Cationic Polymerization of Hexamethylcyclotrisiloxane in Excess Water

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Communication Cationic Polymerization of Hexamethylcyclotrisiloxane in Excess Water

Quentin Barnes 1, Claire Longuet 2 and François Ganachaud 1,*

1 Ingénierie des Matériaux Polymères, CNRS UMR 5223, INSA-Lyon, Univ Lyon, F-69621 Villeurbanne, France; [email protected] 2 IMT—Mines Ales, Polymers Hybrids and Composites (PCH), 6 Avenue De Clavières, F-30319 Alès, France; [email protected] * Correspondence: [email protected]; Tel.: +33-683-021-802

Abstract: Ring-opening ionic polymerization of cyclosiloxanes in dispersed media has long been discovered, and is nowadays both fundamentally studied and practically used. In this short communication, we show some preliminary results on the cationic ring-opening polymerization of

hexamethylcyclotrisiloxane (D3), a crystalline strained cycle, in water. Depending on the catalyst or/and surfactants used, polymers of various molar masses are prepared in a straightforward way. Emphasis is given here on experiments conducted with tris(pentafluorophenyl)borane (BCF), where high-molar polymers were generated at room temperature. In surfactant-free conditions, µm-sized droplets are stabilized by silanol end-groups of thus generated amphiphilic polymers, the latter of which precipitate in the course of reaction through chain extension. Introducing various surfactants in the recipe allows generating smaller emulsions in size with close polymerization ability, but better final colloidal stability, at the expense of low small cycles’ content. A tentative mechanism is Citation: Barnes, Q.; Longuet, C.; Ganachaud, F. Cationic finally proposed. Polymerization of Hexamethylcyclotrisiloxane in Keywords: Piers-Rubinsztajn catalyst; surfactant-free polymerization; polydimethylsiloxane Excess Water. Molecules 2021, 26, 4402. https://doi.org/10.3390/ molecules26154402 1. Introduction Academic Editors: Sławomir Silicones are polymers of broad interest, both on industrial and academic sides. In Rubinsztajn, Marek Cypryk and industry, developments focus on the generation of more and more performing elastomers, Wlodzimierz Stanczyk widely used in numerous applications, e.g., for their thermal resistance or their innocuity [1]. In academia, a great deal of work has recently been conducted on (mostly rediscovered) Received: 24 June 2021 chemistries to generate silicone chains with new functionalities. In the most recent research, Accepted: 16 July 2021 we can cite, in particular, aza-Michael addition [2], thiol-ene click chemistry [3], and Published: 21 July 2021 organocatalyzed polymerization and polycondensation (see, e.g., [4]). A recurrent domain of development concerns the generation of silicone aqueous emul- Publisher’s Note: MDPI stays neutral sions via the ring-opening polymerization of cyclosiloxanes (a recent book published by the with regard to jurisdictional claims in published maps and institutional affil- Dow company summarizes recent comprehension of silicone dispersions [5]). Both cationic iations. and anionic catalysts produce silicone dispersions, but the mechanism by which long polymer chains are formed differs, and is still of debate nowadays [5,6]. Basically, taking the case of octamethylcyclotetrasiloxane (D4), polymerizations in emulsion (fast mixing in the presence of excess surfactant), in microsuspension (pre-generation of nanosized monomer droplets, typically by ultrasonication), or microemulsion (thermodynamically Copyright: © 2021 by the authors. stable nanodroplets) follow very different pathways. In all instances, ring opening proceeds Licensee MDPI, Basel, Switzerland. to propagate chains. Polymerization is stopped by transfer to water, but silanol chains ends This article is an open access article distributed under the terms and can be reactivated to propagate further. Polycondensation between silanol end-groups conditions of the Creative Commons also take place, to increase the molar masses. Side reactions that are generally observed Attribution (CC BY) license (https:// in bulk or solution are likely prominent in water: backbiting that gives rise to a variety creativecommons.org/licenses/by/ of cyclosiloxanes with various reactivities, and intermolecular redistributions between 4.0/). chains that enlarge the molar mass distribution. All the difficulty in these systems is to

