Polymer chain architecture is a critically important chain parameter governing intrinsically the properties and applications of polymers. The rapid developments in "living"/controlled polymerization techniques, particularly the controlled radical polymerization techniques, in the past two decades have enabled the precision synthesis of novel polymers having a great variety of complex yet well-defined chain architectures from various monomer stocks. For polyolefins synthesized via catalytic coordination polymerization, the design of complex chain architectures, however, has only started recently because of the relatively limited advancements in the catalytic "living" olefin polymerization technique. In this regard, the versatile Pd-diimine catalysts have provided some unprecedented opportunities, due to their outstanding features, in rendering successfully a novel class of polyethylenes of various new complex chain architectures through the "living" ethylene polymerization protocol. The complex chain architectures designed to date have included hyperbranched, hybrid hyperbranched-linear, block, gradient and block-gradient, star, telechelic, graft and comb, and surface-tethered polymer brushes. This Feature Article attempts to summarize the recent developments achieved in the area, with an emphasis on the synthetic strategies for the architectural design. These developments demonstrate the great potential for further advancements of this new exciting research area.
Synthesis of polyolefin-based ionomers through direct catalytic copolymerization of olefin with ionic comonomers has been challenging with no prior reports. We demonstrate in this article the first direct synthesis of a class of hyperbranched polyethylene ionomers containing positively charged tetralkylammonium ions and different counteranions ( t e t r a fl u o r o b o r a t e , h e x a fl u o r o p h o s p h a t e , b i s -(trifluoromethane)sulfonimide, or hexafluoroantimonate) by copolymerization of ethylene with tetralkylammonium-containing acrylate-type ionic liquid comonomers (3−5, 7). The use of a Pd−diimine catalyst ( 1), which shows excellent stability toward the highly polar ionic group, is key to the direct synthesis. A detailed study on the catalytic copolymerization and the effects of the polymerization condition on the macromolecular chain parameters of the resulting ionomers has been performed. A range of ionomers differing in ion content and counteranion has been synthesized and characterized. Meanwhile, systematic studies on the structural, thermal, and melt rheological properties of the ionomers have been undertaken. The dramatic effects of ion incorporation on these properties, particularly on the rheological properties, are demonstrated.
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, [(ArNd, 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 narrowdistributed 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.
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