Effect of an Acid Protecting Group on the 'Livingness' of Poly(acrylic acid-ran-styrene) Random Copolymer Macroinitiators for Nitroxide-Mediated Polymerization of Styrene

摘要

Poly(acrylic acid-ran-styrene) (P(AA-r-S)) and poly(tert-butyl acrylate-ran-styrene) (P(tBuA-r-S)) random copolymers previously made with different molar ratios (0, 4.5, and 9 mot %) of additional free nitroxide mediator, N-tert-butyl-N-[1-(diethylphosphono)-2,2-dimethylpropyl]nitroxide (SG1), relative to 2-[[N-tert-butyl-N-[1-(diethylphosphono)-2,2-dimethylpropyl]amino]oxy]- 2-methylpropionic acid (BlocBuilder) alkoxyamine unimolecular initiator were used as macroinitiators for styrene chain extension. The reinitiating experiments were performed in the bulk at 115 degrees C when using P(tBuA-r-S) macroinitiators or 30-50 wt % dioxane solution at 120 degrees C when using P(AA-r-S) macroinitiators. The P(tBuA-r-S)-containing macroinitiators (number-average molecular weight (M) over bar (n) = 13.4-25.8 kg.mol(-1)) with polydispersities (M) over bar (w)/(M) over bar (n) = 1.13-1.57 were capable of initiating chains extended with styrene as indicated by monomodal and narrow molecular weight distributions with (M) over bar (n) = 34.6-66.3 kg.mol(-1) and (M) over bar (w)/(M) over bar (n) ranging from 1.24 to 1.39. Unprotected P(AA-r-S) acrylic acid-containing macroinitiators ((M) over bar (n) = 19.7-28.8 kg.mol(-1)) with (M) over bar (w)(M) over bar (n) ranging from 1.34 to 1.48 also initiated chain extensions with styrene but tended to have much higher (M) over bar (w)/(M) over bar (n) values of 1.50-2.47. In some cases, dead chains that failed to reinitiate were clearly observed from the gel permeation chromatograms. The acrylic acid likely degraded the alkoxyamine as suggested by previous literature, reducing the SG1-active chain end concentration and thereby decreasing the control of the molecular weight distribution. On the basis of these findings, a more desirable route toward higher molecular weight poly(acrylic acid)-containing block copolymers by nitroxide-mediated polymerization is likely done by polymerization of the protected acrylic acid followed by suitable deprotection.