Chemical Science This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. View Article Online EDGE ARTICLE View Journal | View Issue Controlled living anionic polymerization of cyanoacrylates by frustrated Lewis pair based Open Access Article. Published on 01 February 2019. Downloaded on 3/22/2019 11:33:36 AM. Cite this: Chem. Sci., 2019, 10, 3295 All publication charges for this article initiators† have been paid for by the Royal Society of Chemistry Rubén Sáez,a Ciaran McArdle,‡b Fouad Salhi,c Jordi Marquet *a and Rosa M. Sebastián §*a Received 23rd October 2018 Accepted 30th January 2019 A hydrogenated frustrated Lewis pair ([TMPH+][HB(C6F5)3]) promotes controlled living ionic polymerization of cyanoacrylates. Controlled growth of various homopolymeric CAs through sequential DOI: 10.1039/c8sc04729d monomer addition has been achieved, in addition to CA block copolymers with controlled block rsc.li/chemical-science sequences for the first time in the long history of these materials. The emergence of synthetic polymeric materials probably been described since Szwarc reported the anionic polymeriza- constitutes one of the major breakthroughs in materials science tion of styrene, but nowadays living polymerization has also in the 20th century. Any new technology, ranging from been achieved with cationic, radical, metal catalyzed, and composites used in supersonic aircra and high-speed railway group-transfer initiators.6 to components for surgical implants, requires the development Cyanoacrylates, produced in high volume for adhesive of diverse materials with very specic properties, and synthetic applications, are highly reactive monomers that mainly poly- polymers play a critical role.1 The specic structural and merize by anionic initiation through a conjugate addition of molecular parameters (molecular weight and molecular weight anionic nucleophiles (hydroxyl or bromide groups for example) distribution), the presence of functional groups and their or bases such as amines7 and phosphines.8 In these last cases spatial location, or the presence of unreacted monomers, etc. zwitterionic polymerization ensues with a carbanionic active dene the suitability of a polymer for a particular application.2 propagating species.9 The use of tetrabutylammonium salts and So-called ‘living’ polymerization techniques have allowed the uorenyllithium as initiators has also been investigated. In realization of well-dened structures such as block, star, some cases nearly ideal living polymerizations have been macrocyclic, and telechelic copolymers, and represent some of claimed; however, higher than expected molecular weight the most important examples of molecular design and control polymers (as calculated by the initial monomer/initiator ratio), in polymer science.3–5 The initiation step in an ideal living together with broad molecular weight distributions are anionic polymerization is fast as compared with the propaga- observed in most of the studies.10,11 tion reaction, and no transfer or termination reactions occur. The excellent adhesive properties of polycyanoacrylates Active chain ends remain “living” and are only quenched upon (PCA) have enabled their widespread use for many household addition of a terminating agent. The molecular weight of the applications (Super Glue®), and, most recently for applications resulting polymer is given by the monomer/initiator ratio, and in medicine,12 surgery,13 ophthalmology,14 dentistry,15 tissue narrow molecular weight distributions are obtained (Mw/Mn # reconstruction,16 and for drug delivery,17 due to their biocom- 1.1).4 Different mechanistic types of living polymerizations have patibility and biodegradability. Various other applications for CAs include ngerprint detection for forensic applications, a coating for lithographic printing plates, waveguides for optical Chemistry Department, Centro de Innovación en Quı́mica Avanzada (ORFEO-CINQA), Universitat Autònoma de Barcelona, 08193 Cerdanyola del Vallès, Barcelona, Spain. sensors, photoresists and holographic recording media, which E-mail: rosamaria.sebastian@uab.es exploit their rapid polymerizability or the optical properties of b Henkel Ireland Operations and Research Ltd., Tallaght Business Park, Whitestown their polymers.18 However their use as designer materials is Road, Tallaght, Dublin 24, Ireland scarce partly due to the difficulty of controlling their polymeri- c Henkel Corporation, Edici Eureka, Universitat Autònoma de Barcelona, 08193 zation and copolymerization processes. Nevertheless some Cerdanyola del Vallès, Barcelona, Spain anionic copolymerizations of 2-cyanoacrylate monomers have † Electronic supplementary information (ESI) available: Devices, methods and been described,19–21 and a few of these, containing also poly(- procedures, analytical supporting material: NMR