Effect of azoinitiators on nitroxide-mediated photo-living radical polymerization of methyl methacrylate

In order to clarify the initiator factor dominating the molecular weight distribution of the resulting polymer, the nitroxide-mediated photo-living radical polymerization of methyl methacrylate was performed using eight different kinds of azoinitiators: i.e., 2,2'-azobisisobutyronitrile, 2,2'-azobis(2-methylbutyronitrile), 2,2'-azobis(2,4-dimethylvaleronitrile), 1,1'-azobis(cyclohexane-1-carbonitrile), racemic-(2RS,2'RS)-azobis(4-methoxy-2,4-dimethylvaleronitrile), meso-(2RS,2'SR)-azobis(4-methoxy-2,4-dimethylvaleronitrile), dimethyl 2,2'-azobis(2-methylpropionate), and 2,2'-azobis(N-butyl-2-methylpropionamide). The bulk polymerization was carried out at room temperature for 3 h using 4-methoxy-2,2,6,6-tetramethylpiperidine-1-oxyl (MTEMPO) as the mediator in the presence of bis(alkylphenyl)iodonium hexafluorophosphate as the photo-acid generator. All the initiators provided a molecular weight distribution below 1.7 for the MTEMPO/initiator ratio of 2, although at the ratio of unity, about half of the initiators produced the molecular weight distribution around 2.3-3.4. The UV analysis revealed that the initiators having a higher epsilon value tended to more strictly control the molecular weight and provide a higher initiator efficiency. The half-lives of the initiators had little effect on the molecular weight control and initiator efficiency.