0; 001) and ML (F (2,92) = 122.549; p < 0; 001) errors. A first post hoc Holm-Bonferroni procedure [START_REF] Field | Post Doc procedures[END_REF] showed that an increased global distance error was more likely committed when targeting LED 1 rather than LED 2 (Mean error distance = 0.87cm ± SE = 0.15; t (5.661) p < 0.001) and LED 3 (Mean error distance = 1.07cm ± SE = 0.15; t (6.957) p < 0.001). Furthermore, the same Holm-Bonferroni procedure [START_REF] Field | Post Doc procedures[END_REF] showed that the errors were more medial in ML and more likely undershot in AP when targeting LED 1 rather than LED 2 (Mean ML error distance ± SE = -2.68 cm ± 0.19: t (-14.03) p < 0.001. Mean AP error distance ± SE = -1.318 cm ± 0.10; t (-3.986) p < 0.001) and LED 3 (Mean ML error distance ± SE = -2.48cm ± 0.33; t (-13.02) p < 0.001, Mean AP error distance ± SE = -0.954 cm ± 0.33; t =-2.884 p = 0.01). -Qualitative differences in pointing errors Fig. 6 depicts results for θ and ellipse areas for each LED and for all three stimulation types (DC, AC, and sham). Two-way ANOVAs (3 stimulation modalities (DC/AC/SHAM) × 3 LEDs) for repeated measures indicated no interaction effects for ellipse area (F (4.176) = 1.201, p = 0.32). Similarly, no significant main effect of stimulation condition was found (F (2.88) = 1.163, p= 0.317). However, once again a main effect of LED positioning was found (F (2.88) = 8.739, p<0.001). Here, a post hoc Holm-Bonferroni procedure showed that an increased area was more likely to happen when targeting LED 2 rather than LED 1 (Mean ± SE = 2,503 cm 2 ± 0.792; t(2) =3.161, p = 0.004) and LED 3 (Mean ± SE =3.128 cm 2 ± 0.792; t(2) =3.050, p < 0.001). Finally, using the three Watson-Williams multi-sample tests (one per LED), no significant differences due to stimulation type were found for θ for LED 1(F(2) = 0.63; p = 0.533), LED 2 (F(2) = 0.45; p = 0.641) and LED 3 (F(2) = 1; 17; p = 0; 312)).

IV. DISCUSSION

When subjected to both electric and time-varying magnetic fields at powerline frequency, people are susceptible to perceive and report phosphenes. Today, the main hypothesis is that phosphenes result from the membrane potential modulation of the graded potential cells found at the retinal level [START_REF] Saunders | A neurobiological basis for ELF guidelines[END_REF], [START_REF] Attwell | Interaction of low frequency electric fields with the nervous system: the retina as a model system[END_REF]. Interestingly, the vestibular hair cells are also graded potential cells. Consequently, from the perspective of the guidelines, the investigation of the ELF-MF induced Efields on the vestibular system was though legitimate for two reasons: 1) to consider an alternative model providing objective outcome measures and 2) appreciate the performance and safety of the employees working in high ELF-MF environments. These goals are in line with international Standards and Guidelines' intended ambitions to fill the knowledge gaps [START_REF] Icnirp | Gaps in Knowledge Relevant to the 'Guidelines for Limiting Exposure to Time-Varying Electric and Magnetic Fields (1 Hz-100 kHz)[END_REF] needed to answer the health and safety concerns to protect the workers and the public alike while in ELF-MF environments [START_REF]IEEE, IEEE Standard for Safety Levels With Respect to Human Exposure to Electric, Magnetic, and Electromagnetic Fields[END_REF].

Given the very important neurophysiological similarities between the retinal and the vestibular sensory cells [START_REF] Kremer | Usher syndrome: Molecular links of pathogenesis, proteins and pathways[END_REF]- [START_REF] Lagnado | Ribbon Synapses and Visual Processing in the Retina[END_REF] and the fact that EVS above power-line frequencies triggers myogenic responses [START_REF] Forbes | Electrical Vestibular Stimuli Evoke Robust Muscle Activity in Deep and Superficial Neck Muscles in Humans[END_REF], [START_REF] Forbes | Neural mechanisms underlying high-frequency vestibulocollic reflexes in humans and monkeys[END_REF], this study aimed to investigate the impact of EVS at 50 Hz on a human pointing task. As E-Fields and currents trigger the vestibular hair cells [START_REF] Dlugaiczyk | Galvanic vestibular stimulation: from basic concepts to clinical applications[END_REF]- [START_REF] Zenner | Electrically evoked motile responses of mammalian type I vestibular hair cells[END_REF], [START_REF] Norris | Mechanisms and effects of transepithelial polarization in the isolated semicircular canal[END_REF] and vestibular signals affect reaching movements [START_REF] Mars | Vestibular contribution to combined arm and trunk motion[END_REF], [START_REF] Bresciani | Galvanic vestibular stimulation in humans produces online arm movement deviations when reaching towards memorized visual targets[END_REF], we hypothesized that EVS would decrease the arm motor control performance.

Only a main effect related to LED positioning was found in our study. Given that no other effect was recorded, we can only understand this has resulting from biomechanical constraints. Although we tried to standardize the participants' position relative to the table by adjusting the table's height at the hips, the target positions were fixed. As the target positions were not adjusted to the participant's arm length for instance [START_REF] Reichenbach | Reaching with the sixth sense: Vestibular contributions to voluntary motor control in the human right parietal cortex[END_REF], this could have resulted by biomechanically restraining participants when pointing at specific targets.

