Group Transfer Polymerization of (Meth)acrylic Monomers Catalyzed by N-Heterocyclic Carbenes and Synthesis of All Acrylic Block Copolymers: Evidence for an Associative Mechanism

Abstract

N-Heterocyclic carbenes (NHCs), namely, 1,3-bis-(diisopropyl)imidazol-2-ylidene (1) and 1,3-bis(di-tert-butyl)imidazol-2-ylidene (2) were employed as neutral organocatalysts to bring about the group transfer polymerization (GTP) of both methacrylic and acrylic monomers, including methyl methacrylate (MMA), tert-butylacrylate (tBA), and n-butylacrylate (nBA). This could be achieved at room temperature using 1-methoxy-2-methyl-1-trimethylsiloxypropene (MTS) as initiator in polar or apolar medium. In this way, polymethacrylates and polyacrylates with molar masses in the range 10 000-300 000 g 3 mol -1 , corresponding to the initial [monomer]/[MTS] ratio and with polydispersities lower than 1.2, were obtained in quantitative yields. The kinetics of GTP of MMA catalyzed by 1 or 2 was further investigated. Though the first-order kinetic plot ln[M] 0 /[M] versus time deviated from linearity at high monomer conversion, no inhibition period was noted at low monomer conversion. Moreover, the polymerization rate dramatically increased as the concentration of initiator increased, with first-order dependence in initiator. When mixed in 1/1 molar ratio, MTS and NHC 1 did not reveal the formation of enolate-type species by 29 Si or 13 C NMR spectroscopy. Based on these observations, we propose that NHCs activate the silyl ketene acetal initiator and further propagate GTP via an associative mechanism. The fact that ln[M] 0 /[M] does not evolve linearly with time in the terminal phase of the polymerization can be understood by a reduced diffusion of the catalyst to the trimethylsilyl end groups. The proposed associative mechanism can also account for the successful control of NHC-catalyzed GTP of acrylates during which termination reactions such as backbiting or internal isomerization could be drastically minimized. Next, was described the synthesis of all acrylic block copolymers based on polyacrylates and polymethacrylates (e.g., PMMA-b-PnBA-b-PMMA), utilizing the same NHC as catalyst in sequential GTP. It is again argued that such block copolymer formation is favored by an associative mechanism forming highly unstable activated silicon intermediates.

N‐Heterocyclic carbenes for metal‐free polymerization catalysis: an update

“…Initially, there was some disagreement over details of the polymerization mechanism, which was postulated to follow either a dissociative or an associative pathway (Scheme ). Extensive investigations using catalysts 5‐iPr and 5‐tBu strongly favoured the associative mechanism for these NHCs; furthermore it was assumed that more electron‐rich (and silicophilic) compounds like 5 Me ‐iPr might enable dissociative behaviour . The room temperature polymerization of methyl methacrylate (MMA) or tert ‐butyl acrylate in the presence of a GTP transfer agent in THF yielded polymer of controlled molecular weight and weight distribution (typically Đ M ≈ 1.2).…”

“…Extensive investigations using catalysts 5-iPr and 5-tBu strongly favoured the associative mechanism for these NHCs; furthermore it was assumed that more electron-rich (and silicophilic) compounds like 5 Me -iPr might enable dissociative behaviour. 81 The room temperature polymerization of methyl methacrylate (MMA) or tert-butyl acrylate in the presence of a GTP transfer agent in THF yielded polymer of controlled molecular weight and weight distribution (typically Ð M ≈ 1.2). These characteristics also allowed for the synthesis of blockcopolymers.…”

Synthesis of AB block and A2B2 and A3B3 miktoarm star-shaped copolymers using ω-end-functionalized poly(methyl methacrylate) with a hydroxyl group prepared by organocatalyzed group transfer polymerization

“…[27][28][29][30][31][32][33][34][35][36] The living nature of organocatalyzed GTP is advantageous for ω-end-functionalization; for example, we recently reported the quantitative ω-end-functionalization of poly(n-butyl acrylate) and PMMA by organic Lewis acid-catalyzed GTP using α-phenylacrylate derivatives as the terminators. [27][28][29][30][31][32][33][34][35][36] The living nature of organocatalyzed GTP is advantageous for ω-end-functionalization; for example, we recently reported the quantitative ω-end-functionalization of poly(n-butyl acrylate) and PMMA by organic Lewis acid-catalyzed GTP using α-phenylacrylate derivatives as the terminators.…”

Proton‐Transfer Polymerization (HTP): Converting Methacrylates to Polyesters by an N‐Heterocyclic Carbene

“…[14] NHC-mediated step-growth polymerization has been reported as well. [15] NHCs have also been used as alternative nucleophilic catalysts [16] in the chain-growth polymerization of a,b-unsaturated esters such as methyl methacrylate (MMA), through the classic group-transfer polymerization (GTP) initiated by silyl ketene acetals. [17] In addition, such acrylic monomers can be rapidly polymerized by frustrated Lewis pairs consisting of bulky NHC bases, such as the Arduengo carbenes 1,3-di-tert-butylimidazolin-2-yli-dene (I t Bu) and 1,3-di-mesitylimidazolin-2-ylidene (IMes), [18] and the strongly acidic, sterically encumbered Al(C 6 F 5 ) 3 via zwitterionic imidazolium enolaluminate intermediates.…”

“…TPT can form two different types of adducts with aliphatic and aromatic alcohols; with MeOH, TPT forms a stable TPT(OMe)H [21] which can release MeOH to regenerate TPT at 80 8C ( Figure S7), whereas with MP, TPT forms a hydrogenbonded adduct [38] TPT-HOAr ( Figures S8-S11). [39] Because the added phenol doubly benefits the stepgrowth polymerization for producing the desired polyester, we examined the influence of the amount of added MP on the polymerization behavior by fixing the BDMA/TPT ratio (100) and reaction conditions (toluene, 90 8C, 48 h), while varying the MP amount from 0.05 to 1 equiv (relative to TPT, runs [13][14][15][16]. It is noteworthy that the peaks corresponding to PBDMA did not show up at RT, which is consistent with the fact that nucleophilic addition of the enamine to the incoming monomer occurs only at higher temperature ( % 80 8C or higher).…”