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Stereo‑, Temporal and Chemical Control through Photoactivation of Living Radical Polymerization: Synthesis of Block and Gradient Copolymers
journal contribution
posted on 2015-12-17, 09:03 authored by Sivaprakash Shanmugam, Cyrille BoyerNature
has developed efficient polymerization processes, which
allow the synthesis of complex macromolecules 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.