Prior to the pointing task, to make sure our electric stimulations were appropriately applied and strong enough to induce behavioral effects, we used the same 2 mA DC stimulation on all subjects while they were standing eyes closed and feet together. As predicted, our 2 mA DC stimulation destabilized all participants towards the anodal side in the frontal plane (for review see [START_REF] Fitzpatrick | Probing the human vestibular system with galvanic stimulation[END_REF]). Therefore, in this study, before the pointing task, DC was used as a positive control which is defined, herein, as a condition in which specific known effects are expected [START_REF] Johnson | Practical Aspects of Experimental Design in Animal Research Experimental Design : Initial Steps[END_REF]. However, contrary to our hypothesis, our findings showed no increased pointing errors neither with DC nor AC stimulations. A first explanation for our results could be that the 2-mA intensity was too low to directly impact the pointing maneuvers. Indeed, some studies demonstrated arm motor control modulation using 2.5 mA [START_REF] Guerraz | From head orientation to hand control: evidence of both neck and vestibular involvement in hand drawing[END_REF], 3 mA [START_REF] Bresciani | Galvanic vestibular stimulation in humans produces online arm movement deviations when reaching towards memorized visual targets[END_REF] and up to 4 mA [START_REF] Smith | Vestibular feedback maintains reaching accuracy during body movement[END_REF]. Nevertheless, this is surely unlikely. First, in all the aforementioned studies, the participants were stabilized either by being seated [START_REF] Smith | Vestibular feedback maintains reaching accuracy during body movement[END_REF], [START_REF] Guerraz | From head orientation to hand control: evidence of both neck and vestibular involvement in hand drawing[END_REF], or by using a bite-board to prevent possible EVS-induced head motions [START_REF] Bresciani | Galvanic vestibular stimulation in humans produces online arm movement deviations when reaching towards memorized visual targets[END_REF]. To obtain more important vestibular outcomes, greater stimulation intensities are needed with increased body stabilization [START_REF] Fitzpatrick | Probing the human vestibular system with galvanic stimulation[END_REF]. In our study, to try to be as close to an ecological working environment as possible, our participants stood unstabilized while pointing. Second, EVS outcomes have been recorded at intensity much lower than our 2mA stimulations. Vestibulo-ocular outcomes for instance are triggered only with 0.1 mA [START_REF] Severac Cauquil | Short-latency eye movements evoked by nearthreshold galvanic vestibular stimulation[END_REF] and the threshold modulating postural control was found at 0.32 mA [START_REF] Yang | Comparison of postural responses to galvanic vestibular stimulation between pilots and the general populace[END_REF]. Thus, our 2-mA stimulation was expected to trigger responses. One could argue that compared to DC, the sinusoidal aspect of AC helped in lowering the intensity of this stimulation. However, this is also improbable as the intensity of transcranial electric stimulations does not reduce up to 1000 Hz [START_REF] Liu | Immediate neurophysiological effects of transcranial electrical stimulation[END_REF]. Therefore, given that both our DC and AC stimulations were over 6-fold higher than the reported 0.32 mA postural threshold [START_REF] Yang | Comparison of postural responses to galvanic vestibular stimulation between pilots and the general populace[END_REF] and 20 times higher than the 0.1 mA threshold triggering vestibulo-ocular responses [START_REF] Severac Cauquil | Short-latency eye movements evoked by nearthreshold galvanic vestibular stimulation[END_REF], both stimulations were strong enough to impact the vestibular system. EVS-induced vestibular responses follow a craniocentric rule. When the head is facing forward the responses are found in the frontal plane toward the anodal side [START_REF] Fitzpatrick | Probing the human vestibular system with galvanic stimulation[END_REF]. Yet, as the head is turned, the orientation of EVS motor outcomes change accordingly by a similar magnitude [START_REF] Lund | Effects of different head positions on postural sway in man induced by a reproducible vestibular error signal[END_REF], [START_REF] Hlavacka | Postural Responses Evoked by Sinusoidal Galvanic Stimulation of the Labyrinth[END_REF]. Fitzpatrick and Day provided a model in 2004 [START_REF] Fitzpatrick | Probing the human vestibular system with galvanic stimulation[END_REF], revised in 2011 [START_REF] Day | The human semicircular canal model of galvanic vestibular stimulation[END_REF], explaining that the EVS response is mainly explained by summing the six canalithic vectors around which head rotations are perceived. This gives rise to a resultant vector around which the EVS-induced rotation occurs. This resultant vector is oriented backwards in the sagittal plane with an upward component of approximately 18 degrees from Reid's stereotactic plane (the plane joining the ear to the lateral border of the eye) [START_REF] Fitzpatrick | Probing the human vestibular system with galvanic stimulation[END_REF]. This explains why the outcomes given by EVS stimulations produce mainly canalithic outcomes [START_REF] Reynolds | Galvanic vestibular stimulation produces sensations of rotation consistent with activation of semicircular canal afferents[END_REF] with a roll component. In the present study, because workers are not limited in their head movements during their shift, we did not control participant's head orientation. Undeniably, this could have modulated responses in the frontal plane [START_REF] Cathers | Otolith and canal reflexes in human standing[END_REF], [58]. Such changes could be implemented in future protocols. Nonetheless, this would not reflect real-life performance nor give rise to information related to their safety.

Reaching outcomes changes due to vestibular modulation have been hypothesized as either the result of an altered egocentric target location or a neural mechanism stabilizing the arm in space [START_REF] Bresciani | On the nature of the vestibular control of arm-reaching movements during whole-body rotations[END_REF]. EVS impacts cognitive functions in relations to space [START_REF] Reichenbach | Reaching with the sixth sense: Vestibular contributions to voluntary motor control in the human right parietal cortex[END_REF], [START_REF] Hitier | Vestibular pathways involved in cognition[END_REF]- [START_REF] Palla | Ways to investigate vestibular contributions to cognitive processes[END_REF]. Indeed, the vestibular system is highly implicated in spatial orientation [START_REF] Hitier | Vestibular pathways involved in cognition[END_REF] as it plays an important role in space perception [START_REF] Borel | Vestibular syndrome: A change in internal spatial representation[END_REF]- [START_REF] Patel | Galvanic Vestibular Stimulation Induces a Spatial Bias in Whole-body Position Estimates[END_REF] and distance estimation [START_REF] Torok | Up, down, near, far: An online vestibular contribution to distance judgement[END_REF]. EVS also alters the knowledge of arm position in space [START_REF] Smith | Vestibular feedback maintains reaching accuracy during body movement[END_REF], [START_REF] Knox | Do you know where your arm is if you think your head has moved?[END_REF]. Therefore, greater pointing errors could have been expected with both DC and AC EVS stimulations. Besides the vestibular system, pointing to a target, requires the use of other senses such as vision and proprioception. Moreover, the integration of vestibular afferences is multisensory in nature [START_REF] Angelaki | Vestibular System: The Many Facets of a Multimodal Sense[END_REF]. EVS modulate proprioception integration [START_REF] Ferrè | Vestibular modulation of somatosensory perception[END_REF]- [START_REF] Ferrè | Vestibular inputs modulate somatosensory cortical processing[END_REF] which could also have been a factor of increased pointing errors. Regarding vision, our study was carried out in a completely darkened room. Nonetheless, the participants saw the enlightened LED to point at. Therefore, this visual feedback could have helped in modulating the arm trajectory online to compensate for the ongoing EVS effect. Thus, potential greater effects could have been analyzed if the protocol implemented targeting a memorized visual target in the dark. Nonetheless, although workers may work in dark environment such as during the night, or in unlit spaces, they never work blindfolded, nor eyes closed. Thus, modifying our protocol in this sense would have had very little carry over for real life expectations. Furthermore, although it is commonly thought that EVS outcomes decrease when visual input is available, studies show that closing the eyes does not reduce nor abolish responses [START_REF] Bent | Visual-vestibular interactions in postural control during the execution of a dynamic task[END_REF], [START_REF] Welgampola | Vestibulospinal reflexes: Quantitative effects of sensory feedback and postural task[END_REF]. Moreover, other eye-hand coordination tasks necessitating both eyes open and good proprioceptive feedback are influenced by EVS. For instance EVS, with only a 1mA intensity, biases the line bisection task in healthy participants [START_REF] Ferrè | Vestibular modulation of somatosensory perception[END_REF]. The line bisection task is a widely used test to evaluate spatial cognition [START_REF] Jewell | Pseudoneglect: A review and meta-analysis of performance factors in line bisection tasks[END_REF]. During this test, participants draw a vertical line, aligned with their trunk midline, indicating the middle of a horizontal segment. Thus, as in our pointing task, both vision and proprioception are also used during the line bisection task. Interestingly, the line bisection biases were obtained with a much shorter stimulation time. Indeed only a 8-second timeframe was needed to trigger errors [START_REF] Ferrè | Vestibular modulation of somatosensory perception[END_REF]. In contrast, in our study, to try, once again, to analyze how E-Fields could impact workers in their work environment, we chose the much longer period of 3 minutes. During standing protocols, EVS-related responses are rapidly attenuated and vestibular outcomes saturate within the first 40 seconds of stimulation [START_REF] Balter | Habituation to galvanic vestibular stimulation for analysis of postural control abilities in gymnasts[END_REF]. During that 40 seconds period, an 18% decrease in vestibular gain also occur as rapidly as 19 seconds while standing [START_REF] Hannan | Vestibular attenuation to random-waveform galvanic vestibular stimulation during standing and treadmill walking[END_REF]. In our study, to compare the DC, AC, and SHAM conditions, we analyzed the pointing errors during the 3-minute plateau during which the participants had to point 30 times to the randomly lit LEDs. Yet, given this 3min stimulation period, a 15 second ramp gradually increasing the intensity was imposed by the NIC software for participants' comfort. Intensities as low as 0.1 mA start triggering vestibulo-ocular responses [START_REF] Severac Cauquil | Short-latency eye movements evoked by nearthreshold galvanic vestibular stimulation[END_REF]. Therefore, by the time the stimulations' intensity reach the 2-mA peak after the 15 second ramp, the vestibular gain loss could already have reached an 18 % decrease. Although. the first couple of pointing maneuvers could have been impacted by the stimulations, given that each pointing maneuver started every 6 seconds, the average error could have been dampened over time with increased stimulation's habituation. Furthermore, EVS-induced habituation seems to importantly depend on task-dependent mechanisms [START_REF] Hannan | Vestibular attenuation to random-waveform galvanic vestibular stimulation during standing and treadmill walking[END_REF] which could, in our context, be even greater. Indeed, to our knowledge, no specific study as looked specifically at the EVS-habituation mechanisms in pointing tasks.