Molecules 2021, 26, 4402. https://doi.org/10.3390/molecules26154402 https://www.mdpi.com/journal/molecules Molecules 2021, 26, 4402 2 of 9

first understand where the different reactions take place (directly in water, at the droplet interface or inside the droplets), and second, at which paces. Hexamethylcyclotrisiloxane is a monomer of choice when targeting silicone chains with perfectly controlled masses and functionality, as obtained by living anionic polymerization (for a very recent review, see an article just published in this Special Issue [7]). On the other hand, cationic polymerization of D3 has been described mostly in the mid-eighties by two groups led by P. Sigwalt and J. Chojnowsky, and not further exploited (a summary of this more than 10 year’s competition can be found in [8]). Using triflic acid as a superacid, the team of Sigwalt showed that water would retard the polymerization of D3, but would not inhibit it, albeit at a high catalyst content. To our knowledge, ring-opening cationic polymerization of D3 in water has hardly been studied. Early on, Weyenberg et al. showed, in a seminal paper, that D4 or D3 was easily converted into polymers in the presence of dodecylbenzenesulfonic acid [9]. They wrote that ‘polymerization of hexamethylcyclotrisiloxane proceeds at a much faster rate than the cyclotetrasiloxane and, in fact, it is not necessary to pre-emulsify this monomer. Contact of even large crystals of this monomer with DBSA and water at 25 ◦C gave a quantitative conversion to emulsion polymer within 24 hr’. Hemery et al. have later polymerized D3, solubilized in toluene, in emulsion by an anionic polymerization process, where they observed fast generation of polymers with an unlikely broad distribution [10]. Tris(pentafluorophenyl)boron (acronym BCF) was discovered in the early 1960s, and was almost forgotten for 25 years before being rediscovered as a catalyst activator in metallocene-catalyzed olefin polymerization. Its strong Lewis acidity, comparable to those of BF3.OEt2, combined with its air stability and water tolerance, has made it a (co-)catalyst of choice for numerous reactions that are summarized in reviews (e.g., [11]). Since the discovery by Piers et al. that BCF catalyzes the reduction reaction of a silyl ether into alkane in the presence of an hydrogenosilane, this reaction was later patented and published to produce linear silicone chains from alkoxy- and hydrogeno-functionalized silicone molecules [12]. The so-called Piers–Rubinsztajn reaction was studied in detail, particularly in the team of Professor Chojnowsky in a series of papers explaining the mechanism of catalysis. A precision reaction could thus be performed, starting from model molecules, allowing the generation of complex branched structures with exceptional monodispersity (for a review on this, please see [13]). For the record, this Lewis acid was also proved to promote a hydrosilylation reaction, albeit in stoichiometric amount, or, more strangely, oligomerization of electron-withdrawing monomers (typically vinyl methylsulfone or acry- lonitrile) onto SiH functions through a coordinated ate-type intermediate [14]. The team of Chojnowsky has shown that D3 can be open and polymerized by tetramethyldisiloxane (L2H) and other hydrogenosiloxanes in BCF toluene solution, but polymerization does not proceed in the absence of these molecules [15]. In this communication, we propose to show some preliminary results on the cationic polymerization of D3 in excess water. Thanks to our deep knowledge on such processes applied to cyclosiloxanes [6] and vinyl monomers [16], we have selected a variety of Bronsted and (water-tolerant) Lewis acids. We particularly show advanced results on the polymerization of D3 using BCF as a catalyst, and finally propose a brief discussion about a tentative polymerization mechanism.

2. Results and Discussion 2.1. First Screening

Table1 summarizes the different results of D 3 polymerization in excess water, using different catalysts. Basically, molar masses and polydispersity, as well as the ﬁnal contents of polymer, are given here, with selected SEC traces plotted in Figure1. All the experiments were conducted at room temperature, in 10 mL vials, using a magnetic agitation (see conditions in Table1 footnote). We did not speciﬁcally look at the colloidal state of the dispersions here, nor did we follow the kinetics of the reaction. Molecules 2021, 26, 4402 3 of 9

a Table 1. First round of experiments of D3 cationic polymerization in excess water .

D Content Cat.Content Reaction Time M Conv. D Yield Polym. Exp. 3 Catalyst n Ð 3 (wt.%) (wt.%) (h) (kg/mol) (%) (%) 12 - - 100 0 entry 1 30 3 TfOH 12 (After DEDMS 180 2.2 100 100 addition) b 15 1 6 75 1.5 95 85 entry 2 DBSA 30 1 6 63 1.6 90 80 c e entry 3 25 YbDBS3 16 12 4.5 1.9 100 N.D. d e entry 4 25 InDBS3 15 12 4.5 1.9 95 N.D. a Typical procedure: all ingredients are mixed at once in 3 g water into 10 mL vial and a magnetic stirrer. After drying at 90 ◦C, Moleculessamples 2021 were, 26, x recovered FOR PEER in REVIEW toluene and analyzed by SEC; acronyms: TfOH: triﬂic acid; DBSA: dodecylbenzenesulfonic acid; DEDMS: 4 of 9 b c d diethoxydimethylsilane; addition of 5 wt.% of DEDMS; prepared by mixing 3 mol eq of NaDBS with 1 eq of YbCl3.6H2O; prepared by e mixing 3 mol eq of NaDBS with 1 eq of InCl3; LASCs coprecipitate with polymer, so it is not possible to calculate a yield in polymer.

