Hyperbranched-linear polyethylene block polymers constructed with chain blocks of hybrid chain topologies via one-pot stagewise chain walking ethylene 'living' polymerization

摘要

We demonstrate in this article the facile synthesis of a novel range of "treelike" polyethylene block polymers constructed uniquely with chain blocks of hybrid hyperbranched-linear chain topologies from sole ethylene stock. Though chemically identical, the blocks in the polymers are featured with distinctly different chain topologies, varying from hyperbranched to linear. This synthesis is achieved uniquely through one-pot stagewise chain walking ethylene "living" polymerization with a Pd-diimine catalyst, [(ArNC(Me)-(Me)CNAr)Pd(CH_3)(N≡CMe)]~+SbF _6~- (Ar = 2,6-(iPr)_2C_6H_3) (1), under varying conditions. It takes advantage of the combined outstanding features of the Pd-diimine catalyst in ethylene polymerization the "living" polymerization behavior at a broad range of ethylene pressure and temperature and the capability of topology tuning by changing both parameters. In this stagewise "living" polymerization technique, the polymerization condition (ethylene pressure and temperature) is varied from stage to stage to grow blocks of different desired topologies while with maintained "living" behavior. With this technique, diblock polymers, containing a hyperbranched first block and a linear second block with controllable narrow-distributed sizes, have been obtained through two-stage polymerizations using the growth order of "hyperbranched-first" with the first stage at 1 atm/15 °C and the second stage at 27 atm/5 °C. The distinct block structure in these diblock polymers is verified based on the fact that their intrinsic viscosity data follow consistently the combination rule found with conventional diblock polymers. In addition, the synthesis of triblock polymers, composed of a hyperbranched first block, a medium-compact second block, and a linear third block, is also demonstrated through three-stage polymerization involving the first stage at 1 atm/15 °C, the second at 3 atm/15 °C, and the third at 27 atm/5 °C.