spectra, GPC traces and MALDI-TOF chromatograms. See DOI: 10.1039/c8sc04729d ethylene glycol) blocks, show certain control of the nal poly- ‡ Actual address: CTO-Anitica Technologies s.l., Edici Eureka, Universitat mer architecture achieved by a nearly living oxyanion-initiated Autònoma de Barcelona, 08193 Cerdanyola del Vallès, Barcelona, Spain. polymerization (Mw/Mn > 1.2).22,23 Random cyanoacrylate § All authors contributed equally. This journal is © The Royal Society of Chemistry 2019 Chem. Sci., 2019, 10, 3295–3299 | 3295 View Article Online Chemical Science Edge Article copolymerization with vinyl monomers has been reported by 1, entries 3–5). The higher the amount of initiator used (80 : 10), radical,24,25 and metal catalysed initiators.26 Recent studies the faster was the generation of the lower molecular weight showed that the use of RAFT agents did not promote ideal living daughter polymers. This rate was reduced using freshly distilled block polymerization of CA.27 monomers. Only through the use of a non-ionic initiator, PPh3 This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. To control the polymerization of CA a reduction of the (zwitterionic polymerization), the formation of daughter poly- exceptionally fast propagation rate is required. To the best of mers was avoided, but without control of the process (Table 1, our knowledge, the concept of “group transfer” in anionic entry 6). polymerization has never been applied before for CA. Frustrated In order to obtain a controlled living polymerization of CA we Lewis pairs (FLPs), showing unusual reactivity,28 have been sought an initiator with good nucleophilic properties (Lewis recently used as promoters for the polymerization of acrylic base) but, also with the ability to stabilize the propagating anion Open Access Article. Published on 01 February 2019. Downloaded on 3/22/2019 11:33:36 AM. monomers.29 In these cases, sterically encumbered Lewis acid (Lewis acid) thereby reducing the propagation rate. The ideal and Lewis base moieties work cooperatively to activate the initiator must further reduce the formation of daughter poly- monomer substrate and participate in chain initiation (1,4- mers, chain transfer, and termination steps while maintaining conjugate additions are observed), as well as chain propagation the living character of the polymeric chain end using a group and termination/transfer events. Hydrogenated FLPs obtained transfer polymerization type approach. In this sense, we opted from hydrogen splitting reactions can reduce selectively the to study three species (NBu4Br, NBu4OH and NaSCN) that alone double bond in the a-position of a carbonyl group.30 With these polymerize CA in an uncontrolled manner (Table 2, entries 1, 3 interesting characteristics in mind we were curious to learn if and 5), now in the presence of B(C6F5)3. However, these FLPs or their hydrogenated derivatives could initiate, in combinations formed classical Lewis pairs (CLPs) and the a controlled manner, and participate somehow in the propa- reaction did not proceed (Table 2, entries 2, 4 and 6). Clearly gation steps of CA polymerization. In this work several Lewis passivation of the nucleophile by the Lewis acid should be pairs and FLPs have been studied and applied as promoters for avoided for control polymerization to succeed. This led us to the polymerization of various CA monomers. A hydrogenated hypothesize that FLPs could be the ideal initiators for derivative of a FLP has allowed us to achieve for the rst time controlled CA polymerization. The combination of boranes or a controlled living ionic polymerization of CA that also enables alanes with sterically hindered amines, phosphines or carbenes the construction of block copolymers with designed structures. has been recently applied to the polymerization of regular To gain familiarity with CA chemistry, the polymerization of acrylic monomers.29 We decided to use B(C6F5)3 (alane deriva- n-butyl cyanoacrylate (BCA) initiated by BuLi (80 : 1, BCA : BuLi) tive is shock and air/moisture sensitive) in combination with was rstly studied in THF at 25  C to reproduce the experiments PPh3, pyridine and 2,2,6,6-tetramethylpiperidine (TMP) as FLP of Pepper.31 The choice of CA type and solvent is an important initiators for the highly electron decient cyanoacrylic mono- factor for subsequent unambiguous GPC characterization. A mers. In two experimental combinations with these initiators, high molecular weight polymer was formed (105–106 Da) within polymerization occurs, but without control (Table 2, entries 7 very short reaction times but the appearance of daughter low and 8). Nevertheless, the mixture of B(C6F5)3/TMP, yielding polymers (10 000–15 000 Da) was observed through time as polymers of 18 900 Da (Đ ¼ 1.6), that although of higher previously reported, due to degradative