YbDBS3

DBSA + 30wt.% D3

TfOH + DEDMS

15 20 25 30 35 40 elution time (min)

FigureFigure 1.1.Selected Selected SECSEC chromatogramschromatograms ofof samplessamples polymerizedpolymerized with with various various acids acids in in the the ﬁrst first set set of of polymeriations.polymeriations. NegativeNegative peakpeak atat 3636 min min is is due due to to the the ﬂow flow marker. marker. For For the the record, record, D D3 and3 and D D5 5were were separatelyseparately analyzedanalyzed by by SEC SEC and and came came out out at at 34.5 34.5 and and 34 34 min, min, respectively. respectively.

2.2. The Case first of experiments BCF conducted with triflic acid in large quantities showed that solid D is rapidly consumed to give a totally transparent solution. We could not track the 3 The origin of this second set of experiments comes from a study of the condensation presence of polymer by precipitation of a sample aliquot in excess methanol, nor any other reactions of alkoxy- and silanol-functionalized telechelic polymers in water [17]. When cycles that would have generated oil stains on the vial’s walls. After the addition of a slight starting from the model molecules, tetramethyldisiloxane and dimethyldimethoxysilane, quantity of diethoxydimethylsilane (DEDMS, 0.2 eq. of initial D3) and agitation during 12 h, we observed, the rapid generation of cyclosiloxanes of various sizes, from D3 to D7, towe observed a precipitate at the bottom of the tube assigned to a polymer of high molar gether with some polymer. The former cycle gradually disappeared with time, whereas mass (Mn of 180,000 g/mol by SEC). DEDMS introduced in triflic acid aqueous solution the larger ones would accumulate in the reactor. This intriguing observation prompted us in absence of D3 did not produce a polymer, in agreement with a previous study [17]. We thento further concluded study that the trifliccationic acid ROP opens of D the3 in cycle water to catalyzed generate water-solubleby BCF, in the oligomersabsence of that any convertother siloxane- into polymer or silane-based by acid-catalyzed molecules. condensation Note that betweenwe preliminary silanol andchecked ethoxysilane that BCF groups.does not Note promote that thisD3 polymerization reaction is very in different toluene from overnight the polycondensation (not shown). reaction of bis- silanol-terminatedA typical experiment PDMS long, consisted hydrophobic of introducing oligomers D3 powder that occurs straightaway exclusively in ata test-tube droplet interfacescontaining [18 an]. aqueous solution of the catalyst, at room temperature and under magnetic agitationChanging (see formulation a molecular in superacid Table 2, entry to an 5). acidic surfactant, DBSA, after only 6 h of reaction, we could detect the presence of polymers in the test tubes. SEC curves give molar Table 2. Second round of experiments of D3 cationic polymerization in excess water a. masses of around 70,000 g/mol, with a larger content of small cycles at a larger D3 content

Cont.(about State 10 wt.% of at theReaction end Time of the polymerization).Mn SinceYield at thePolym. time we wereCycles looking but D3 for Exp. Surf. b Đ (wt.%) Dispersion c (h) (kg/mol) (%) (wt.% Content) 12 26 2.0 56 D5 and above (41) entry 5 - - H 72 144 2.2 89 D4, D5, D6 (7) entry 6 Lauric acid 0.1 µE 12 51 2.4 88 D4, D5, D6 (11) entry 7 SDS 0.1 µE 12 14.4 2.2 73 D4, D5 (27) 6 3 1.4 47 D6 (3) entry 8 Brij 98 0.5 E 12 25 2.3 65 D4, D5 and above (35) entry 9 DTAB 0.1 H 12 13 1.5 39 D4 (61) a Typical procedure: all ingredients are mixed at once into a 10 mL vial equipped with a magnetic stirrer. Water = 5 g; D3 = 0.5 g (10 wt.%); BCF = 24 mg (0.5 wt.%). Samples are precipitated in methanol, dried, recovered in toluene and analyzed by SEC. b Acronyms: SDS: sodium dodecyl sulfate; DTAB: dodecyl trimethyl ammonium bromide; c H: heterogeneous; µE: microemulsion; E: emulsion.

Even if D3 first resides as solid chunks at the top of the water phase, it is then gently incorporated with time. A white emulsion (Figure 2a), of about 1.5 µm in size (Figure 2b),

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cycle-free emulsions, we did not further explore this path; we are currently pursuing some experiments to check how fast and efﬁcient this polymerization is. We also tested some rare earth Lewis acids (ytterbium and indium chloride salts) combined with sodium dodecylbenzene sulfonate (NaDBS), to generate so-called Lewis acid surfactant complexes (LASCs), as reported before [16]. Even in large excesses, as tested here, these catalysts produced exclusively oligomers of molar masses around 4500 g/mol. Note that methanol also precipitated the LASC catalyst, so that it cannot be easily separated from the oligomers. According to the price of the catalysts used here, and the short oligomers produced, this alley was not pursued.

