Polymerizing epoxides after cyclic esters remains a major challenge, though their block copolymers have been extensively studied and used for decades. Reported here is a simple catalytic approach based on a metal-free Lewis pair that addresses the challenge. When the Lewis acid is used in excess of a base, selective (transesterification-free) polymerization of epoxides occurs in the presence of esters, while selectivity toward cyclic esters is achieved by an oppositely biased catalyst. Hence, one-pot block copolymerization can be performed in both ester-first and ether-first orders with selectivity being switchable at any stage, yielding ether-ester-type block copolymers with unlimited ordering of sequences as well as widely variable compositions and architectures. The selectivity can also be switched back and forth several times to generate a multiblock copolymer. Experimental and calculational results indicate that the selectivity originates mainly from the state of catalyst-activated hydroxy species.
Keywords: Lewis acids; chemoselectivity; organocatalysis; polymers; ring opening.
© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.