retro-Michael reactions molecular weight than expected (12 250 Da based on the ratio (Table 1, entries 1 and 2).32–34 Other simple initiators such as sec- [monomer : initiator]), gave an indication that some control BuLi, Bu2CuLi and LiI were also tested in order to see if any was taking place (Table 2, entry 10). The sterically hindered effect on the rate of the initiation step or the limitation of amine TMP, used alone, was able to initiate CA polymerization daughter polymer formation could be observed, however giving rise to high molecular weight polydisperse polymers aer similar results were obtained according to GPC analysis (Table 15 min (54 280 Da, Đ 2.2, Table 2, entry 9). In this case the formation of the degradation product daughter polymers was observed with time. However, when a stoichiometric amount of Table 1 Summary of BCA polymerization performed in THF at 25  C B(C6F5)3 was present in the mixture with TMP (Table 2, entry using several nucleophiles 10), lower molecular weight and stable polymers were obtained, Entry Initiator Time (min) Calc. Mna Exp. Mnb Đb showing an important effect of free borane in the process as a potential “group transfer agent”. 1 BuLi 20 12 250 8990/3.2  105 1.5/1.4c In order to achieve better control we contemplated the 2 BuLi 240 12 250 15 144/5.0  105 1.7/1.3d possibility of increasing the initiation rate using a hydride atom 3 sec-BuLi 120 12 250 18 580/3.4  105 1.7/1.3 from a hydrogenated FLP as the initiator. As described in the 4 Bu2CuLi 120 12 250 7699/3.1  105 1.7/1.4 5 LiI 120 12 250 5050/1.4  105 1.2/1.4 literature, these compounds have the ability to reduce double 6 PPh3 120 12 250 3.5  105 1.7 bonds in the a-position of a carbonyl group.35 With this aim a [TMPH+][HB(C6F5)3] was prepared following the procedure Calculated through the experimental ratio [M]/[I] (80 : 1) assuming that every initiator molecule initiates one polymer chain. Expressed described in the literature,36 and it was used to polymerize BCA, in g mol1. b Experimental data obtained by GPC, when more than showing much better control of the reaction (Table 2, entry 11 one peak is observed, the data related to each polymeric species are and Scheme 1). The propagation rate was reduced, the experi- included from low to high molecular weight. The same pattern is extended to the corresponding dispersities and the relative peak mental molecular weights were quite similar to the calculated areas. c Area of each peak: 53/47. d Area of each peak: 89/11. ones together with narrower molecular weight distributions 3296 | Chem. Sci., 2019, 10, 3295–3299 This journal is © The Royal Society of Chemistry 2019 View Article Online Edge Article Chemical Science Table 2 Summary of BCA polymerizations performed in THF at 25  C using the combination of a Lewis base and Lewis acid, and Lewis bases alone for comparison Entrya Initiator Time (min) Calc. Mna Exp. Mnb Đ This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. 1 NBu4Br 30 12 250 2.4  105 1.6 2 NBu4Br/B(C6F5)3 30 12 250 271 1.01 3 NBu4OH 30 12 250 4000/9.8  104 2.4/1.5 4 NBu4OH/B(C6F5)3 30 12 250 252 1.01 5 NaSCN 30 12 250 3.3  105 1.6 6 NaSCN/B(C6F5)3 30 12 250 252 1.01 7 PPh3/B(C6F5)3 30 12 250 2.6  105 3.5 Open Access Article. Published on 01 February 2019. Downloaded on 3/22/2019 11:33:36 AM. 8 Pyr/B(C6F5)3 30 12 250 91 150 3.5 9 TMP 15 12 250 54 280 2.2 10 TMP/B(C6F5)3 30 12 250 18 900 1.6 11 [TMPH+][HB(C6F5)3] 15 12 250 11 050 1.17 a Calculated through the experimental ratio [M]/[I] (80 : 1) assuming that every initiator molecule initiates one polymer chain. Expressed in g mol1. b Experimental data obtained by GPC, when more than one peak is observed, the data related to each polymeric species are included from low to high molecular weight. The same pattern is extended to the corresponding dispersities and the relative peak areas. including comparison with BuLi initiation at similar reaction times). The reaction was carried out with different ratios of BCA : [TMPH+][HB(C6F5)3], being possible to obtain polymers of different degrees of polymerization (DP 5, 35, 100, 215, 320 and 655 – MW  100 kg mol1) with narrow polydispersities (1.02, 1.08, 1.09, 1.09, 1.07 and 1.11 respectively) (see ESI†). A negative dependence of the polymerization rate on temperature was observed in accordance with Pepper's studies performed on zwitterionic polymerization mechanisms some years ago and Scheme 1 Proposed intermediates in the synthesis of poly(butyl consistent with a complex composite initiation process that cyanoacrylate) initiated by [TMPH+][HB(C6F5)3]. precedes propagation. This happened exclusively when tertiary amines were used (termed by Pepper “slowly initiated, non- terminated polymerizations”).38 Working at 60  C, it was grati- (1.17), and the formation of daughter polymers did not occur. fying to see a