Living Radical Polymerization Immobilized on Wang Resins:  Synthesis and Harvest of Narrow Polydispersity Poly(methacrylate)s

Wang resin has been transformed into an initiator for copper(I)-mediated living radical polymerization of methacrylates at initiator loading of 0.9 and 3.5 mmol g-1. The immobilized initiator was characterized by ATR FTIR, gel phase 13C NMR, and solid-state CP/MAS 13C NMR using two different spinning frequencies as well as a TOSS pulse sequence. The immobilized initiator has been used to prepare poly(methyl methacrylate), PMMA, homopolymer, and poly(methyl methacrylate)-block-poly(benzyl methacrylate-co-methyl methacrylate), P(MMA)-block-P(BzMA-co-MMA), block copolymers. The poly(methacrylate)s have been harvested from the insoluble resin by a simple trifluoroacetic acid, TFA, wash which selectively cleaved the activated benzyl ester linkage, so as to facilitate analysis. At an initiator loading of 0.9 mmol g-1 the Mn increases linearly with conversion with kinetics following first-order behavior in monomer as would be expected for living polymerization. After 3 h a 61.9% conversion of MMA is reached, with the isolated polymer chains having an average number molar mass, Mn, of 8200 and a polydispersity, PDI, of 1.18. High conversions, >90%, lead to considerable increases in Mn and PDI. Moreover, small amounts of “free” chains present in the supernatant, ca. 5−8% after 4 h of reaction time, were found. The morphology of the beads was monitored by SEM with the integrity being maintained throughout the transformations. Attempts to prepare true block copolymers via a two-stage process involving isolation of the Wang resins with the first block and subsequent reuse to attach the second block were not satisfactory. However, a one-shot addition of BzMA at high conversion of MMA allowed the synthesis of P(MMA)-block-P(BzMA-co-MMA) with a narrow molar mass distribution, as confirmed with SEC, DSC, and NMR. The paper demonstrates that Wang immobilized chemistry can be used to prepare excellent polymers maintaining the characteristics of analogous homogeneous living radical polymerizations while allowing for catalyst removal by simple washing procedures. The potential for automation of this chemistry for high throughput synthesis has been demonstrated.