自从Stephan和Erker两位科学家提出“位阻型(Frustrated)Lewis酸碱对”概念以来,Lewis酸碱对的催化化学得到极大的关注。近年来,人们也发现Lewis酸碱对在催化极性乙烯基单体和内酯单体聚合中有着重要的应用。Lewis酸碱对催化极性乙烯基单体聚合可形成具有高分子量和窄分子量分布的聚合物,而催化活性与所使用的Lewis酸碱对关系密切,最有效的Lewis酸是Al(C6F5)3和B(C6F5)3,Lewis碱是有机磷、氮杂环卡宾和氮杂环卡宾烯和膦腈超强碱,可聚合的单体包括甲基丙烯酸甲酯、γ-甲基-α-亚甲基-γ-丁内酯、α-亚甲基-γ-丁内酯、 丙烯酸正丁酯、 N,N-二甲基丙烯酰胺、 N,N-二苯基丙烯酰胺、 乙烯基磷酸二乙酯、2-乙烯基吡啶、2-异丙烯基-2-氧 NFDA1 唑林以及非对称的极性二乙烯基单体。聚合过程包括:链引发、链增长和链终止。链引发过程是通过Lewis酸、Lewis碱和单体相互作用形成两性离子,链增长是通过双金属活化单体加成方式进行,链终止通过两种途径:1)增长聚合物链中活化的酯氧负离子对相邻酯中羰基碳原子的亲核进攻形成δ-戊内酯;2)增长聚合物链中活化的烯酯碳负离子对倒数第三个酯中羰基碳原子的亲核进攻形成β-酮酸酯。Lewis酸碱对催化内酯单体聚合可形成线形和环状聚合物,所使用的Lewis酸为Zn(C6F5)3、有机铝、氯化铟,Lewis碱为有机胺。
The catalytic chemistry of Lewis pairs has attracted an explosive level of interest since the “frustrated Lewis pairs” (FLPs) concept was uncovered through the seminal works of Stephan and Erker. Recently, FLPs has been shown to efficiently promote the polymerization of lactones and polar vinyl monomer. Lewis pairs are highly active for polymerization of polar vinyl monomer, affording typically high molecular weight polymers with relatively narrow molecular weight distributions. Especially effective systems are the Lewis pairs (LPs) consisting of the LA Al(C
6F
5)
3 (or B(C
6F
5)
3) and strong LBs, such as phosphines and
N-heterocyclic olefins,
N-heterocyclic carbenes and phosphazene superbases, for polymerization of methacrylates, acrylamides, 2-vinyl pyridine, 2-isopropenyl-2-oxazoline, α-methylene-γ-butyrolactones, diethyl vinylphosphonate, as well as renewable dissymmetric divinyl polar monomers. Chain initiation involves cooperative addition of LPs to the monomer to generate zwitterionic active species, chain propagation proceeds via a bimetallic, activated-monomer addition mechanism, and chain chain-termination via two pathways: one that proceeds via intramolecular backbiting cyclization involving nucleophilic attack of the activated antepenultimate ester group of the growing chain by the C-ester enolate active chain end to generate a cyclic β-ketoester chain end, and the other that proceeds via intramolecular backbiting cyclization involving nucleophilic attack of the activated adjacent ester group of the growing chain by the O-ester enolate active chain end to generate a δ-valerolactone chain end. Lactones are also polymerize to produce linear or cyclic polymer using FLPs consisting of the LB amine and LAs, such as Zn(C
6F
5)
2, alkylaluminum, and indium chloride.
Contents
1 Introduction
2 Thedevelop of Lewis acid and Lewis base polymerization systems
3 The kind of FLP polymerization systems
4 The polymerizationmechanism of FLP polymerization systems
4.1 Chain initiation and chain propagation
4.2 Chain termination
5 Conclusion and outlook
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