Frequency control of crossover reactions in concurrent cationic vinyl-addition and ring-opening copolymerization of vinyl ethers and oxiranes: specific roles of weak Lewis bases and solvent polarity
Frequency control of crossover reactions in concurrent cationic vinyl-addition and ring-opening copolymerization of vinyl ethers and oxiranes: specific roles of weak Lewis bases and solvent polarity
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DOI：
10.1039/c5py00152h
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发表时间：
2015-07
影响因子：
4.6
通讯作者：
Arihiro Kanazawa;S. Aoshima
中科院分区：
文献类型：
--
作者：
Arihiro Kanazawa;S. Aoshima
研究表明，在烷基乙烯基醚（VE）与环氧乙烷的阳离子加成开环共聚反应中，弱刘易斯碱和溶剂极性是导致交叉反应发生的主要原因.弱刘易斯碱如乙酸乙酯和1，4-二氧六环促进从氧化异丁烯或一氧化丁二烯衍生的增长物质到烷基VE单体的交叉反应，这可能是通过氧鎓离子的开环更频繁地产生碳阳离子。开环反应产生的碳正离子的特异性--对VE单体的偏好或对环氧乙烷单体的厌恶--是改变交叉频率的另一个因素。然而，弱刘易斯碱对VE衍生的增长物种与每个单体的相对反应性几乎没有影响。相比之下，溶剂极性对促进从VE衍生的传播末端到环氧乙烷的交叉具有显著影响，而从环氧乙烷衍生的传播末端的交叉的频率不受影响。与两种环氧乙烷相比，反应条件对使用一氧化异戊二烯的共聚影响很小，一氧化异戊二烯是一种通过开环产生更稳定、共振稳定的碳阳离子的环氧乙烷。讨论了在不同条件下乙烯酯和环氧乙烷的相对反应性，在一个嵌段共聚物中每个单体单元的平均数和单体竞聚率方面。
Weak Lewis bases and solvent polarity are demonstrated to be highly responsible for the frequency of crossover reactions in the concurrent cationic vinyl-addition and ring-opening copolymerization of alkyl vinyl ethers (VEs) and oxiranes. Weak Lewis bases such as ethyl acetate and 1,4-dioxane promote crossover reactions from isobutylene oxide- or butadiene monoxide-derived propagating species to an alkyl VE monomer, potentially through the more frequent generation of carbocations via the ring opening of the oxonium ion. The specificity of the carbocation that results from the ring-opening reaction—a preference for VE monomers or an aversion to oxirane monomers—is another factor that changes the crossover frequency. Weak Lewis bases, however, have little effect on the relative reactivity of the VE-derived propagating species to each monomer. In contrast, solvent polarity has a significant effect on the promotion of crossover from the VE-derived propagating end to an oxirane, while the frequency of the crossover from the oxirane-derived propagating end is not affected. In contrast to the two oxiranes, the reaction conditions have little effect on the copolymerization using isoprene monoxide, which is an oxirane that generates a more stable, resonance-stabilized carbocation through ring opening. The relative reactivities of VEs and oxiranes under various conditions are discussed in terms of the average number of each monomer unit in one block of the copolymers and in terms of the monomer reactivity ratios.