V. CONCLUSION

The intensity used herein was 2-mA. This likely translates to 0.16 V/m peak at the canalithic system [START_REF] Thomas | Understanding current flow in Galvanic Vestibular Stimulation: A Computational Study*[END_REF]. Such value is much higher than the 0.075 V/m peak synaptic modulation threshold used in the international guidelines for the ELF-MF range [START_REF]IEEE, IEEE Standard for Safety Levels With Respect to Human Exposure to Electric, Magnetic, and Electromagnetic Fields[END_REF]. Yet, although only 0.1mA (0.008 V/m) impacts the vestibular system [START_REF] Severac Cauquil | Short-latency eye movements evoked by nearthreshold galvanic vestibular stimulation[END_REF] we did not record specific behavioral modulation during the pointing task in our study. Nonetheless, 2-mA EVS above powerline frequency impacts the vestibular system [START_REF] Forbes | Electrical Vestibular Stimuli Evoke Robust Muscle Activity in Deep and Superficial Neck Muscles in Humans[END_REF], [START_REF] Forbes | Neural mechanisms underlying high-frequency vestibulocollic reflexes in humans and monkeys[END_REF]. Therefore, until proven otherwise, from a Standards/Guideline's perspective, this argues against potential powerline ELF-MF adverse effects at this E-Fields levels. Nonetheless, further studies will have to concentrate on potential mid-and long-term health-related issues. Altogether, our results could be due to habituation mechanisms decreasing the EVS impact explaining why this modulation did not translate behaviorally through the chosen pointed task. Thus, from both performance and safety perspectives, workers should particularly pay close attention to security procedures when first emersed in ELF-MF environment and wait a couple of minutes for the habituation process to take place to ensure optimal safety and performance during their work shift. Finally, to date, phosphene should remain the main model on which both the international guidelines and Standards should be based.

ETHICAL STANDARD

The study was conducted according to the guidelines of the Declaration of Helsinki and approved by the Institutional Review Board of Euromov-DHM laboratory at the University of Montpellier, France (IRB # 2001D approved on October 7 th , 2020). , where he is a Scientist since 2007. He has expertise in the fields of neurosciences, kinesiology, and biophysics applied to the study of neurostimulation and neuromodulation. His research interests mainly relate to the effects of specific electric and magnetic stimuli (DBS, transcranial magnetic stimulation, and time-varying magnetic fields) on human brain processing, motor control, and cognitive functions. He was a Board Member and then secretary of the BEMS society (from 2013 to 2020). He organized the BioEM2019 international conference in Montpellier, France. He is currently Canadian chair for URSI commission K, co-chair of the subcommittees 3 and 4 within the IEEE-ICES TC95, and chair or the Editorial Working Group in charge of the next Standards revisions in the Low Frequencies. He is also co-chairing the Non-Ionizing Radiations Task Group of IRPA with Dr. Julien Modolo (International Radio Protection Association).

FIGURE 1 .

 1 FIGURE 1. Schematic experimental setup seen from above. All LEDs were embedded in the wooden table. The red dot (LED-R) signifies the starting point. Each green LED indicates the pointing targets to aim at. Each target was distanced 30 cm away from LED-R. LED-1 was set in line with LED-R. Starting with LED-1, LED-2 and LED-3 were consecutively set 45° clockwise.

FIGURE 2 .

 2 FIGURE 2. Classical binaural bipolar montage used for the direct and the alternating vestibular stimulations. In both cases the electrodes (green circles) are set at the back on the mastoid processes. For DC stimulations, the cathode is arranged at the back of the right mastoid process and the anode is put at the back of the left mastoid process.

FIGURE 3 .

 3 FIGURE 3. Schematic representation of the protocol. The entire session consists of three distinct runs, each encompassing three 3.5 min randomized blocks done in complete darkness, separated by 30 sec rest periods. During each randomized block, the participants pointed 30 times to a randomly lit LED during a 3 min stimulation period. Between runs, the light was turned back on for 3 min to provide more rest.

FIGURE 4 .

 4 FIGURE 4. Graphical representation of the pointing errors dependent variables. In all panels, the green dot represents a LED target, and the blue dots embody the pointing coordinates for one trial. A) black dot symbolizes the mean pointing score. The red dotted line quantitively represents the Euclidean distance describing the Global error which can then be decomposed along both the X and Y axes. B) The red line represents the main direction of the pointing errors in space at an angle θ symbolized by the grey shaded area. C) The blue ellipse is an example of the measure of error variability using the ellipse area (blue shaded zone).

FIGURE 5 .

 5 FIGURE 5. Global pointing errors occurring during the three types of stimulation for the first LED 1 (A), LED 2 (B) and LED 3. Each blue dot represents the value of the Euclidean distance in cm between a LED and a pointing performance. The red diamond embodies the mean global pointing error along with the standard deviation.

FIGURE 6 .

