甲基丙烯酸甲酯自由基聚合歧化终止机理

Disproportionation Termination Mechanism in the Free-radical Polymerization of Methyl Methacrylate

首先用过氧化二苯甲酰(BPO)引发甲基丙烯酸甲酯(MMA)自由基聚合合成聚甲基丙烯酸甲酯(PMMA),然后用偶氮二异丁腈(AIBN)为引发剂,研究PMMA中由歧化终止生成的末端双键与苯乙烯(St)的共聚合反应行为。采用体积排除色谱(SEC)、核磁共振波谱(NMR)对聚合物进行了分析表征。结果表明:与单体MMA相似,PMMA在化学位移δ=6.20和5.47处有对应于聚合物末端双键氢的NMR信号。远程异核和近程异核相关NMR,^(13)C-NMR和DEPT-135 NMR分析和自由基共聚合实验确证:MMA自由基聚合的双基歧化终止为单一的链自由基末端β位甲基氢自由基转移机理,生成1,1-二取代甲基丙烯酸酯型双键。除双基歧化终止反应外,体系还明显地存在苯甲酰初级自由基和苯初级自由基与链自由基间的初级终止反应。

Poly(methyl methacrylate)(PMMA) was prepared through free-radical polymerization using BPO as the initiator. Size-exclusion chromatography and several NMR analysis methods, including ^(1)H-NMR,^(13)C-NMR, DEPT135-NMR, heteronuclear multiple-bond correlation, and heteronuclear single quantum correlation NMR were used to analyze the structure of the end vinyl group from disproportionation termination to disclose the mechanism. ^(1)H-NMR measurement indicates that there are two NMR proton resonances at δ=6.20 and 5.47 ppm, the other NMR methods confirm that there is only one type of vinyl group formed from disproportionation and the transferred hydrogen radical comes from the side methyl group at the chain end. The polymerization behavior of the end vinyl group in PMMA provides further evidence to support the conclusion from NMR measurements. In addition, there is also marked primary termination besides disproportionation in the polymerization.