Unique behavior of nitroxide biradicals in the controlled radical polymerization of styrene
Unique behavior of nitroxide biradicals in the controlled radical polymerization of styrene
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DOI：
10.1021/ma0116856
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发表时间：
2002-03-12
期刊：
影响因子：
5.5
通讯作者：
Vairon, JP
中科院分区：
文献类型：
--
作者：
Huang, WL;Chiarelli, R;Vairon, JP
使用各种binitroxides，与两个自由基网站的相似的反应性，作为介体在苯乙烯的受控自由基聚合进行了检查和所涉及的反应的速率常数测定在130 degreesC。在聚合的早期阶段观察到受控自由基聚合的典型特征，导致形成在核处含有二氮氧化物的双臂大分子。然而，在较高的转化率，连续的分解反应导致的双臂大分子断裂成一个死链具有不饱和端基和一个由改性的二氮氧化物封端的活性链。后者具有通过转化成羟胺而失活的游离氮氧位点。这种副反应的程度远大于在经典的氮氧自由基介导的苯乙烯可控自由基聚合。这个特征被分配给生长链的结构。由于它们在核中含有二硝基氧，活化反应产生增长链和硝基氧，另一个聚合物链通过第二个烷氧基胺键保持连接。因此，增长自由基的氮氧化物的失活是两个大分子物种之间的双分子过程。这种独特的情况下，烷氧基胺均裂解离的速率常数没有显着的影响，但更重要的是降低了重组的速率常数。后者比130 ℃下克里思与聚苯乙烯自由基复合的测定值低30倍。缓慢的重组被分配到空间位阻，并导致在聚合介质中的氮氧化合物的浓度异常高，连同一个大浓度的传播自由基。这两个自由基的高浓度的一个后果是增强的氢转移率从活性物种的持久性自由基，导致烷氧基胺键断裂，因此，臂分离。
The use of various binitroxides, with both radical sites of similar reactivity, as mediator in the controlled radical polymerization of styrene was examined and the rate constants of the reactions involved were determined at 130 degreesC. Typical features of a controlled radical polymerization were observed in the early stage of the polymerization, leading to the formation of two-arm macromolecules containing the binitroxide at the core. At higher conversion, however, continuous decomposition reaction resulted in a break of the two-arm macromolecules into a dead chain having an unsaturation end group and a living chain capped by a modified binitroxide. The latter had the free nitroxide site inactivated by conversion into hydroxylamine. The extent of this side reaction was much larger than in classical nitroxide-mediated controlled radical polymerization of styrene. This feature was assigned to the structure of the growing chains. As they contain the binitroxide at the core, the activation reaction produces a propagating chain and a nitroxide to which another polymeric chain remains attached by the second alkoxyamine bond. Thus, deactivation of the propagating radical by nitroxide is a bimolecular process between two macromolecular species. This unique situation had no significant effect on the rate constant of the alkoxyamine homolytic dissociation, but more importantly decreased the rate constant of recombination. The latter was 30 times lower than that determined for the recombination of TEMPO with polystyryl radical at 130 degreesC. The slow recombination was assigned to steric hindrance and resulted in an unusually high concentration of nitroxide in the polymerization medium, together with a large concentration of propagating radicals. A consequence of the high concentration of both radicals was the enhanced rate of hydrogen transfer from the active species to the persistent radical, leading to alkoxyamine bond breaking and, hence, to arm separation.