Cooligomerization of γ-butyrolactone with (meth)acrylates catalyzed by N-heterocyclic carbene: Low possibility of hybrid copolymerization

Highlights

• Cooligomerization of γ-butyrolactone (γBL) with (meth)acrylates was catalyzed by an N-heterocyclic carbene (NHC).

• Graft cooligomers were formed via transesterification.

• Contrary to the previous report, the poly(ε-caprolactone-co-methyl methacrylate) is also proposed to be a graft copolymer.

Abstract

Copolymerization between lactones and vinyl monomers has significant implications for the synthesis of new polymeric materials; however, a highly reactive cyclic monomer, e.g., ε-caprolactone (CL), leads to a copolymeric product but with a complex structure remaining to be elucidated. Herein, we present cooligomerizations of γ-butyrolactone (γBL), a low-polymerizable cyclic monomer, with (meth)acrylates catalyzed by an N-heterocyclic carbene (NHC) under conditions in which no reaction occurs in the absence of either comonomer. Several experimental studies indicate that the product is not a hybrid cooligomer with an ester linkage in the main chain but a graft cooligomer. The reaction mechanism involves the generation of the γBL enolate by the NHC to initiate oligomerizations of γBL and (meth)acrylates followed by the transesterification of oligo (γBL) with an ester unit of (meth)acrylates. These results prompted us to investigate the previously reported structure of copolymer of CL and methyl methacrylate obtained by a phosphazene base catalyst. The hydrolysis experiment revealed that the copolymeric product turned out not to be a hybrid random copolymer.

Introduction

Hybrid copolymerization of cyclic monomers, e.g., lactones and epoxides, with vinyl monomers, e.g., (meth)acrylates and vinyl ethers, leads to the production of new structures of polymers. In contrast to conventional vinyl polymers, hybrid copolymers may be degradable because of the presence of functional groups, ester or acetal groups, in the main chain, which can contribute to sustainable polymer recycling. However, cross propagation between cyclic and vinyl monomers is generally difficult because of a large difference in reactivities of monomers and propagating species. A recent successful example is the cationic hybrid copolymerization of oxiranes and cyclic acetals, with vinyl ethers and styrenes catalyzed by Lewis acids reported by Kanazawa and Aoshima et al. [[1], [2], [3], [4], [5], [6], [7]], [[1], [2], [3], [4], [5], [6], [7]] [[1], [2], [3], [4], [5], [6], [7]] in which the occurrence of the copolymerizations was proved not only by detailed structural analysis using NMR and MALDI-TOF-MS but also acid hydrolysis of the acetal moiety of the main chain. You et al. reported hybrid copolymerization combining radical vinyl addition of acrylamides and anionic ring-opening polymerization of thiiranes [8]. Zhang et al. reported anionic hybrid copolymerization of lactones and (meth)acrylates catalyzed by a phosphazene [[9], [10], [11]]. The ¹H and ¹³C NMR analysis of the copolymer showed different signals from the homopolymers, and one glass transition temperature was observed in a DSC measurement; however, the possibility of transesterification of methacrylates with polyester to form copolymeric product cannot be excluded.

Organocatalysis by N-heterocyclic carbenes (NHCs) has been extensively explored for two decades [12,13]. In the field of polymer science, NHCs are used as efficient nucleophilic catalysts or initiators for ring-opening and vinyl addition polymerizations [14]. Among the NHCs shown in Fig. 1, IMes and IiPr selectively catalyzed ring-opening polymerization of lactones or lactides [[15], [16], [17]]. In contrast, there are several types of reactions of polar vinyl monomers by NHCs. Specifically, TPT and IMes catalyzed tail-to-tail dimerization [[18], [19], [20], [21], [22], [23]] and cyclotetramerization [24] of polar vinyl monomers, respectively, through deoxy-Breslow intermediates. On the contrary, ItBu promoted polymerization of polar vinyl monomers in the presence of an alcohol as an initiator or Lewis acids as an activator [[25], [26], [27]]. Thus, we have focused on ItBu as a potential catalyst for hybrid copolymerization.

γ-Butyrolactone (γBL), a five-membered cyclic ester, has recently received much attention as a low-polymerizable and biomass-derived monomer [28]. Despite the unfavorable thermodynamics for its ring-opening polymerization because of the low ring strain energy, catalytic systems using phosphazene [[29], [30], [31]], N-heterocyclic olefin [32], urea/alkoxide [33], and La complex [34] have been recently developed to produce high molecular weight polymers. Because the hybrid copolymerization of highly reactive lactones with polar vinyl monomers produced a complex copolymeric product, we have conceived the use of the low-polymerizable γBL as a monomer to simplify the copolymerization process leading to an understanding of the mechanism for the hybrid copolymerization. In this paper, we report the copolymerization of γBL with (meth)acrylates catalyzed by ItBu and its mechanistic elucidation.