 6 FIGURE 6. Qualitative representation of the pointing errors in space for all three LEDs and all three stimulations (SHAM, DC, AC). The area of the red ellipses is a representation of the measure of error variability. The red ellipses are oriented along an angle θ, indicating the main direction of the pointing errors.

  signal processing in 1995. The same year, he obtained a Master's degree in image processing from the University of Toulon, France. He received his PhD from the same university in 1999, studying specific nonlinear and chaotic dynamical systems. His main research interests are in the area of nonlinear time series analysis techniques, with applications to neurophysiological signals and human movement data.Dr. ALEXANDRE LEGROSis Director of the Bioelectromagnetics and Human Threshold Research Group at the Lawson Health Research Institute; Associate Professor with the Departments of Medical Biophysics, Medical Imaging, and Kinesiology of Western University in London ON, Canada; and a scientist at EuroMov-DHM, University of Montpellier, France, where he is duplicating his Canadian laboratory and developing new collaborative research projects involving human nervous system responses to electric and magnetic stimulations. Dr. Legros received the Ph.D. degree in human movement sciences in 2004 from the University of Montpellier. He completed a first postdoctoral fellowship on electrical deep brain stimulation (DBS) in patients suffering from dystonic syndromes (Neurosurgery unit of Guy de Chauliac Hospital, Montpellier, France), and a second postdoctoral fellowship in the Bioelectromagnetics Group of the Lawson Health Research Institute (London ON Canada)

This work was supported by the

ANDRES CARVALLO received the engineer in telecommunications degree specialized in signal, imaging, embedded systems, and automatic control from the University of IMT Atlantique, France in 2013 and his PhD degree in biomedical signal processing from the University of Rennes 1, France in 2018. He has been working as a researcher in the University of Montpellier since 2020. His research interests include exposure limits to extremely low frequency electric and magnetic fields, human experimentation, and dosimetry studies.


Abdulrahman Albeladi

Moman Akhlaq

Timothy F L Mckenna
email: timothy.mckenna@univ-lyon1.fr

Akhlaq Moman

Performance of Post Phthalate Ziegler Natta Catalysts with Activity Limiting Agents for Propylene Gas Phase Polymerization

Keywords: Ziegler-Natta catalysts, gas phase polymerization, activity limiting agent, polypropylene, post phthalate Catalysts

published or not. The documents may come   L'archive ouverte pluridisciplinaire

Introduction

The combination of the highly exothermic nature of Ziegler Natta (ZN) catalyzed polymerizations and the need to operate at high polymerization rates make the possibility of thermal runaway a real risk, particularly in gas phase reactors. Thermal runaway in fluidized bed reactors while the catalyst is still highly active lead to many problems ranging from increased fouling to complete loss of fluidization due to the formation of agglomerates. For propylene polymerization, the use of activity limiting agents (ALA), also called thermal runaway reducing agents (TRRA) have proven to be quite effective in deactivating the catalyst at elevated reaction temperatures: higher than 80 °C which provides an effective solution to the mentioned problems. This effect is generally referred to as the self-extinguishing property of a catalyst system [1] . Such modifiers are electron donating compounds similar to the external electron donors that are used in propylene polymerization to control stereoregularity but have not been proven to be effective tacticity control agents on their own [2] , hence, mostly they are injected in addition to the classical tacticity control agents such as alkoxysilanes.

In an overview of the recent advances in fluidized bed polymerization technology, Cai et al. [3] presented the advantage of using DOW's (the technology now is owned by W.R. Grace) proprietary Advance Donor Technology (ADT) with Ziegler Natta catalysts for propylene polymerization where the catalyst deactivates, or, as the authors coined it, it self-extinguishes as the temperature increases. They stated that such optimization of catalyst kinetics reduces particle agglomeration by killing the active sites before the particle temperature can reach the softening point. Another advantage they have also mentioned is the increased flexibility in the reactor operating envelope, such as increasing the production rate without decreasing the polymer bed inventory or working at higher fluidized bed densities while maintaining a high production rate.

Chen and Campbell [4] in their patent (US 7,491,670 B2, entitled "A self limiting catalyst composition and propylene polymerization process) disclosed a catalyst composition that includes an Activity Limiting Agent (ALA) to control polymerization in addition the normal modifiers used for propylene polymerization (i.e., co-catalyst, and selectivity control agents). This ALA renders the catalyst almost inactive at elevated temperatures. They discussed that the use of para ethoxy ethyl benzoate (PEEB) as an external donor with third generation ZN catalysts that provides the desired selfextinguishing property with respect to reaction temperature, but that is ineffective for tacticity control with the later generations of ZN catalysts. They presented many examples for combinations of different selectivity control agents, including silanes, and ALAs such as dibutylsebacate (DBS), and methytrimethylacetate (MTMA) that seem to lead to a significant (self-extinguishing) decrease in catalyst activity at high reaction temperatures.

Looking at their data, one can see when the selectivity control agents are solely used, the catalyst still retains enough activity at temperatures close to the softening temperature of polypropylene which may lead to agglomeration and fouling, but this was not the case when ALAs were used.

Cai et al. [5] disclosed a gas phase polymerization process where the fluidized bed density is higher than 128 kg/m 3 . Operating at high bed density for a fluidized bed reactor means lower superficial gas velocity which in turn means less heat is removed from the bed. On the other hand, as long as one maintains stable reactor operation, a high bed density means high space time yields that are economically desirable. However, as in the mentioned patent, the inventors employ a mixed external donor system where one of these donors is an ALA which allows for operating the reaction beds at higher bed densities without encountering continuity issues. The use of the ALA provides the needed protection from temperature excursions as the catalyst is deactivated at elevated temperatures before reaching the softening point of the polymer. They presented two examples of using DBS with homo-polymerization and random copolymerization of propylene in vertically and horizontally stirred reactors, and compared these to two similar examples without the ALA.

They showed that after a problem occurring to the agitator, the kill gas is immediately introduced and an attempt to start agitation is done after fixing the problem. Agitation could have not been started when no ALA is used due to the blocking of the reactor with agglomerates and chunks, which was not the case when the ALA was used and agitation in this case was re-started without issues.

In a patent by Basell, Morini et al. [2] highlighted two negative aspects of using ALAs to control the reaction at elevated temperatures: the drop in base catalyst activity at normal operating temperatures, and the drop in stereoregularity when attempting to lower the mixed donor ratio to the catalyst. In their patent, they disclosed a catalyst composition for polymerization of propylene containing a diether internal donor that exhibits the desired self-extinguishing property without the addition of specific donors like in the case of using ALAs. They presented several examples whereby showing the residual activity of their catalyst system drops to as low as 43% at 90 °C.

Several other patents disclose novel mixtures of external donors and processes

where an ALA is used, including in the production of impact propylene copolymers [6] , in the production of high melt flow propylene polymers [7] , in the production of ethylene based polymers [8] , the discontinuous addition of thermal runaway control agents in commercial production of polypropylene [9] , the use of monoether compounds as ALAs [10] , utilizing fluorinated fatty acids as ALAs [11] , and an improved multi-zone olefins polymerization process operating in deep condensed modes employing several thermal runaway control agents [12] .

Although there is a significant surge in patent applications disclosing processes and compounds where the desired self-extinguishing property can be achieved in olefin polymerization reactors since 2005 (c.f. Figure 1), there are no clear hypotheses of their mechanism of action. Vipin Raj et al. [13] conducted DFT studies on two novel ALAs and concluded that interactions between the ALA and dimeric TEA leads to a decomposition reaction with the help of another TEA molecule, producing either a ketone or an aldehyde.

Similarly, the interaction of the ALA with Ti on the 110 surfaces of MgCl2 produces more favorably a ketone. The decomposition reactions products lead to the poisoning of active sites as both ketones and aldehydes are known to be able to poison Ziegler Natta active sites. They also explained the efficacy of one ALA over the other in their study by thermodynamic binding preferences supported by the computational study. Such decomposition reactions have been experimentally observed as a result of the reaction between the alkyl aluminum and esters by Chien et al. [14] where they also suggested the pathways by which such reactions occur and the role of temperature in the decomposition rates. In this work, we are evaluating the performance of a 6 th generation Ziegler Natta catalyst for propylene polymerization in gas phase developed by SABIC with three activity limiting agents. The ALAs were chosen from the cited patents and have been proven to work effectively with the corresponding catalyst systems used in the aforementioned patents. The main property we will look at is the so-called self-extinguishing property of the catalyst system. This will be conducted by varying the amounts of ALAs and combinations with the selectivity control agent at different reaction temperatures. We will also take a closer look at the polymer microstructure and powder morphology (mainly the bulk density and fines content) to identify any eventual effects of ALA, if any.