2.2. The Case of BCF The origin of this second set of experiments comes from a study of the condensation reactions of alkoxy- and silanol-functionalized telechelic polymers in water [17]. When starting from the model molecules, tetramethyldisiloxane and dimethyldimethoxysilane, we observed, the rapid generation of cyclosiloxanes of various sizes, from D3 to D7, together with some polymer. The former cycle gradually disappeared with time, whereas the larger ones would accumulate in the reactor. This intriguing observation prompted us to further study the cationic ROP of D3 in water catalyzed by BCF, in the absence of any other siloxane- or silane-based molecules. Note that we preliminary checked that BCF does not promote D3 polymerization in toluene overnight (not shown). A typical experiment consisted of introducing D3 powder straightaway in a test-tube containing an aqueous solution of the catalyst, at room temperature and under magnetic agitation (see formulation in Table2, entry 5).

a Table 2. Second round of experiments of D3 cationic polymerization in excess water .

Reaction Cont. State of M Yield Polym. Cycles but D Exp. Surf. b Time n Ð 3 (wt.%) Dispersion c (kg/mol) (%) (wt.% Content) (h) 12 26 2.0 56 D and above (41) entry 5 -- H 5 72 144 2.2 89 D4,D5,D6 (7) entry 6 Lauric acid 0.1 µE 12 51 2.4 88 D4,D5,D6 (11) entry 7 SDS 0.1 µE 12 14.4 2.2 73 D4,D5 (27) 6 3 1.4 47 D (3) entry 8 Brij 98 0.5 E 6 12 25 2.3 65 D4,D5 and above (35) entry 9 DTAB 0.1 H 12 13 1.5 39 D4 (61) a Typical procedure: all ingredients are mixed at once into a 10 mL vial equipped with a magnetic stirrer. Water = 5 g; D3 = 0.5 g (10 wt.%); BCF = 24 mg (0.5 wt.%). Samples are precipitated in methanol, dried, recovered in toluene and analyzed by SEC. b Acronyms: SDS: sodium dodecyl sulfate; DTAB: dodecyl trimethyl ammonium bromide; c H: heterogeneous; µE: microemulsion; E: emulsion.

Even if D3 ﬁrst resides as solid chunks at the top of the water phase, it is then gently incorporated with time. A white emulsion (Figure2a), of about 1.5 µm in size (Figure2b), is generated, and remains almost so until the end of the reaction (we can track a slight enlargement in the size distribution with time, see Figure2b). Zeta potential measurements give surface values of about –65 mV (Figure2c). We suspect that the silanol groups of the oligomers/polymers protruding at the surface of the droplets stabilize them, as proposed earlier by Vincent et al. [19] and ourselves [17]. Only when the content of the silanol groups becomes too small that the polymer precipitates and deposits on the ﬂask wall (Figure2a). Molecules 2021, 26, x FOR PEER REVIEW 5 of 9

is generated, and remains almost so until the end of the reaction (we can track a slight Molecules 2021, 26, x FOR PEER REVIEW 5 of 9 enlargement in the size distribution with time, see Figure 2b). Zeta potential measurements give surface values of about –65 mV (Figure 2c). We suspect that the silanol groups of the oligomers/polymersis generated, protruding and remains at almost the su sorface until of th thee end droplets of the reaction stabilize (we them, can track as pro-a slight posed earlier by Vincentenlargement et al. in [19] the sizeand distributionourselves [17].with tiOnlyme, see when Figure the 2b). content Zeta potential of the silanol measure- groups becomes mentstoo small give surfacethat the values polymer of about pr –65ecipitates mV (Figure and 2c). depositsWe suspect on that the the flask silanol wall groups (Figure 2a). of the oligomers/polymers protruding at the surface of the droplets stabilize them, as proposed earlier by Vincent et al. [19] and ourselves [17]. Only when the content of the silanol Molecules 2021, 26, 4402 5 of 9 groups becomes too small that the polymer precipitates and deposits on the flask wall a b c (Figure 2a). 15

