Fully alternating and regioselective ring-opening copolymerization of phthalic anhydride with epoxides using highly active metal-free Lewis pairs as a catalyst†
A recent study has been directed to the design of metal-free Lewis pair catalysts for ring-opening alternating copolymerization (ROAP) reactions to enhance both activity and selectivity. While the simplest types of organic bases/Lewis bases (for example: PPN+Cl−, DMAP, DBU and TBD) are able to copolymerize anhydride-epoxide in a non-living and non-quantitative manner, the introduction of Lewis acids radically changes this behaviour. In this study, various Lewis acids such as B(C2H5)3, Al(CH)3, Et2Zn and nBu2Mg in combination with various Lewis bases such as PPN+Cl−, DMAP, DBU and TBD were tested as Lewis pair catalysts for anhydride-epoxide ring-opening copolymerization (ROCOP) studies. Based on the observed results, the B(C2H5)3/PPNCl pair stood out as the most active and effective Lewis pair for the perfectly alternating and regioselective controlled ROCOP of various epoxides (cyclohexene oxide, CHO; tert-butyl glycidyl ether, tBGE and 2-benzyloxirane, BO) with phthalic anhydride (PA). Medium to high molecular weight linear poly(anhydride-co-epoxide)s (Mn up to 57.5 kg mol−1) are achieved, and most of them exhibit narrow molecular weight distributions (Mw/Mn as low as 1.07). However, in the presence of strong Lewis acids (Al(CH)3, Et2Zn and nBu2Mg) and neutral Lewis bases (DMAP/DBU/TBD) this broad applicability is offset by the lack of control over the polymerizations, including the side reactions as a consequence of strong acidity/alkalinity. Hence, the ideally suitable acidity/alkalinity and matched size of the Lewis pair are considered crucial for the effective copolymerization of PA and epoxides. In addition, from P(PA-alt-tBGE) copolymers, hydroxyl-functionalized poly(ester-alt-glycerol)s were successfully synthesized by deprotection of the t-butyl groups.