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Mechanism of the cationic ring opening polymerization of cyclosiloxanes-interpretation of results

机译：环硅氧烷阳离子开环聚合的机理-结果解释

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Mechanism of the cationic ring opening polymerization of cy-closiloxanes is still not fully understood.For example,the role of tertiary silyloxonium ions as the intermediates in this process is still controversial.Formation of trisilyloxonium ions generated from hexamethylcyclotrisil-oxane,D_3,and octamethylcyclotetrasiloxane,D_4,was observed by Si NMR spectroscopy at low temperatures and the ring opening polymerization (ROP) of these monomers in the presence of such ions was demonstrated.Kinetics of the polymerization of cyclic oxymultisilylene,[(Me_2Si)_2O]_2,~2D_2,and the direct observation of trisilyloxonium ions in the polymerization system under low nucleophilic conditions is in favour of this mechanism [equations (3),(4),(8)].The results of sequence analysis in the polymer formed by cationic ROP of cyclotrisiloxanes,containing different siloxane units in the molecule (Markov statistics-Table 1,Schemes B and C),were interpreted in terms of the chain extension mechanism involving reversible deactivation,in which the cyclic trisilyloxonium ion is not a persistent active propagation center but only a transient intermediate in each act of monomer addition.Polymerizations of strained cyclotrisiloxanes (such as D_3),unstrained cyclotetrasiloxanes (such as D_4) and cyclic oxymultisilylenes (such as ~2D_2) seem to proceed according to the common mechanism.The differences in kinetics and thermodynamics of the polymerization of these monomers can be explained by the differences in the ring strain and in the nucleophilicity of these monomers relative to that of the oxygen atoms in a polymer.
机译：环氯硅氧烷阳离子开环聚合的机理尚不完全清楚。例如，叔硅氧鎓离子在此过程中作为中间体的作用仍存在争议。六甲基环三硅氧烷，D_3和八甲基环四硅氧烷生成的三硅氧鎓离子的形成通过Si NMR光谱在低温下观察到D_4，并且证明了这些单体在这些离子存在下的开环聚合（ROP）。环氧基多亚甲硅烷基[（Me_2Si）_2O] _2，〜2D_2的聚合动力学，并且在低亲核条件下在聚合体系中直接观察到三甲硅烷基氧鎓离子有利于这种机理[式（3），（4），（8）]。阳离子ROP形成的聚合物的序列分析结果分子中包含不同硅氧烷单元的环三硅氧烷（马尔可夫统计量，表1，方案B和C），根据涉及以下分子的扩链机理进行了解释：可逆的失活，其中环状三甲硅烷基氧鎓离子不是持久的活性扩散中心，而是在单体添加的每个行为中只是一个过渡中间体。应变环三硅氧烷（如D_3），不应变环四硅氧烷（如D_4）和环状氧代多甲硅烷基（如因为〜2D_2）似乎是按照共同的机理进行的。这些单体聚合的动力学和热力学的差异可以用环应变和相对于氧原子中氧原子亲核性的差异来解释。聚合物。

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