c a b 10

5 intensity (mV) intensity

0 -200 -150 -100 -50 0 Zeta potential

Figure 2. Colloidal state of D3 suspension polymerization: (a) metastable emulsion, where polymer stains can finally be seen on the Figure 2. Colloidal state of D suspension polymerization: (a) metastable emulsion, where polymer stains can ﬁnally be top of the wall; (b) averageFigure particle 2. Colloidal size at state different of D3 suspension times3 of polymerization: reaction to highlight (a) metastable the emulsionemulsion, where stability polymer (every stains 15 canmin finally in the be follow-seen on the top ofseen the wall; on the (b top) average of the wall;particle (b) size average at different particle times size atof different reaction timesto highlight of reaction the emulsion to highlight stability the emulsion (every 15 stability min in (everythe following order: red, black, green, blue); (c) zeta potential of emulsion after 6 h on three different samples (average value of −65 ± 15 mV). ing order:15 min red, in black, the following green, blue); order: (c) red, zeta black, potential green, of emulsion blue); (c) zetaafter potential6 h on three of emulsiondifferent samples after 6 h (average on three value different of − samples65 ± 15 mV). (average value of −65 ± 15 mV). Typical SEC tracesTypical are given SEC traces in Figure are given 3a, in along Figure with 3a, along interpretations with interpretations of it; theof it; average the average molar masses aremolar reportedTypical masses in SEC are Table tracesreported 2 are at givenintwo Table indifferent Figure2 at two3a, reactiondifferent along with reactiontimes. interpretations times.It can It be ofcan it;seen be the seen averagehere here thatmolar polymerization masses are reported of D3 occurs in Table quite2 at smoothly, two different generating reaction polymers times. Itof can very be high seen molar here that polymerization of D3 occurs quite smoothly, generating polymers of very high molar massthat polymerization (typically 150,000 of D g/mol).3 occurs This quite is smoothly,typical of generatingcationic polymerization polymers of very in emulsion high molar of mass (typically 150,000 g/mol). This is typical of cationic polymerization in emulsion of cyclosiloxanesmass (typically [6]. 150,000 We were g/mol). not capable This is typical of characterizing of cationic polymerizationthe chain-end of in the emulsion polymers of cyclosiloxanes [6].ofcyclosiloxanes Wesuch were high molarnot [6]. capable Wemasses; were however,of not ch capablearacterizing the of fact characterizing that the molar chain-end mass the chain-end increases of the ofwith polymers the time polymers let us of such high molarthinkof suchmasses; that high the however, molar polymers masses; chainsthe however, fact do notthat theclose molar fact ends that mass here. molar increasesSimilar mass results increases with were withtime observed time let us let for us think that the polymersthethink system that chains thestarting polymers do from not chainslinear close precursors doends not closehere. [17]. ends Similar here. results Similar resultswere wereobserved observed for for the system starting from linear precursors [17]. the system startinga from linear precursors [17]. b D4 cycles D3 D4 a b D6 cycles D5 D4D3 D6

D6 D5 D4 21 22 23 24 D6 retention time (min) polymer

21 22 23 24 retention time (min) polymer D5 D9

D3 D7 D8 10 15 20 25 D5 D9 retention time (min) Time (min) Figure 3. (a) SEC chromatograms of samples taken after ½ day (blue curve), 1 day (green olive curve) D3 D7 and 3 days (red curve). Zoom on the zone of small cycles is given on the inset. (b) DGC-MS8 analyses 10 15of D3/BCF system 20after 12 h of reaction. 25 Abundances are relative, since peaks appear larger as molar massretention of the time cycle (min) increases. Time (min)

FigureFigure 3. (a) 3. SEC (a) chromatograms SEC chromatograms of samples of taken samples after taken1/2 day after (blue ½ curve), day (blue 1 day curve), (green olive 1 day curve) (green and olive 3 days curve) (red We can also track, on the SEC trace, a rapid generation of small cycles (typically D5 curve).and Zoom 3 days on the (red zone curve). ofand small above)Zoom cycles which ison given the contents onzone the of inset. growsmall (b) slowly GC-MScycles analyseswithis given time. of on D Intermediate3 /BCFthe inset. system ( bmacrocycles after) GC-MS 12 h of analyses reaction. are visible Abundancesof D3/BCF are relative,system sinceafteron peaksthe 12 SECh appearof tracereaction. larger after asAbundances 3 molardays massof reaction, of ar thee relative, cycle showing increases. since that peaksbackbiting appear and larger intermolecular as molar re- mass of the cycle increases.distribution are retarded, but occur in this polymerization system (Figure 3a). To gain We can also track, on the SEC trace, a rapid generation of small cycles (typically D5 and betterabove) insight which into contents the course grow slowlyof polymerization, with time. Intermediate we have injected macrocycles the intermediate are visible sample on the We can also SECtrack, trace on after the 3 SEC days oftrace, reaction, a rapid showing generation that backbiting of small and intermolecular cycles (typically redistribution D5 and above) whichare contents retarded, grow but occur slowly in this with polymerization time. Intermediate system (Figure macrocycles3a). To gain are better visible insight on the SEC trace afterinto the 3 days course of of reaction, polymerization, showing we have that injected backbiting the intermediate and intermolecular sample in a GC/MS re- apparatus (Figure3b). D 6 and D9 appear together in larger proportions than other cycles, distribution are retarded, but occur in this polymerization systemF (Figure 3a). To gain except for D4. We observed a similar accumulation of D3x cycles building (x = 2,3,4) in F better insight intothe the anionic course polymerization of polymerization, of D3 in we miniemulsion, have injected before the a intermediate backbiting reaction sample occurs F F extensively and generates intermediate cycles (D4 ,D5 . . . ), but no macrocycles [20].

