Development of Living Radical Emulsion Polymerization

从理论上预测，在特定条件下，在乳液或分散聚合体系中进行的退行链转移介导的活性自由基聚合（LRP）将延长自由基的寿命和增加退行链转移的频率。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预计会产生更长的链、更低的多分散性和更高的链端活性（活度）分数。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。在分散系统中获得的显著结果将在下面报告。在水相溶剂（75%乙醇）、分散剂（聚丙烯酸）和温度（70℃）的优化条件下，PMMA的数均分子量约为1.0 × 10^5，多分散性指数M_w/M_n约为1.1，验证了上述理论。然而，PMMA颗粒之间存在一定的相互粘附，这是一个有待解决的问题。在苯乙烯聚合中，聚乙烯醇吡咯烷酮被用作分散剂，没有观察到颗粒的这种粘附。聚合得到粒径可控且均匀的聚苯乙烯颗粒（通常为2μm）。对于MMA和苯乙烯，聚合都在短时间内完成，这意味着有效自由基浓度非常高，从而提高了退化链转移型LRP的性能，并更好地利用了LRP。

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.