Modification of surfaces with covalently grafted polymer brushes is a widely used technique to tailor surface properties such as wettability, biocompatibility, corrosion resistance and lubricity. In the current work, surface-tethered poly(methyl methacrylate) (PMMA) layers were produced via surface-initiated photoiniferter-mediated photopolymerization (SI-PMP). Photoiniferters, based on dithiocarbamate chemistries are often utilized as photoinitiators for “living” radical polymerizations in bulk or solution (1). However, it has been found that, due to a low concentration of deactivating radicals, SI-PMP of methyl methacrylate suffers from irreversible termination reactions, which lead to cessation of polymerization (2). To prevent irreversible termination tetraethylthiuram disulfide (TED), a source of dithiocarbamyl radicals, was added to the monomer solution prior to photopolymerization. The studies of the impact of TED on the growth of PMMA by SI-PMP reveal interesting trade-offs: dithiocarbamyl radicals generated from TED reduce the rate of growth of the layers and decreases the extent of irreversible termination reactions; however, the dithiocarbamyl radicals generated can initiate polymerization in solution, resulting in monomer consumption and a lower rate of propagation. PMMA layers synthesized at various TED concentrations were also reinitiated using styrene as a monomer to investigate the effect of TED concentration on the extent of irreversible termination. The reinitiation efficiency increased as [TED] was increased, indicating that extent of irreversible termination reactions can be reduced by increasing [TED]. These studies highlight the dramatic impact of TED on growth of polymer layers by SI-PMP and show that it is necessary to supply a source of deactivating radicals to obtain a controlled polymerization; however, this enhancement comes at the cost of reducing the rate of polymerization and layer growth. To improve the rate of SI-PMP, the various alternatives, such as increasing photoiniferter concentration and light intensity are currently being explored in our laboratory.

References:

1.Otsu, T.; Yoshida, M.; Tazaki, T. Macromol. Chem. Rapid Commun. 1982, 3, 133-140. 2.Rahane, S. B.; Kilbey, S. M., II; Metters, A. T. Macromolecules, 2005, 38, 8202-8210.