Development of Living Radical Emulsion Polymerization

理论上预测，在特定条件下，在乳液或分散聚合体系中进行的简并链转移介导的活性自由基聚合(LRP)将延长自由基寿命和增加简并链转移的频率。换句话说，与在均相体系中进行的LRP相比，在非均相体系中进行的LRP可以得到更长的链，具有更低的多分散度和更高的链端活性(活性)。为了验证这些理论预测，在乳液和分散体系中进行了甲基丙烯酸甲酯(MMA)和苯乙烯的聚合反应。通过对水相溶剂(75%乙醇)、分散剂(聚丙烯酸酯)和温度(70℃)的优化，得到了数均相对分子质量约为1.0x10^5、多分散指数M_w/M_n约为1.1的聚甲基丙烯酸甲酯，验证了理论。然而，PMMA粒子之间存在一定的相互粘连，这是一个有待解决的问题。在以聚乙烯吡咯烷酮为分散剂的苯乙烯聚合中，没有观察到粒子的粘附性。聚合反应得到尺寸可控且均匀的聚苯乙烯颗粒(通常为2μm)，由定义良好的聚苯乙烯组成。对于MMA和苯乙烯，聚合在很短的时间内完成，这意味着有效自由基浓度非常高，从而提高了简并链转移类型LRP的性能和更好地利用。

It is theoretically predicted that the degenerative chain transfer-mediated living radical polymerization (LRP) carried out in an emulsion or dispersion polymerization system under specific conditions will provide an elongated radical lifetime and an increased frequency of degenerative chain transfer. In other words, LRP carried out in the inhomogeneous system is predicted to give longer chains with a lower polydispersity and a higher fraction of chain end activity (livingness) than LRP carried out in a homogeneous system. In order to test these theoretical predictions, polymerization of methyl methacrylate (MMA) and styrene were carried out in an emulsion and a dispersion system. Notable results obtained in the dispersion system will be reported below.Optimization of the aqueous-phase solvent (75% ethanol), dispersant (polyacrilic acid), and temperature (70℃), resulted in PMMA with a number-average molecular weigh about 1.0 x 10^5 and a polydispersity index M_w/M_n about 1.1, verifying the theory. However, some mutual adhesion of the PMMA particles was observed, which remains to be a problem to be solved. In the styrene polymerization, in which polyvinyl pyrrolidone was used as a dispersant, no such adhesion of the particles was observed. The polymerization gave polystyrene particles of controllable and homogeneous size (typically 2μm) composed of well defined polystyrene. For both MMA and styrene, the polymerization was completed in a short time, meaning that the effective radical concentration was exceptionally high, and thus increasing the performance, and making better use, of the degenerative chain transfer type of LRP.