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in a GC/MS apparatus (Figure 3b). D6 and D9 appear together in larger proportions than other cycles, except for D4. We observed a similar accumulation of D3xF cycles building (x = 2,3,4) in the anionic polymerization of D3F in miniemulsion, before a backbiting reaction F F Moleculesoccurs2021, 26, 4402extensively and generates intermediate cycles (D4 , D5 …), but no macrocycles [20]. 6 of 9

2.3. Introducing Surfactants in the Recipe As we noted before,2.3. Introducing the simplest Surfactants system in the Recipedescribed above is heterogeneous, i.e., micron-sized droplets areAs slowly we noted converted before, the into simplest a polymer system described film, precipitating above is heterogeneous, on the walls i.e., micron- of the test tube as sizeda function droplets of are time. slowly We converted tried intoto ause polymer different ﬁlm, precipitating surfactants on to the ensure walls of thea test stabilization of thetube dispersion as a function throughout of time. We triedthe to process, use different while surfactants still tocarrying ensure a stabilizationout the of the dispersion throughout the process, while still carrying out the polymerization. Table2 polymerization. Tablesummarizes 2 summarizes the different the trialsdifferent we made, trials and we Figure made,4 shows and the Figure corresponding 4 shows SEC the traces. corresponding SEC traces.

a b

DTAB D4

Brij 98 D5

SDS D6 D9 L5 L6 Lauric acid

No surf 12 14 16 18 20 22 24 elution time (min) Time (min) FigureFigure 4. ( a4.) SEC(a) SEC traces traces of different of different trials done trials in presence done in of presen surfactant,ce of compared surfactant, to pristine compared one. For to polymerizationpristine one. For polymerization conditions, see Table 2; (b) GC-MS analyses of D3/BCF/Brij system after 12 h of conditions, see Table2;( b) GC-MS analyses of D3/BCF/Brij system after 12 h of reaction. reaction. Using lauric acid, a translucent dispersion, typical of a microemulsion state, was Using lauric acid,obtained a translucent (average droplet dispersion, size of 30typical nm measured of a microemulsion by DLS, not shown). state, Polymerization was ob- appeared to be quite fast, but led to the formation of larger contents of D4 and D5 than tained (average dropletwithout size a surfactantof 30 nm (more meas thanured 10 by wt.%). DLS, Lauric not shown). acid also Polymerization allowed rather large ap- molar peared to be quite fast,masses but to led be to produced, the formation around 55,000of larger g/mol, contents while keepingof D4 and the D colloidal5 than with- stability of out a surfactant (morethe dispersion.than 10 wt.%). Lauric acid also allowed rather large molar masses to be produced, aroundWith 55,000 SDS, g/mol, a similar while microemulsion keeping was the formed, colloidal and stability polymerization of the was disper- likely faster sion. than without a surfactant. A polymer of a molar mass of around 15,000 g/mol was produced, together with a large load of small cycles (almost 30 wt.%). The same polymolec- With SDS, a similarularity microemulsion as for the other rounds was wasform observed,ed, and typically polymerization around two. was DTAB likely did notfaster produce than without a surfactant.stable dispersions, A polymer and of reactions a molar led mass to a rapid of around generation 15,000 of a largeg/mol load was of Dpro-4 (above duced, together with60%), a large together load with of small polymers cycles of a low(almost molar 30 mass wt.%). (13 kg/mol). The same The polymolecu- fact that only D4 is larity as for the othergenerated rounds here was was observed, not expected, typically and remains around unexplained. two. DTAB did not produce Not shown here is a trial with PVA, where an emulsion was formed, but polymer- stable dispersions, izationand reactions did not proceed led to because a rapid of generation complexation of of a BCF large with load the alcohol of D4 groups(above of the 60%), together withdispersant. polymers Brij of 98a alsolow complexes molar mass the catalyst(13 kg/mol). through The the oxygenfact that atoms only from D4 ethylene is generated here wasglycol, not expected, but does notand inhibit remains polymerization. unexplained. Molar masses increase with time, together Not shown herewith is thea trial content with of PVA, small cycleswhere from an Demulsion4 to macro was ones. formed, Looking but at the polymeri- GC/MS of the zation did not proceedsample because taken after of complexation 12 h (Figure4b), weof BCF can notice with the the presence alcohol of groups tentatively of assignedthe linear disilanol oligomers, in addition to the same cycles observed before (D6 and D9). This dispersant. Brij 98 alsoconfirms complexes the formation the ofcataly molecularst through intermediates the oxygen before theyatoms cycle from back. ethylene glycol, but does not inhibit polymerization. Molar masses increase with time, together 2.4. Proposed Polymerization Scheme with the content of small cycles from D4 to macro ones. Looking at the GC/MS of the sample taken after 12 h (FigureD3 polymerizes 4b), we can via notice a cationically the presence catalyzed of process tentatively in excess assigned water andlinear at room temperature. The initial screening showed that both Bronsted acids and Lewis acids disilanol oligomers,catalyze in addition the reaction, to the albeitsame atcycles different observed paces and before for final (D6 endand results.D9). This The con- fact that firms the formationtriflic of molecular acid opens intermediates the cycle into small before water-soluble they cycle oligomers, back. but does not convert them