Experimental

Materials

The catalyst used in this study is a sixth generation (phthalate free) Ziegler Natta catalyst developed by SABIC. The titanium content of this catalyst is around 3 wt% and it is formulated with two different internal electron donors. Triethylaluminum purchased from Witco was diluted to around 1 M in n-heptane which is purified using Mbraun solvent purification unit. The external electron donor (ED): n-propyltrimethoxysilane (N-Donor) was purchased from ABCR and stored over molecular sieves after bubbling for a couple of hours using Argon to get rid of free oxygen. The ALAs: isopropyl myristate (IPM), methyl trimethylacetate (MTMA), and dibutyl sebacate (DBS) were purchased from ABCR and were treated similarly to the electron donor. Both the ED and ALA compounds were diluted in purified heptane before introduction to the polymerization experiments.

Polymerization Setup and Procedure

The polymerization setup and procedures of preparation, conducting the experiments, and post treatment of the polymers are the same as described in a previous publication [15] .

Polymer Characterization

The polymers in this study were characterized for molecular weight using GPC, stereoregularity using 13 C NMR, bulk density using a standard volume recipient, and particle size distribution using a Malvern Mastersizer 3000 particle analyzer. All the characterization methods have been described in a previous publication [15] .

Diffuse Reflectance Infrared Transmittance (DRIFT)

DRIFT analyses were performed using a Nicolet 6700 FTIR where data are collected using OMNIC software. The treated samples were placed in the airtight DRIFT cells with a Classification: General Business Use CaF2 window inside the glove box. For each spectra, 64 scans were collected with a resolution of 4 cm -1 . All the treatments of the catalyst for DRIFT analysis were performed at room temperature.

Experiments

The three main variables in this study are reaction temperature, use of electron donor (an alkoxysilane), and the ALA/Ti ratio. All the core experiments of this work were conducted with 20 g of NaCl as a seedbed under a propylene pressure of 8.0 barg, and 2.0 mol% of hydrogen. The molar ratios for the co-catalyst and the ED to Ti were 51, and 3, respectively. As shown in Table 1, experiments 1 to 3 are the reference experiments where N-Donor is used and the reaction temperature is varied at three levels: 58, 78, and 94 °C. A second set of experiments was carried out, as shown in Table 2, with the objective of discriminating the possible reactions of ALA during the gas phase polymerization of propylene. This set of experiments was conducted using the same catalyst with 6 barg of propylene, g of NaCl as seedbed, where no hydrogen nor a selectivity control agent (alkoxysilane; namely N-Donor) have been introduced in an attempt to minimize the variables. All the experiments were conducted with one ALA, dibutylsebacate (DBS) and triethylaluminum (TEA) as the co-catalyst. The four main variables considered here are the Al/Ti ratio, ALA/Ti ratio, reaction temperature, and method of addition for the co-catalyst and the ALA. The methods of addition of modifiers will be described below:

1. Method A: the catalyst is pre-contacted with the ALA and then vacuum dried, the catalyst then is introduced to the reactor one hour after the pre-contacting step, and the co-catalyst is injected before the start of polymerization.

2. Method B: the catalyst is pre-activated with the co-catalyst which is then vacuum dried before introduction to the reactor with a time of contacting around 10 minutes, then the reaction is started without an ALA until around minute 30 where it is introduced.

3. Method C: similar to method B (catalyst pre-activated with Al/Ti of 2) except that the ALA added during the reaction is mixed with the co-catalyst to make up the total Al/Ti of 50 and injected at around minute 15 of the reaction.

4. Method D: similar to method B except that the ALA addition is done at around minute 15 of the reaction followed by a TEA injection around minute 25.

Results and Discussions

Reaction Rates

Temperature Effect with and without N-Donor (ED)

The reaction rate profile for this catalyst exhibit two types of behavior when Ndonor is used as shown in Figure 3-A, at the low temperature (i.e. 56 °C), it shows the typical high initial activity followed by decay, whereas at the higher reaction temperatures, 78 and 94 °C, there are two regions; a very short mild decay followed by a build up kinetic profile. The appearance of a second peak in the reaction rate has been reported by some researchers for ethylene and ethylene-propylene polymerizations where mainly two explanations have been put forth: the changes in Ti oxidation state and the role of hydrogen in re-oxidation of Ti [16] , and perhaps the existence of two different types of active sites [17] . This behavior could be of a chemical origin where reactivation of some catalytic sites takes place later on during polymerization resulting in an increased rate of activation over deactivation. The external donor role in this behavior is important which could be participating in the activation of such sites as such behavior is not observed when no external donor is used as shown in Figure 3-B. The reaction rates are higher in the absence of the electron donor by a maximum increase of around 30% at the lowest reaction temperature which is due to the deactivating ability of the N-donor which has 3 alkoxy groups, two of which can participate in the deactivation of catalytic sites [18] . However, at the highest temperature, the reaction rate without an external donor is significantly decreased. This might be due to self poisoning of the catalyst with one of the internal donors or its decomposition products. This catalyst has two types of internal donors, one that is not affected by the co-catalyst, and the other one which can be removed by the co-catalyst, we believe it is possible for this second donor to poison the active sites at the higher temperature because of the possible higher rate of removal of this second internal donor at elevated temperatures by the cocatalyst.

Temperature Effect with IPM

The effect of different IPM molar ratios to Ti at different temperatures is shown in Figure 4-A, B, C, andD. At the lowest IPM molar ratio, the reaction rate at the lowest temperature does not change but as the temperature increases, the activity decreases reaching a significant drop of 67% at the highest reaction temperature as shown in Table 3. Moreover, all the kinetic curves are showing a decay type with two distinct regions: a quick decay until around 10 minutes followed by a very stable region with the rate of decay slowing down as the reaction temperature is increased. Increasing the molar ratio of IPM to 1.0 results in a very mild decrease in activity while the reaction rates are practically the same as shown in Figure 4-B. Similarly, the highest ALA/Ti molar ratio we have used in this study of 9 shows similar trends for the lower temperatures while the significant impact of ALA on the reaction rate is observed at the highest reaction temperature, 94 °C, as shown in Figure 4-C.

Using IPM in absence of N-donor leads to around 20% increase in the catalyst productivity as shown in Figure 4-E, while the reaction rate curves at 58, and 78 °C show similar behavior to the rest of experiments where we have used N-donor. However, at the highest temperature, we see the appearance of a second activation evident by the slightly increasing reaction rate after the first 5 minutes of reaction.