unimodal GPC trace obtained aer 5 minutes of To the best of our knowledge this is the rst example of reaction. At this temperature the monomer was not fully controlled living ionic polymerization of CA at practical consumed, but it completely disappeared when experiments temperatures even if the polydispersities must be further were conducted at 25  C (faster reaction). reduced. The hybridoborate anion, [HB(C6F5)3], as the coun- An estimation of the relative polymerization rate was made terion of metallacyclic cations, has been previously proposed to (see ESI†). Calorimetric studies were performed for reactions at catalyze the polymerization of conjugated polar alkenes, not 25  C but 1H NMR experiments were used to study the kinetics including cyanoacrylates, by an H-shuttling mechanism in at 60  C. A standard polymerization reaction using PPh3 as the a non-controlled manner, a role completely different to that initiator at 25  C was used as the rate reference. A reduction of proposed in this work.37 rate by at least two orders of magnitude was observed when the A real living polymerization requires specic features such as [TMPH+][HB(C6F5)3] initiator was used at the same tempera- full consumption of the monomer and initiator molecules, ture. This rate was reduced by almost a further two orders of absence of irreversible termination and chain transfer reac- magnitude (i.e. four orders of magnitude with respect to the tions, initiation faster than propagation (almost inconceivable reference polymerization) when the reaction was carried out at for CAs), targeted molecular weight, dispersities below 1.1, capability of growing in a controlled manner upon consecutive addition of monomers and the possibility to obtain block copolymers. An optimization of the different variables affecting the polymerization procedure of BCA was performed. A freshly distilled monomer and pure initiator were required to avoid the presence of oligomers in the nal polymer. The addition order was also crucial; a concentrated solution of the initiator in THF was added in one shot directly to the solution of the monomer Fig. 1 GPC traces of poly(n-butyl cyanoacrylate) (4.48 mmol distilled in THF. Under these conditions a polymer was obtained in less BCA in THF) initiated by [a] BuLi (0.056 mmol in THF, Exp. Mn  12 250 than 5 minutes showing polydispersities lower than 1.1, and Da) and [b] [TMPH+][HB(C6F5)3] (0.021 mmol in THF, Exp. Mn  32 678 daughter polymers were not observed aer 60 min (Fig. 1, Da) at 25  C. This journal is © The Royal Society of Chemistry 2019 Chem. Sci., 2019, 10, 3295–3299 | 3297 View Article Online Chemical Science Edge Article 60  C. These results would support the proposed mechanism of controlling the polymerization by lowering the polymerization rate through stabilization of the anionic chain end. Moreover, under these conditions, a linear evolution was observed in the This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Mn versus conversion-plot (see ESI†), supporting a controlled anionic living polymerization. Two possible explanations can be envisaged to justify the inverse dependence of polymerization rate on temperature observed in our case. One would refer to the existence of a complex equilibrium composite process that precedes the Open Access Article. Published on 01 February 2019. Downloaded on 3/22/2019 11:33:36 AM. actual initiation, related to the behavior described by Pepper and Ryan for zwitterionic polymerizations of CA with tertiary amines,38 but with the important difference that in our case we achieve total control of the polymerization. The second would Fig. 2 MALDI-TOF MS of the reaction crude of the stoichiometric reaction between BCA and [TMPH+][HB(C6F5)3] in THF (negative be related to the variation of dielectric constant of the solvent mode), using DCTB as the matrix. (THF) with temperature.39 Hence, higher temperatures (lower dielectric constant of THF) would favor the formation of contact ion pairs, thus contributing to the reduction of the polymeri- zation rate. Discrimination between those mechanistic expla- nations merits further studies that presently do not constitute the aim of this work. No evidence was obtained for the presence of boron atoms linked to the polymer chains using various analytical techniques, most likely due to the low concentration of this atom in the sample. Other analyses were also performed in order to identify the formation of compounds of type A (Scheme 1) as promoters of the polymerization. A stoichiometric reaction between BCA and [TMPH+][HB(C6F5)3] in THF at room temperature furnished a white solid that was analyzed by NMR. The disappearance of the double bond signals of BCA and the appearance of a new CH3 signal at 1.70 ppm in 1H NMR and 15.2 ppm in 13C NMR were observed. In the 11B NMR spectrum the disappearance