Molecules 2021, 26, x FOR PEER REVIEW 7 of 9

2.4. Proposed Polymerization Scheme

D3 polymerizes via a cationically catalyzed process in excess water and at room tem- Molecules 2021, 26, 4402 7 of 9 perature. The initial screening showed that both Bronsted acids and Lewis acids catalyze the reaction, albeit at different paces and for final end results. The fact that triflic acid opens the cycle into small water-soluble oligomers, but does not convert them into poly- intomers, polymers, seems to seemsindicate to that indicate a condensation that a condensation reaction of reaction silanols of is silanolsnot likely is in not these likely con- in theseditions. conditions. This is certainly This is due certainly to the due absence to the of absence an interface, of an interface,where this where reaction this generally reaction generallytakes place takes [6,18]. place This [6 also,18]. s Thiseems also to confirm seems to that, conﬁrm in contra that,st in to contrast the previously to the previously proposed proposedemulsion polymerization emulsion polymerization of cyclosiloxane of cyclosiloxane [5], small hydrophilic [5], small hydrophilic oligomers oligomersdo not chain- do notextend chain-extend in water. inIt would water. Itbe would worth be in worth the future in the to future mix together to mix together D3, DEDMS, D3, DEDMS, and a non- and aionic non-ionic surfactant surfactant from the from first the place, ﬁrst with place, a view with aof view generating of generating a stable alatex stable while latex gaining while gaininghigh molar high mass molar silicone mass siliconepolymer. polymer. DBSA and BCFBCF catalysiscatalysis holdsholds thethe followingfollowing comparablecomparable features:features: fast polymerization,tion, largelarge molarmolar massmass polymers,polymers, andand aa fairfair loadload ofof smallsmall cycles,cycles, asas expectedexpected fromfrom suchsuch cyclosiloxane cationic process. This This most most likely likely shows shows that that BCF BCF acts acts here here principally principally as as a aBronsted Bronsted catalyst. catalyst. Ring Ring opening, opening, one one to totwo two condensation condensation steps, steps, and and back-cyclization, back-cyclization, to- togethergether with with true true ring-opening ring-opening polymerization, polymerization, take take place place here. here. When adding lauric acidacid toto thethe BCFBCF system,system, anan acidicacidic surfactantsurfactant thatthat isis tootoo weakweak toto participateparticipate toto thethe reaction,reaction, thethe resultsresults matchmatch perfectly perfectly with with the the DBSA-catalyzed DBSA-catalyzed ones, ones, as follows:as follows: molar molar masses masses of typically of typi- 55cally to 7055 kg/mol,to 70 kg/mol, stable stable dispersions dispersions and cycle and cycle contents contents of 10 wt.%.of 10 wt.%. It would It would be interesting be inter- toesting introduce to introduce both DBSA both andDBSA BCF and in theBCF recipe, in the torecipe, see if to synergy see if synergy occurs; we occurs; plan towe conduct plan to suchconduct process such soon. process soon. IntroducingIntroducing otherother surfactantssurfactants thatthat could interactinteract together with BCF led to more com- plexplex results.results. In all thethe cases,cases, largerlarger contentscontents ofof cyclescycles werewere observed,observed, andand polymerizationpolymerization waswas generallygenerally fasterfaster thanthan inin thethe absenceabsence ofof surfactants.surfactants. Molar masses were rather low, around 1515 kg/mol,kg/mol, which which may may be due to the larger content of waterwater insideinside thethe monomermonomer droplets, due to the presence of excess D (this cycle is more polar than silicone chains). droplets, due to the presence of excess D44 (this cycle is more polar than silicone chains). TheThe typetype ofof dispersionsdispersions dependdepend onon thethe contentcontent andand naturenature ofof thethe surfactant,surfactant, butbut thisthis waswas not studiedstudied inin detaildetail here. here. Figure Figure5 summarizes5 summarizes the the reaction reaction taking taking place place in this in this process process for the exemplary case of BCF. for the exemplary case of BCF.

