Alternating, Gradient, Block, and Block-Gradient Copolymers of Ethylene and Norbornene by Pd-Diimine-Catalyzed "Living" Copolymerization

We demonstrate, in this article, the facile synthesis of a broad class of low-polydispersity ethylene-norbornene (E-NB) copolymers having various controllable comonomer composition distributions, including gradient, alternating, diblock, triblock, and block-gradient, through "living''/quasiliving E-NB copolymerization facilitated with a single Pd-diimine catalyst (1). This synthesis benefits from two remarkable features of catalyst 1, its high capability in NB incorporation and high versatility in rendering E-NB "living'' copolymerization at various NB feed concentrations ([NB](0)) while under an ethylene pressure of 1 atm and at 15 degrees C. At higher [NB](0) values between 0.42 and 0.64 M, E-NB copolymerization with 1 renders nearly perfect alternating copolymers. At lower [NB](0) values (0.11-0.22 M), gradient copolymers yield due to gradual reduction in NB concentration, with the starting chain end containing primarily alternating segments and the finishing end being hyperbranched polyethylene segments. Through two-stage or three-stage "living'' copolymerization with sequential NB feeding, diblock or triblock copolymers containing gradient block(s) have been designed. This work thus greatly expands the family of E-NB copolymers. All the copolymers have controllable molecular weight and relatively low polydispersity (with polydispersity index below 1.20). Most notably, some of the gradient and block-gradient copolymers have been found to exhibit the characteristic broad glass transitions as a result of their possession of broad composition distribution. (C) 2012 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 672-686