摘要
含碳官能基的聚硅氧烷除本身具有许多优良性能外，还可利用其官能基的反应活性，将聚硅氧烷引入其他聚合物，以形成具有特殊性能的各种聚合物，是一大类在许多领域发挥重大作用并迅速发展的有机硅聚合物，也是近来比较活跃的研究领域。然而，许多碳官能基的引入使得聚硅氧烷的热稳定性降低，影响了在某些方面的使用。研究搞清该类聚合物的热降解规律及其影响因素，对于控制热降解、更好的利用该类化合物以及改进和研制具有更优异性能的聚合物具有十分重要的意义。
本文使用密度泛函理论(DFT)和过渡态理论(TST)，以含有2-4个硅氧链节的二甲基硅氧烷和羟烷基二甲基硅氧烷的低聚体为模型，在B3LYP／6-311++G(3df，3pd)／／B3LYP／6-31G(d)水平上，首次对羟丙基聚二甲基硅氧烷、羟乙基聚二甲基硅氧烷和端羟基聚二甲基硅氧烷的热降解反应以及连结在不同原子上的甲基和苯基对热降解反应的影响进行了系统的理论研究。揭示了反应机理，进行了动力学和热力学分析。
主要成果有：
一、首次对α-硅醇的热重排反应进行了理论研究。结果表明：
1、硅基甲醇H_3SiCH_2OH在加热条件下可发生两种dyotropic重排反应。一种是Bro-ok重排反应，硅基由碳原子向氧原子迁移，同时氧上的氢原子向碳原子迁移，经由两个三元环构成的具有桥环结构的过渡态，生成甲氧基硅烷；另一种是羟基迁移反应，羟基由碳原子向硅原子迁移，同时硅上的一个氢原子向碳原子迁移，经由两个三元环构成的具有桥环结构的过渡态，生成甲基硅醇。两个反应过渡态中的硅均为五配位。在G3水平上两反应的活化能分别为344.5和236.8kJ／mol。动力学和热力学均有利于羟基重排反应。当硅基甲醇加热时，重排的主要产物是甲基硅醇。
2、连接在中心碳原子上的乙烯基可显著降低羟基迁移反应的活化能并增大反应速率，但是对Brook重排反应几乎没有影响。
3、连接在硅原子上的烃基可显著降低Brook重排反应的活化能并增大反应速率，对羟基迁移反应几乎没有影响。隧道效应对所研究反应的速率没有数量级水平的影响。
4、在MP2(full)／6-31G(d)、MP2(full)／6-311G(d，p)、B3LYP／6-31G(d)和B3LYP／6-311++G(3df，3pd)／／B3LYP／6-31G(d)等理论计算模型中，B3LYP／6-311++G(3df，3pd)／／B3LYP／6-31G(d)对反应活化能的预测值与G3水平的计算值最接近。
In spite of possessing a variety of unique and superior properties, carbofounctionalized polysiloxanes play important roles in modifitions of organic polymers, such as the copolymerization of polysiloxanes and organic polymers, with the aid of the reaction activity of the carbofunctional groups, to endow the organic polymers with the excellent properties of the polysiloxanes. However, because of the decomposition or rearrangement under certain conditions, their applications are limited to some extent. Understanding their rearrangement conditions and mechanisms will be of great important values for the control and application of these compounds.
In this dissertation, theoretical studies on thermal degradation or rearrangement reactions of hydroxylpropyl polydimethylsiloxanes, hydroxylethyl polydimethylsioxanes and hydroxyl-ended poly-dimethylsiloxanes and effects of methyl and phenyl attached to different atoms on the reactions were carried out by employing polydimethylsiloxane, hydroxylpropyl polydimethyl sioxane or hydroxylethyl polydimethylsioxanes oligomers containing three or four silicon atoms as model compounds. For all reactions under study, the reaction mechanisms were revealed through Density Functional Theory (DFT) and molecular orbital theory at the B3LYP/6-311++G(3df,3pd)//B3LYP/6-31G(d) level, and reaction rate constants were evaluated by using Transition State Theory (TST) with direct dynamic method. The important and valuable results are summarized as follows: 1. On the thermal rearrangement reactions of α-silylalcohols
(1) Two dyotropic reactions may occur when α-silylalcohols are heated. One is via the Brook rearrangement reaction, where the silyl group migrates from carbon atom to oxygen atom coupled with a simultaneous migration of a hydrogen atom from oxygen atom to carbon atom passing through a double three-membered ring transition state, forming alkyloxysilane. In the other rearrangement, the reactant undergoes a hydroxyl group migration from carbon atom to silicon atom coupled with a simultaneous migration of a hydrogen atom from silicon atom to carbon atom via a double three-membered ring transition state, forming alkylsilanol. The hydroxyl group migration reaction is preferred to the Brook rearrangement reaction both thermodynamically and kinetically.
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