科研之友
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摘要
Due to the steric hindrance effects in combination with stability of the tertiary benzylic alpha-methylstyryl radical, there is a dynamic equilibrium between the monomer alpha-methylstyrene (AMS) and its polymer (PAMS) when the temperature is greater than 61 degrees C (the ceiling polymerization temperature). Based on this unique feature, a novel strategy to prepare copolymers of AMS having liable bonds as potential macromolecular free radical initiators for synthesizing block and graft copolymers has been successfully developed in our laboratory. By conventional free radical polymerization, a series of AMS copolymers, including copolymers with (meth)acrylate, acrylic acid, styrene and maleic anhydride were synthesized. Typically, with the increasing of AMS fraction in monomer feed, the rate of copolymerization was significantly retarded and the molecular weight of the copolymers was reduced. However, the copolymer yield could be as high as 90% (w) with the increased addition of initiator, up to 4% (w), and the molar fraction of AMS structural unit in AMS copolymers could be up to 25% (mol%). It has been demonstrated that the copolymers containing AMS structural units are efficient free radical initiators when the temperature is greater than 80 degrees C (much better higher than 90 degrees C). These copolymers could be exploited as macroinitiator in preparing block copolymers and core-shell polymer particles by bulk, solution and emulsion polymerization processes. In addition, the experimental results demonstrated that the molecular weight of copolymer products prepared with AMS copolymers as macromolecular initiators increased steadily with the monomer conversion. Though the polymerization initiated by AMS copolymers was not a well-controlled living system yet, it showed some characteristics of living polymerization. The ESR spectrum presented direct evidence of the generation of carbon centered radicals in the products of copolymer of AMS with glycidyl methacrylate (PAG) heated with N-t-butyl-alpha-phenylnitrone at 90 degrees C in toluene. Besides initiating the polymerization of vinyl monomer to prepare diblock copolymer, the AMS copolymers offered a practical pathway to synthesize grafting polymers in melting state. For example, with the addition of PAG in the PP/Nylon melten blending, it has been demonstrated a significant in situ compatibilization effect and the formation of graft polymer of PAG and PP. Furthermore, the AMS copolymers could also be used to modify MWCNT by free radical grafting onto mechanism. Instead of competing with the other existing controlled free radical polymerization technologies, the AMS copolymer method, with competitive cost and no small molecular residues, offers an alternative tool for polymer chemists to develop block copolymers on an industrial scale for some applications, such as dispersant and compatibilizer.
