Synthesis of Hyperbranched Poly(phenylacetylene)s Containing Pendant Alkyne Groups by One-Pot Pd-Catalyzed Copolymerization of Phenylacetylene with Diynes

We report in this article the convenient synthesis of a class of hyperbranched poly(phenylacetylene)s (HBPPAs) containing various branching densities and different contents of pendant alkyne groups through one-pot chain-growth copolymerization of phenylacetylene (PA) with a diyne comonomer (1,3-or 1,4-diethynylbenzene (DEB)). The polymerization was facilitated with the use of an in situ generated cationic diphosphine-ligated Pd(II) catalyst system. Serving effectively as difunctional cross-linker in the polymerization, the diyne comonomer first undergoes monoinsertion to render pendant alkyne groups, which can be further enchained to generate branching structures. A systematic investigation has been undertaken to study the effects of various polymerization parameters, including diyne/PA feed ratio, diyne type, temperature, and solvent, on the polymerization, polymer structure and topology. With either 1,3- or 1,4-DEB, a convenient tuning of polymer topology from linear to hyperbranched can be achieved by simply increasing diyne/PA feed ratio. Relative to 1,3-DEB, 1,4-DEB is more effective in rendering branching structures since the pendant alkyne groups suffer less steric effect from the polymer backbone and are thus more reactive. As to the solvent, a dichloromethane/methanol mixture (at vol. ratio of 13:3) was shown to better help the formation of branching structures than methanol alone as the resulting polymers can dissolve well in the former while precipitate in the latter. Because of their possession of the valuable pendant alkyne groups, these polymers have also been demonstrated for their use as building blocks in the synthesis of core shell structured star polymers containing a HBPPA core and polystyrene arms through their Cu-catalyzed click reaction with an azide-ended polystyrene.