Nitroxide-Mediated Synthesis of Poly(poly(ethylene glycol) acrylate) (PPEGA) Comb-Like Homopolymers and Block Copolymers

摘要

The kinetics of poly(ethylene glycol) acrylate (PEGA) nitroxide-mediated polymerization in bulk were studied at two temperatures (115 and 125 degrees C) and at initial molar ratios r = 0.05-0.21 of added free nitroxide (N-tert-butyl-N-[1-diethylphosphono-(2,2-dimethylpropyl)] nitroxide (SG1) relative to the unimolecular initiator 2-methyl-2-[N-tert-butyl-N-(diethoxylphosphoryl-2,2-(dimethylpropyl)ami nooxy]propionic acid (Bloc-Builder) (r = [SG1](0)/[BlocBuilder](0)). Bulk polymerizations were more controlled at 115 degrees C with r = 0.10-0.21 giving polymers with polydispersity indices ((M) over bar (w)/(M) over bar (n)) as low as 1.17 and a linear increase in number average molecular weight (M) over bar (n) with conversion up to about 50%. The k(p)K values (k(p) = propagation rate constant, K = equilibrium constant) for PEGA at 115 degrees C were 1.9 x 10(-5) s(-1) to 2.9 x 10(-5) s(-1) and at 125 degrees C were 6.7 x 10(-5) to 8.3 x 10(-5) s(-1). PEGA polymerizations in dimethylformamide (DMF) or anisole solution were most controlled when conducted in 25 wt % solutions, r similar to 0.15 and lower temperatures (95-115 degrees C) but side reactions were prevalent and control was not significantly better than in bulk. Chain extension of a comb-like poly(PEGA) (PPEGA, (M) over bar (n) = 8.1 kg/mol, (M) over bar (w)/(M) over bar (n) = 1.20) with styrene in 50 wt % DMF solution at 115 degrees C to produce a PPEGA-poly(styrene) (PPEGA-PS) block copolymer indicated sufficient "livingness" of the PPEGA chain ends as observed by the steady increase in (M) over bar (n) and monomodal molecular weight distribution as determined by size exclusion chromatography although some broadening of the molecular weight distribution was evident ((M) over bar (n) = 62.3 kg/mol and (M) over bar (w)/(M) over bar (n) = 1.53). H-1 NMR spectroscopy revealed resonances corresponding to the styrenic and PEGA segments and differential scanning calorimetry revealed two distinct glass transition temperatures T-g indicative of the PPEGA (T-g approximate to -70 degrees C) and PS (T-g approximate to 98 degrees C) segments.