聚乙二醇接枝聚醋酸乙烯酯的聚合动力学研究

Kinetic Study of Vinyl Acetate Graft Polymerization on PEG

以过氧-2-乙基己酸叔丁酯为引发剂,进行了聚乙二醇(PEG)存在下的醋酸乙烯酯(VAc)本体接枝聚合,考察了各VAc:PEG配比下聚合速率、PEG上PVAc的接枝率、PVAc的接枝效率、PEG的被接枝效率以及接枝共聚物的分子量随聚合时间的变化规律。结果发现,随配方中VAc配比的增加,聚合速率与极限转化率明显增加,聚合反应出现明显的自动加速现象;当VAc:PEG配比较大时,综合速率参数K随聚合时间不断上升,当VAc:PEG配比较低时,K随聚合时间呈下降趋势。各VAc:PEG配比下PVAc的接枝率几乎都随转化率的增大而线性地增加,接枝效率则几乎不随聚合时间而变;接枝率的大小正比于VAc:PEG配比,接枝效率则普遍较高,低VAc:PEG配比时,接枝效率可达95%以上;PEG的被接枝效率虽随转化率和VAc:PEG配比的增加而明显增加,但总体仍较低,表明聚合体系中有相当多一部分PEG未被接枝。高的VAc接枝效率和低的PEG被接枝效率表明,该GraftFrom接枝聚合体系中,可能存在着GraftOnto的接枝聚合机理。根据定义用■=■nPEG1+GrGd来计算PEG-g-PVAc的数均分子量,发现■随聚合转化率的增大而减小,并逐渐趋于常数,配方中VAc浓度越大■越大;由GPC测试、PVAc普适校正计算发现,接枝聚合产物的分子量分布指数在1.5～2.5之间,且随单体浓度和转化率的增加而增大。这些均与聚合体系的凝胶效应有关。

Using tert-butyl peroxy-2-ethyl hexanoate （TPRO） as initiator, the vinyl acetate （VAc） bulk grafting polymerization in the presence of polyethylene glycol （PEG） to get the PEG-g-PVAc was studied. The time evolution of polymerization rate, grafting ratio, grafting efficiency and molecular weight of the grafted copolymer were investigated respectively for various weight ratios of VAc to PEG. The results show that, with increasing VAc＇PEG ratio, the polymerization rate and final conversion ratio are both enhanced, and an observable automatic acceleration phenomenon begins to appear. The overall reaction rate coefficient K increases with reaction time at higher VAt：PEG ratio, while a tendency of decrease of K with reaction time at lower VAt：PEG ratio was observed. At almost all VAc：PEG ratios, the gaffing ratio was found to increase linearly with the conversion ratio, and the grafting efficiency keeps almost unchanged throughout the whole conversion. The grafting ratio is in proportion to VAc：PEG ratio, while the grafting efficiency is generally very high and can reach above 95% at low VAt：PEG ratio. Although the PEG grafted efficiency increases with both monomer consumption and rising VAc＇PEG ratio, the overall PEG grafted efficiency is low, it indicates that a great amount of undrafted PEG exists. The phenomenon of high VAc grafting efficiency accompanying with low PEG grafted efficiency indicates that the ＇Graft From＇ polymerization system may undergo a ＇Graft Onto＇ mechanism. According to the definition, the PEG-g-PVAc number-average molecular weights were calculated, and the results show that the number-average molecular weights reduce at first with the increase of conversion ratio, and then approach to a constant; the higher the VAc：PEG ratio, the higher the number-average molecular weight. Via GPC analysis and PVAc universal calibration, the PDI （polydispersity index） of the grafted copolymer was determined to be between 1.5 - 2.5 and it aggrandizes with the increase of VAc concentration and conversion. It might be considered that, all these phenomena are related to the gel effect in the polymerization system.