One of the parameters we are using in this study to measure the extent of catalyst self-extinguishing property in the percentage of activity decrease at 94 °C in relation to the standard experiment at the same temperature. As we can see in Table 3 for IPM, even at the lowest ratio of ALA/Ti, a 67% drop in activity takes place compared to the standard system (STD) while the activity at the lowest temperature is not affected. Further increasing the amount of IPM increases the activity drop to 74, and 93 %, however, at the highest ratio of IPM where the best self-extinguishing performance is seen, it comes at the expense of activity at the lower temperature (i.e., normal operating temperatures) with a drop of around 19% compared to the standard experiments. Finally, using IPM without an external donor does not seem to be very effective in controlling the activity at higher temperatures with the activity dropping around 40% while activity at the lowest temperature increased by 23%. The increase in activity at the lower temperatures is due to the lower total ED+ALA molar ratio to Ti which lowers the poisoning effect of those external bases leading to an increase in activity. However, the inefficient behavior of the ALA when no external donor is used is due to two factors: the increase of TEA concentration reacting with IPM which will be discussed later, and the lower ratio of ALA to Ti.

13

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Temperature Effect with MTMA

MTMA is a monoester similar to IPM but is a smaller molecule in comparison and has been reported to show a self-extinguishing property with ZN catalyst systems [4] .

Similar to IPM, the kinetic behavior of the catalyst with MTMA at the lowest temperature as seen in Figure 5 exhibits the classical rate decay over time, however, at the higher temperatures, 78 and 94 °C for ALA/Ti of 0.3 and 3.0, we start to see the two peaks reaction rate where a second reactivation peak appears followed by very minimal decay as shown in Figure 5-A, andB. This behavior does not seem to be present in absence of the external donor with MTMA, as shown in Figure 5-C.

The efficacy of MTMA in inducing self-extinguishing of the catalyst can be observed in Figure 5-E and Table 4 where a significant drop of activity can be seen, even at the lowest ALA/Ti. However, the highest activity drop is observed at the highest ALA/Ti with about 96% loss of activity compared to the standard catalyst experiment at the same temperature (i.e., experiment number 3). Though the behavior of retarding activity at higher temperatures seems to be similar to IPM, the negative influence on the activity at the lowest temperature is quite significant where 50% of the activity is lost as shown in Table 4 compared to the standard system with an increase of 30% compared to the loss with IPM at the highest ALA/Ti. We believe this effect is dependant on two main factors:

the reactivity of MTMA with TEA, and the higher diffusivity of the MTMA. We will discuss in a later section of this work the possible mechanisms of actions for ALA which will shed some light on some of the differences in their self-extinguishing behavior. Moreover, MTMA seems to be quite effective when used in absence of the external donor contrary to IPM where we see 80% drop in activity at 94 °C while the negative effect on activity at 58 °C is only a 14% decrease.

Table 4. Percentage of changes in catalyst productivity with MTMA as an ALA.

Experiments Set

Temperature Effect with DBS

The reaction rates for the experiments where DBS is used are shown in Figure 6-A,B,C, and D. Similar to the reference experiments, we can see a reactivation period for the lower ALA/Ti ratios that disappears when ALA/Ti is increased to 9. DBS shows a similar behavior in terms of inducing the self-extinguishing property as IPM with one exception, the drop in activity at 58 °C is more than that of IPM as shown in Table 5. It is possible that having two functional groups leads to more catalyst deactivation at the lower temperatures especially if the TEA only binds to one of them leaving the other free to adsorb on the active site. Another interesting difference is the significant increase in activity when DBS is used alone, one of the reasons could be the stronger ability of DBS to control isotacticity compared to the other ALAs while having a lower chance of deactivating the catalyst as compared to the ED which has three alkoxy groups which means higher chance of deactivating the catalyst [18] .

Table 5. Percentage of changes in catalyst productivity with DBS as an ALA.

Experiments Set

Polymer Microstructure and Morphology

The microstructure (molecular weight and tacticity) and morphology (bulk density and fines content) analyses were performed on the most meaningful subsets of experiments, which are listed below:

 The reference experiments: 1 to 3.

 The reference experiments without an external donor: 4 to 6.

 ALAs experiments with the highest ALA/Ti, which is 9.

 ALAs experiments in absence of the external donor.

Molecular Weight

Molecular weight results are shown in Table 6 where the number average molecular weight (Mn), weight average molecular weight (Mw), and molecular weight distribution (MWD) are reported for the analyzed experimental subsets. As we can see in the table, both Mn and Mw decrease for all experiments as the reaction temperature increases. There are mainly two factors that underlie such a trend: the transfer reactions, and the concentrations of active sites with different tacticities. The increase in reaction temperature leads to an increase in the transfer reactions especially with the co-catalyst [19] . Additionally, the drop in isotactic sites concentration, demonstrated by the isotactic pentads results which will be discussed next, that are known to produce higher molecular weight leads to the observed drop in Mw. We can also observe this in the molecular weight results of the experiments where no external donor was used where we see on average a 25% decrease in Mw compared to the reference experiments (i.e., experiments 1 to 3).

The MWD results are comparable for all experiments where we do not see any major effect of ALAs or temperature except for MTMA when used with the external donor where a slight broadening is observed due to the atypical response in effect of temperature on Mn and Mw. Additionally, when DBS is used in absence of external donor, the MWD shows mild narrowing at the reaction temperature of 78 °C. It is possible that ALAs participate in transfer reactions which could explain some of the subtle differences observed.

Stereoregularity

One of the important properties of polypropylene is the stereoregularity of the polymer, which is measured using various characterization techniques, most notably for quick indication of tacticity, Xylene Soluble (XS) are usually performed in industrial plants.

However, for more detailed and accurate quantification of the stereo-errors in the polymer, 13 C NMR is used which we have utilized in this study. The assignment of the pentads was performed as described elsewhere [20] . In addition to the isotactic pentads, mmmm, we are reporting the Meso Sequence Length (MSL). MSL indicates the length of isotactic portions of the polymer chain which correlates to the isotactic pentads but is affected significantly with stereo-errors, and is calculated using the equation that follows [21][22][23] . The catalyst used in this study has two internal donors to control the isotacticity of the polymer, one of them is hardly affected by the TEA whereas the other one can be extracted by TEA, and because of this we can see that the mmmm pentads fluctuate within a narrow range as shown in Table 7. For all the experiments, the temperature seems to negatively affect the isotactic pentads, which is most likely due to the extraction of one of the internal donors by the co-catalyst, TEA. It is also possible that as temperature increases, TEA decomposition reactions are more favored [START_REF] Malpass | Industrial Metal Alkyls and Their Use in Polyolefin Catalysts[END_REF] , which lowers the efficiency of the external donor in isotacticity control, as the complex between TEA and the external donor is thought to play an important role in regulating isotacticity [START_REF] Sacchi | [END_REF] .

The standard catalytic system with N-donor as an external donor gives a maximum mmmm content of 93%, which drops to 91% at the highest polymerization temperature as shown in Table 7. However, in absence of the external donor, the isotactic pentads did not drop significantly which as we mentioned earlier is due to the inability of TEA to extract one of the internal donors, thus, retaining a high isotacticity. A major difference when the external donor is not used is the drop in the MSL which indicates the presence of higher stereo-errors in the absence of the external donor; this correlation is shown in Figure 7 for all the experiments where the MSL correlates very well to the mr diads which represent the stereo-irregular diads. The addition of the ALAs with the external donor increased the isotactic pentads marginally but the significant increase can be observed in the MSL which is indicative of the ability of the ALAs, when added with an external donor, to control the polymer stereoregularity. On the other hand, using the ALAs without an external donor leads to a similar content of isotactic pentads as in the experiments without an external donor. This shows that the ability of ALAs to regulate tacticity is nonexistent in absence of an external donor. Nevertheless, the highest isotacticity in terms of mmmm pentads and MSL is observed when MTMA and DBS are used with the external donor.

