Cooligomerization of γ-butyrolactone with (meth)acrylates catalyzed by N-heterocyclic carbene: Low possibility of hybrid copolymerization
Graphical abstract
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.
Section snippets
Materials
ItBu was synthesized according to the previous literature [35]. γBL (Tokyo Chemical Industry Co., Ltd. >99%), ε-caprolactone (CL) (Tokyo Chemical Industry Co., Ltd. >99%), benzyl methacrylate (BnMA) (Tokyo Chemical Industry Co., Ltd. >98%), benzyl acrylate (BnA) (Tokyo Chemical Industry Co., Ltd. >98%), tert-butyl methacrylate (tBuMA) (Tokyo Chemical Industry Co., Ltd. >98%), tert-butyl acrylate (tBA) (Tokyo Chemical Industry Co., Ltd. >98%), methyl methacrylate (MMA) (Wako Pure Chemical
Results and discussion
We initially examined the homopolymerizations of γBL and BnMA by a catalytic amount of ItBu. No reactions of γBL or BnMA occurred in the presence of 5 mol% ItBu as a catalyst in bulk and in CH2Cl2 at room temperature for 4 h. Lowering the temperature to −60 °C in the reaction of γBL by 3 mol% of ItBu produced poly (γBL) with Mn of 2900 and Mw/Mn of 2.9 in 62% conversion. Then, we investigated the copolymerization of BnMA and γBL under similar reaction conditions at room temperature. Remarkably,
Conclusions
We have investigated the cooligomerization of γBL with (meth)acrylates catalyzed by ItBu. No respective reactions of γBL and (meth)acrylates occurred, while cooligomeric product was obtained at moderate to high conversions under the similar conditions. The product structure was directly analyzed using 1H, 13C NMR, ESI-MS, and SEC. The signals corresponding to the copolymer sequences were detected in ESI-MS without sequences of homooligomers, oligo (γBL) and oligo (MMA). A kinetic study
CRediT authorship contribution statement
Takuhiro Nomura: Investigation, Methodology, Data curation, Validation, Writing – original draft. Makoto Ryukan: Initial, Investigation, Methodology. Shin-ichi Matsuoka: Conceptualization, Visualization, Supervision, Writing – original draft, Writing – review & editing. Masato Suzuki: Writing – review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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