of the B–H doublet signal observed at 23.9 ppm and the appearance of a new signal corresponding to B–O species at 4.02 ppm supported the existence of compounds of type A. Also in 19F NMR a clear shi of the signals was observed showing the new environment of uorine atoms in agreement with the expected structures. Fig. 3 GPC traces of the polymers obtained in THF by three Moreover direct evidence of the formation of A was obtained by consecutive additions of BCA ([a] at 25  C, [c] at 60  C), ECA [d], MECA MALDI-TOF MS analysis in the negative mode, where its signal at [e], and BCA + ECA + BCA [f] at 60  C. Chart [b] shows the calorimetric 666.1 g mol1 was detected. Another interesting signal was traces corresponding to polymerizations in chart [a]. All reactions were identied at 819.2 g mol1 corresponding to the dimer linked to initiated by [TMPH+][HB(C6F5)3]. the boron atom (Fig. 2). In our opinion, these data constitute direct evidence of the mechanism proposed for the living anionic polymerization process seemed to be resistant to signicant polymerization of cyanoacrylates promoted by [TMPH+] amounts of water present in the media (10  [initiator]), but [HB(C6F5)3], namely, hydride initiated polymers growing always below the threshold range in which water could initiate through a boron end-capping mediated controlled propagation, itself CA polymerization. Acid pre-treated glassware or poly- and shown in a simplied version in Scheme 1. propylene asks were required. In order to study the scope of the reaction two other CA The living character of the polymer chain ends was conrmed monomers were polymerized under the best conditions found, by the preparation of polymers by up to three sequential mono- ethyl cyanoacrylate (ECA) and b-methoxyethyl cyanoacrylate mer additions of either the same or different monomers (BCA, (MECA). The molecular weight of the obtained polymers was in ECA or MECA) (Fig. 3a and c–f). To the best of our knowledge agreement with the expected ones and narrow polydispersities trace (f) in Fig. 3 represents the rst ever example of controlled were obtained (1.06 and 1.10 respectively, Fig. 3d and e, rst living ionic polymerization of CAs to produce bespoke block addition) indicating a generality of use of such initiators with sequences even though these monomers rst emerged for study this class of monomers. in the late 1940's.40 Closer agreement between designed molec- Although the synthesis of [TMPH+][HB(C6F5)3] was per- ular weights and experimental ones was achieved once more formed in a glove box to avoid oxygen and moisture, the living working at 60  C, where narrow polydispersities (#1.1) were 3298 | Chem. Sci., 2019, 10, 3295–3299 This journal is © The Royal Society of Chemistry 2019 View Article Online Edge Article Chemical Science achieved in all cases. It was also possible to maintain the living 12 A. J. Domb and N. Kumar, Biodegradable Polymers in Clinical polymer species dormant in the solid state. Use and Clinical Development, John Wiley and Sons. Inc., New Jersey, USA, 2011. Conclusions 13 F. Scognamiglio, A. Travan, I. Rustighi, P. Tarchi, This article is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported Licence. S. Palmisano, E. Marsich, M. Borgogna, I. Donati, N. de To summarize, a hydrogenated frustrated Lewis pair ([TMPH+] Manzini and S. Paoletti, J. Biomed. Mater. Res., Part B, [HB(C6F5)3]) initiator has allowed the controlled living ionic 2016, 104, 626. polymerization of highly reactive CA monomers (BCA, ECA and 14 T. Kim and B. V. Kharod, Curr. Opin. Ophthalmol., 2007, 18, 39. MECA) in THF. Homopolymers could be prepared by successive 15 P. A. Leggat, U. Kedjarune and D. R. Smith, Ind. Health, 2004, additions of monomers as could be block copolymers, and in all 42, 207. Open Access Article. Published on 01 February 2019. Downloaded on 3/22/2019 11:33:36 AM. cases with narrow polydispersities. Formation of daughter 16 D. Severian and V. Popa, Polymeric Biomaterials, CRC Press, polymers was avoided. An inverse dependence of the polymer- Boca Ratón, USA, 2013, vol. II. ization rate on temperature was observed with the hydroge- 17 S. Saranya and K. V. Radha, Polym.-Plast. Technol. Eng., 2014, nated FLP initiator, being four orders of magnitude lower than 53, 1636. the rate given by a standard reference PPh3 initiator at 60  C. 18 J. Woods, Polycyanoacrylates, Encyclopedia of Polymer Science Polymerization reaction intermediates containing living anions and Technology, John Wiley & Sons Inc., 2001, vol. 3. stabilized by B(C6F5)3 were detected by MALDI-TOF MS which 19 M. Stein, J. Appl. Polym. Sci., 1992, 46, 2217. supports the group transfer mechanism proposed. 20 C. Y. Huang and Y. D. Lee, Int. J. Pharm., 2006, 325, 132. 21 I. Szanka, A. Szanka, S. Sen, N. Nugay and J. 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