Figure 5. Proposed generic mechanistic scheme for the BCF-catalyzed D3 cationic polymerization in Figure 5. Proposed generic mechanistic scheme for the BCF-catalyzed D cationic polymerization in excess water. Here, the catalyst was omitted for sake of clarity. 3 excess water. Here, the catalyst was omitted for sake of clarity.

3. Materials and Methods

Hexamethylcyclotrisiloxane (D3, 95%) was either kindly given by Bluestar Silicones (1st set of experiments) or purchased from ABCR (2nd set of experiments). Triﬂic acid

(reagent grade, 98%), 4-DBSA (mixture of isomers, >95%), DTAB (>98%), SDS (98%) and lauric acid (≥98%) were all purchased from Sigma-Aldrich (Saint Louis, MO, USA). Brij Molecules 2021, 26, 4402 8 of 9

98 (hydroxyl titration: 50 to 65 mg KOH/g) came from ACROS organics (The Hague, Netherlands). Tris(pentafluorophenyl)borane (B(C6F5)3, purity 97%) was obtained from Lancaster (Watd Hill, MA, USA). Here, two series of experiments were performed at different times and locations. In the first set of experiments, size exclusion chromatography, SEC, was carried out using a Malvern Viscotek (Malvern, UK) GPC Max apparatus equipped with three Shodex columns (KF-804, -805, and -806). Detection systems were a refractive index and differential viscom- etry detectors. Toluene (HPLC grade, provided by Sigma-Aldrich) was eluted at 1 mL/min with diisopropylethylamine as flow marker [21]. In the second set of experiments, a Spectra Physics (Andover, MA, USA) apparatus with two PL gel columns (5 µm particles size, 300 mm length, with the following two pores sizes: one with 50 Å and one with 100 Å) and a Styragel HR2 column (7.8 mm internal diameter × 300 mm length) were used. An SP8430 differential refractometer achieved the detection. The toluene was eluted at a flow rate of 0.8 mL/min using diethylether as a flow marker. In both systems, the temperature for the SEC column set and the detector chamber was 35 ◦C to ensure stable baselines, high chromatographic efficiency, and consistent results. The standards used to calibrate the SEC were polystyrene standards. Gas chromatography coupled with a mass spectrometer (GC/MS) was done on a 6890 N apparatus from Agilent Technologies (Santa Clara, CA, USA), equipped with an electrospray mass detector Agilent 5973 N and an apolar capillary column HP5-MS 30 m × 0.25 mm (stationary phase made of a film of diphenyldimethylpolysiloxane 5%, 0.25 µm). Conditions used were as follows: initial temperature 45 ◦C during 2 min, temperature ramp of 2 ◦C/min up to 50 ◦C then 10 ◦C/min up to final temperature, 250 ◦C, set during 10 min. Peak integration were corrected with factors inherent of each silicone species, according to the procedure published elsewhere [22]. Particle sizes were determined by dynamic diffraction of a laser beam on a Nanotrac NPA 250 device (Microtrac Inc., Montgomeryville, PA, USA), typically in a size range between 8 nm and 6.54 µm. The light dispersed by the particles entails a Doppler effect, due to Brownian motion. The Microtrac® Windows Software amplified, filtered, and mathematically treated this signal is to produce a size distribution.

4. Conclusions

To summarize, D3 is the cyclosiloxane of choice to generate silicone dispersions of very high molar mass polymers, a priori not reachable neither by conventional polycondensation in microsuspension nor emulsiﬁcation. The fact that this monomer performs polymerization in almost all the catalytic systems screened here opens large avenues in the search for the ideal polymerization process that would hopefully not produce small cycles, such as D4 and D5, now targeted in the ‘Registration, Evaluation and Authorisation of Chemicals’ (REACh) European regulation.

Author Contributions: Conceptualization, F.G.; methodology, Q.B., C.L. and F.G.; formal analysis, C.L. and F.G.; writing—original draft preparation, Q.B. and C.L.; writing—review and editing, F.G. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. Conﬂicts of Interest: The authors declare no conﬂict of interest. Sample Availability: Samples are not available. Molecules 2021, 26, 4402 9 of 9

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