Resin Morphology

The powder morphology is quite important for gas phase processes and can be evaluated using several characterization techniques, some of the most commonly used in industrial plants are powder settled bulk density and fines content (less than 45 µm particles for this scale and conditions). It is always desired to maximize the bulk density and minimize the content of fines in gas phase fluidized bed reactors. As we can see in

Possible Mechanisms of ALAs

To investigate the possible mechanisms of ALA with the catalyst we have used, we have performed a series of experiments that we have described in the experimental section, experiments 19 to 26. Additionally, the catalyst was analyzed using DRIFT without any treatment (R530 curve in Figure 10), and with DBS treatment (DBS curve in Figure 10). The treatment with DBS was done in a glove box at room temperature where a ratio of DBS/Ti of 9 was used, after the DBS is contacted with the catalyst under mixing, the catalyst is dried and sampled for the DRIFT analysis. The DRIFT spectra as shown in Figure 10 has the following distinct peaks that are identified based on the functional groups of the internal donors and the DBS:

1. Peaks 1024 and 1060 belong to the first internal donor used in this catalyst.

2. Peak 1611 belongs to the second internal donor.

3. The peak appearing after DBS treatment, 1690, belongs to DBS.

4. Peaks between 3400 and 3600 belong to traces of OH on the catalyst surface [26] .

The addition of DBS does not influence the internal donors used in this catalyst; however, DBS seems to bind to the catalyst surface. Since no Ti vacancies are available in a non activated catalyst as is the case here, the DBS is most likely bound to the Mg. As we will see from the experiments next, the DBS binding to the Mg does not seem to influence the selfextinguishing property of the catalyst. reference to the earlier experiments described in the prior sections, at the highest temperature with DBS, we would expect a complete extinction of the polymerization before 60 minutes which is not the case. This indicates that treating the catalyst with DBS does not affect the self-extinguishing property of the catalyst significantly. This is most likely the case as the DBS is bound to Mg where the effect is limited to stereoregular control. In the second set of experiments shown in the same figure, B, addition of the DBS during the polymerization where no TEA is present as the catalyst is preactivated and dried in the glove box before injection shows that the DBS is able to retard the polymerization independent of the polymerization temperature. This means that the DBS is able to adsorb on the active site directly and suppress activity completely. To further distinguish which reaction is more critical with the DBS, some TEA was mixed with the DBS before injection during the reaction as seen in graph C, where at the lower temperature, the reaction proceeds as if there was no DBS added, however, at the higher temperature, we get the same effect as in graph B where the reaction rate goes to zero after the addition of the DBS mixed with TEA. Though we have no direct evidence, these experiments strongly suggest that the most critical reaction happens between the DBS and TEA, where either this reaction is not favored at elevated temperatures which allows the free DBS molecule to adsorb on the active site and suppress the activity, or the decomposition reaction that occurs between the TEA and the DBS is causing the observed deactivation, however, the decomposition reaction doesn't explain the catalyst deactivation by DBS at the lowest reaction temperature where the catalyst is pre-activated with TEA and dried unless the decomposition can occur at the active sites without the aid of the co-catalyst at low temperatures.

Finally, to understand how reversible this retarding reaction by DBS is, we have performed the last set of experiments shown in graph D, where DBS is added in the middle of the reaction which leads to complete suppression of the reaction at both operating temperatures, followed by two injections of TEA at a molar ratio of 48 to Ti. We can see in the case of the lower temperature, TEA seems to reactivate the catalyst slowly but steadily, even in the case of the higher temperature, we can see some reactivation, but it is extremely low. Though we cannot rule out the decomposition of DBS into some poisonous compounds when reacting on the active species, however, it is difficult to imagine such a decomposition reaction is fast acting especially at the lower reaction temperature of around 36 °C. Instead, it is more likely that the DBS is adsorbing directly on the active species in absence of TEA and rendering it inactive. Based on the experiments performed in this study and in addition to the decomposition reactions that are favored at higher temperatures, we believe the following reactions might take place as well when an ALA is introduced to the reaction environment:

 A reversible ALA-TEA complex formation that is favored at low temperatures.

 Reacting with the Mg in an analogous way to the alkoxysilanes which are used as external donors; this reaction could be the reason ALAs exhibit mild stereoregular control.

 Adsorption on the alkylated active Ti species which leads to the complete suppression of the reaction at higher temperatures.

In addition to the expected reactions that take place with the ALAs, the diffusion of these molecules could play an important role as most of them are quite large molecules as we have seen a difference in the behavior of the smaller molecule we have used in this study, MTMA, compared to the aliphatic mono and diesters, IPM and DBS. Additionally, the number of functional groups could play a significant role in the reactions taken place, for example, we know from studies conducted on alkoxysilanes (i.e. external donors) that an increased number of functional groups leads to increased and unselective deactivation of active species [18] .

Conclusions

The performance of three activity limiting agents (ALA) with a sixth generation Ziegler Natta catalyst system was evaluated during the gas phase polymerization of propylene. The retarding effect of ALAs on the catalyst at elevated temperatures in order to prevent reactor instabilities in the case of thermal runaways were also studied.

Additionally, we have investigated the effects of ALAs on the polymer microstructure, and the morphology of the powder reactor product. Finally, we have looked at the possible pathways by which the retarding effect of the ALA is taking place.

reflected in the isotactic pentads and more so in the MSL. The increase in isotacticity is more pronounced with MTMA, and DBS, than with IPM. Moreover, the use of ALAs without an external donor does not lead to any significant reduction in the isotacticity of polypropylene. Finally, it seems that besides the suspected decomposition reactions' products that could poison the active sites at higher temperatures, the ALA itself is capable of reversibly deactivating the active site, this reaction is seemingly prevented when the ALA complexes with the co-catalyst which is probably more favored at lower temperatures.

Figure 1 .

 1 Figure 1. Patent applications for ALA and self-extinguishing catalyst formulations.

Experiments 4 2 .Figure 2 .

 422 Figure 2. Chemical structures for the ALAs used in this study: A. Isopropylmyristate (IPM), B. methyltrimethylacetate (MTMA), and C. dibutylsebacate (DBS).

Figure 3 -

 3 Figure3-C shows the catalyst productivity as a function of temperature for the two sets of experiments: with and without an external donor, the activity in both cases follow a decreasing trend but the decrease in the presence of the ED at 94 °C compared to 58 °C is around 26% whereas it is around 89% in the absence of the ED. This behavior could mean the catalyst itself exhibit a self-extinguishing property in the absence of external donors.

Figure 3 .

 3 Figure 3. Experimental reaction rates for: A. the reference experiments with N-Donor and no ALA, B. No N-Donor nor ALA, and C. the catalyst productivities for these experiments plotted against reaction temperature.

Figure 4 .

 4 Figure 4. Experimental reaction rates for: A. ALA/Ti=0.3 with N-donor, B. ALA/Ti=3.0 with Ndonor, C. ALA/Ti=9.0 with N-donor, D. ALA/Ti=3.0 without N-donor, and E. the catalyst productivities for these experiments plotted against reaction temperature.

Figure 5 .

