Stereo-, Temporal and Chemical Control through Photoactivation of Living Radical Polymerization: Synthesis of Block and Gradient Copolymers

摘要

Nature has developed efficient polymerization processes, which allow the synthesis of complex macro-molecules with a perfect control of tacticity as well as molecular weight, in response to a specific stimulus. In this contribution, we report the synthesis of various stereopolymers by combining a photoactivated living polymerization, named photoinduced electron transfer-reversible addition-fragmentation chain transfer (PET-RAFT) with Lewis acid mediators. We initially investigated the tolerance of two different photoredox catalysts, i.e., Ir(ppy)_3 and Ru(bpy)_3, in the presence of a Lewis acid, i.e., Y(OTf)_3 and Yb(OTf)_3, to mediate the polymerization of N,N-dimethyl acrylamide (DMAA). An excellent control of tacticity as well as molecular weight and dispersity was observed when Ir(ppy)_3 and Y(OTf)_3 were employed in a methanol/toluene mixture, while no polymerization or poor control was observed with Ru(bpy)_3. In comparison to a thermal system, a lower amount of Y(OTf)_3 was required to achieve good control over the tacticity. Taking advantage of the temporal control inherent in our system, we were able to design complex macromolecular architectures, such as atactic block-isotactic and isotactic-block-atactic polymers in a one-pot polymerization approach. Furthermore, we discovered that we could modulate the degree of tacticity through a chemical stimulus, by varying [DMSO]_0/[Y(OTf)_3]_0 ratio from 0 to 30 during the polymerization. The stereochemical control afforded by the addition of a low amount of DMSO in conjunction with the inherent temporal control enabled the synthesis of stereogradient polymer consisting of five different stereoblocks in one-pot polymerization.