基于氮杂环烯烃的Lewis酸碱对催化二氢香豆素基聚酯合成的机理研究

Mechanistic Study on the Synthesis of Dihydrocoumarin-based Polyester Catalyzed by N-Heterocyclic Olefins-based Lewis Pairs

通过选用氮杂环烯烃(NHO)和三乙基硼(TEB)作为二元协同催化剂,实现了3,4-二氢香豆素(DHC)与环氧化合物在Lewis酸碱对聚合体系作用下的交替共聚.对比研究了酸碱催化剂的取代基变化对于催化活性、交替共聚的化学选择性、环氧开环的区域选择性的影响;另外,通过活性物种结构表征、MALDI-TOF MS端基分析以及核磁反应等,深入研究了NHO的环外双键不同甲基取代情况对于聚合机理的影响,从而筛选获得具有独特的非对称双头引发能力的NHO2-DHC/TEB体系,且证明了该体系同时可以实现分子内酯交换反应的完全抑制,避免了环状聚酯副产物的产生,从而实现双头引发获得DHC基线形聚酯材料可控合成,为聚酯基三嵌段聚合物、热塑性弹性体等高效合成提供基础.

To alleviate the energy and environment pressures resulted from the consumption of petroleum-based polymers,it is of great significance to synthesize the bio-based polyesters with degradability,sustainability and environmental friendliness.Various types of polymerization methods have been developed to accomplish such synthesis.Here it is shown that Lewis pair composed of N-heterocyclic olefin(NHO)and organic boron to synergistically catalyze the alternating copolymerization of 3,4-dihydrocoumarin(DHC)and epoxide by Lewis pair polymerization(LPP)for the first time.The effects of the substituents of Lewis acids(LA)and Lewis bases(LB)on the catalytic activity,chemoselectivity of alternating copolymerization versus homopolymerization of epoxides,regioselectivity of epoxides have been systematically studied.It turned out that the weaker acidity of BEt3is beneficial for the alternating copolymerization activity relative to BPh3and B(C6F5)3.After heating at80-100°C,such LPP system can achieve 52%-97%conversion of DHC in 12 h,89%-100%polyester selectivity and 93%-97%regioselectivity.In combination with the experimental details including the identification of active species and mono-initiator,MALDI-TOF MS analysis of low molecular weight polyesters,and NMR reactions,the influences of exocyclic double bond of NHO with varying methyl substitution on polymerization mechanism were studied thoroughly.It turned out that NHO1 with two methyl groups substituted at the exocyclic double bond initiates the polymerization via alkaline,whereas NHO2 with one methyl group substituted at the exocyclic double bond initiates the polymerization via nucleophilicity.It is noted that the NHO2-DHC/TEB LP exhibit a unique,asymmetric dual initiation capability of synthesizing linear polyesters.Due to the complete inhibition of intramolecular transesterification by NHO2-DHC/TEB LP,there is no formation of cyclic polyester by-products,thus realizing the controllable synthesis of dual-initiated linear polyesters.This strategy provides important experimental and theoretic foundation to the rapid synthesis of DHC related polyester-based triblock polymers,thermoplastic elastomers and etc.