 5 Figure 5. Experimental reaction rates for: A. ALA/Ti=0.1 with N-donor, B. ALA/Ti=1.0 with Ndonor, C. ALA/Ti=3.0 with N-donor, D. ALA/Ti=3.0 without N-donor, and E. the catalyst productivities for these experiments plotted against reaction temperature.

Figure 6 .

 6 Figure 6. Experimental reaction rates for: A. ALA/Ti=0.3 with N-donor, B. ALA/Ti=3.0 with Ndonor, C. ALA/Ti=9.0 with N-donor, D. ALA/Ti=3.0 without N-donor, and E. the catalyst productivities for these experiments plotted against reaction temperature.

Figure 7 .

 7 Figure 7. MSL plotted as a function of mr diads (i.e., stereo-irregular diads).

Figure 8 -Figure 8 .Figure 9 .

 889 Figure8-A, the bulk density follows a similar decreasing trend with reaction temperature for all the experiments. The decrease in bulk density as the reaction temperature increases is perhaps related to changes in the fragmentation of the growing catalyst particles, which is clearly affected by the initial reaction rate as shown in Figure9-A, and B. The polymer produced using this catalyst system shows a high bulk density overall, however, due to the mechanically stable structure of the catalyst support, higher initial reaction rates result in a more uniform fragmentation, whereas lower initial reaction rate probably leads to a nonuniform process of fragmentation which is reflected on the lower bulk densities. For example, when DBS is used without an external donor where the highest initial activities are observed at 58 and 78 °C, we can see a clear significant improvement in bulk density compared the standard experiments where the external donor is used.

Figure 10 .

 10 Figure 10. DRIFT spectra of the catalyst with and without DBS treatment.

Figure 11 .

 11 Figure 11. DBS experiments with different methodologies; A. experiments number 19 and 20, B. experiments number 21 and 22, C. experiments number 23 and 24, and D. experiments number 25 and 26.

Table 1 .

 1 Experiments on the effects of different ALAs in gas phase propylene polymerization.

Table 2 .

 2 Experiments with DBS with different methods of additions for Al and ALA.

	SN	Al/Ti	ALA/Ti	T (°C)	Al and ALA Addition
					Method
	19	50	9	36	Method A
	20	50	9	96	Method A
	21	2	9	36	Method B
	22	2	9	96	Method B
	23	2 and 48	9	36	Method C
	24	2 and 48	9	96	Method C
	25	2 and 48	9	36	Method D
	26	2 and 48	9	96	Method D

Table 3 .

 3 Percentage of changes in catalyst productivity with IPM as an ALA.

Table 6 .

 6 Molecular weight characteristics.

	SN	ED	ALA	ALA/Ti	Mn (kDa)	Mw (kDa)	MWD
	1	N	None	0	40	217	5.4
	2	N	None	0	33	166	5.0
	3	N	None	0	24	124	5.3
	4	None	None	0	32	163	5.1
	5	None	None	0	27	138	5.2
	6	None	None	0	13	83	6.2
	13.1	N	IPM	9.0	31	213	6.9
	14.1	N	IPM	9.0	29	190	6.5
	15.1	N	IPM	9.0	26	130	5.0
	16.1	None	IPM	3.0	35	191	5.5
	17.1	None	IPM	3.0	29	152	5.2
	18.1	None	IPM	3.0	20	115	5.7
	13.2	N	MTMA	9.0	39	301	7.7
	14.2	N	MTMA	9.0	22	164	7.6
	15.2	N	MTMA	9.0	22	113	5.2
	16.2	None	MTMA	3.0	30	187	6.3
	17.2	None	MTMA	3.0	28	147	5.2
	18.2	None	MTMA	3.0	15	103	6.9
	13.3	N	DBS	9.0	43	233	5.3
	14.3	N	DBS	9.0	39	212	5.4
	15.3	N	DBS	9.0	26	151	5.8
	16.3	None	DBS	3.0	34	177	5.3
	17.3	None	DBS	3.0	34	142	4.2
	18.3	None	DBS	3.0	21	104	4.9

Table 7 .

 7 isotactic pentads and MSL results.

	SN	ED	ALA	ALA/Ti	mmmm	MSL
	1	N	None	0	93	85
	2	N	None	0	92	72
	3	N	None	0	91	65
	4	None	None	0	92	70
	5	None	None	0	91	63
	6	None	None	0	89	47
	13.1	N	IPM	9.0	94	99
	14.1	N	IPM	9.0	94	101
	15.1	N	IPM	9.0	93	88
	16.1	None	IPM	3.0	92	73
	17.1	None	IPM	3.0	92	70
	18.1	None	IPM	3.0	90	52
	13.2	N	MTMA	9.0	95	138
	14.2	N	MTMA	9.0	94	119
	15.2	N	MTMA	9.0	94	115
	16.2	None	MTMA	3.0	92	69
	17.2	None	MTMA	3.0	91	61
	18.2	None	MTMA	3.0	90	52
	13.3	N	DBS	9.0	95	145
	14.3	N	DBS	9.0	94	110
	15.3	N	DBS	9.0	93	84
	16.3	None	DBS	3.0	93	67
	17.3	None	DBS	3.0	92	65
	18.3	None	DBS	3.0	92	60

Classification: General Business Use

Classification: General Business Use

The activity of this catalyst system when used with an external donor decreases mildly as the reaction temperature increases from 58 to 94 °C, however, when we introduce the ALAs even at the lowest ALA/Ti of 0.3 along with the external donor, we observe a decrease in the catalyst productivity of 67, 50, and 17 % in comparison to the standard system productivity at the same temperature for IPM, MTMA, and DBS, respectively. Further increasing ALA/Ti intensifies this retarding effect of activity at the highest reaction temperature, for instance at an ALA/Ti of 9.0, the drop in productivity at the highest temperature in comparison to the standard system is 93, 96, and 96 % for IMP, MTMA, and DBS, respectively. This significant drop in catalyst activity at the high reaction temperature is desirable in order to prevent the softening of the polymer which subsequently leads to the formation of agglomerates. However, this effect comes at the expense of a drop in catalyst productivity at the normal operating temperature of 58 °C; 19, 50, and 40 % for IPM, MTMA, and DBS. An optimum balance between the retardation effect of catalyst activity at higher temperatures and the least negative effect at the normal operating temperatures is achieved at an ALA/Ti of around 3.0. We have only tested three levels of ALA/Ti in a very wide range, the balance of activity at normal operating temperatures and the desired retardation effect can be adjusted for the operation of an industrial plant in favor of retardation when the process is facing some instabilities, and towards the least negative impact on activity at normal temperatures when the process is operating smoothly. Furthermore, the retarding effect is still observed when we use the ALA without an external donor at an ALA/Ti ratio of 3.0, we do see the desirable retarding effect of activity but to a lesser extent than when the external donor is used except for MTMA; the achieved drop in productivity is 40, 80, 50 % for IPM, MTMA, and DBS, respectively. Nonetheless, when the ALAs are used without the external donor, the activity at 58 °C increases by 23, and 98 % for IPM, and DBS, respectively, and drop by 14 % for MTMA.

The use of ALAs with this catalyst system does not lead to any significant effects on the bulk density of the powder, and the fines content. Similarly, no significant effects were observed on the molecular weight averages and distributions. However, when the ALA are used with the external donor, we observe a mild increase in the isotacticity which is