Control of Branching in Radical Polymerization Based on New Concept

甲基丙烯酸甲酯（MMA）的不可聚合二聚体可自由基加成产生加合物自由基，该加合物自由基太稳定而不能相互反应，也不能加成到单体中。加合自由基主要与增长自由基发生偶联，通过二聚体单元连接两条聚合物链。二聚体的加成和偶联可用于自由基聚合中无凝胶分支的形成。本文研究了α-甲基乙烯基化合物的二聚体如甲基丙烯酸甲酯（MMA）、甲腈（MAN）、α-甲基苯乙烯和对位取代α-甲基苯乙烯的反应性。其中，MAN的二聚体仅参与加成/偶联以连接两个聚合物链。聚苯乙烯自由基加成到MMA二聚体上，主要与加成自由基发生偶联。然而，加合自由基也通过加合自由基的β-断裂产生带有不饱和端基的聚苯乙烯。采用可聚合加成-断裂链转移剂和双官能加成-断裂链转移剂将二聚体结构引入聚合物中。这些转移剂含有一个和两个α-烷硫基甲基丙烯酰基作为链转移基团。在可聚合转移剂存在下，MMA发生聚合反应，得到具有可链转移侧基的聚MMA，该侧基进一步参与加成/断裂链转移，将聚MMA链连接起来。侧链也可以涉及分支点。发现苯乙烯在聚（MMA）存在下的聚合产生超支化嵌段共聚物。双官能团链转移剂在聚甲基丙烯酸甲酯的两端引入了碳碳双键。这些双键用于聚苯乙烯自由基的加成/偶联，以提供支化的嵌段共聚物而不凝胶化。

A non-polymcrizable dimer of methyl methacrylatc (MMA) is feasible to radical addition to yield an adduct radical which is too stable to react mutually nor to add to monomer. The adduct radical primarily undergoes coupling with propagating radical to link two polymer chains through the dimer unit. The addition and coupling involving the dimer could be employed for gel-free branch formation in radical polymerization. The reactivities of various dimers of α-methyl vinyl compounds such as MMA, metherylonitrile (MAN), α-methylstyrene, and p-substituted α-methylstyrene were examined. Among these, the dimer of MAN participated only in addition/coupling to connect two polymer chains. Polystyrene radical added to the MMA dimer and mainly couple with the adduct radical. However, the adduct radical also yield the polystyrene bearing the unsaturated end group through β-fragmentation of the adduct radical. The dimer structure was introduced into polymer using a polymerizable addition-fragmentation chain transfer agent and a bifunctional addition-fragmentation chain transfer agent. These transfer agents involved one and two α-alkylthiomethylacryloyl groups as chain transferable groups. The polymerization of MMA in the presence of polymerizable transfer gent gave the poly(MMA) with the chain transferable side group which was further involved in addition/fragmentation chain transfer to link poly(MMA) chain. The side chain could also involve branching points. The polymerization of styrene in the presence of the poly(MMA) was found to result in a hyper branched block copolymer. The bifunctional chain transfer agent introduced the carbon carbon double bond to both end of poly(MMA). These double bonds were employed for addition/coupling of polystyrene radical to afford a branched block copolymer without gelation.