液态29Si NMR研究TEOS/DDS混合体系的氨催化水解动力学

原位引入有机组分对氧化硅体系改性是合成有机-无机杂化硅材料的重要方法.利用原位的29Si液体核磁,研究了甲醇为溶剂、氨水催化条件下的四乙氧基硅烷（TEOS）和二甲基二乙氧基硅烷（DDS）原位共水解的动力学过程.通过改变反应体系中氨和水的浓度,拟合出单体及中间产物浓度随时间的变化曲线,得到了TEOS和DDS各自的水解速率常数以及相应各反应物的反应级数.与单前驱体水解一致的是,在双前驱体系中TEOS和DDS自身的反应级数仍保持一级,但是氨和水的反应级数都有不同程度的增大.与单前驱体水解速率方程相比,混合体系中TEOS的水解速率常数增大.同时,DDS在双前驱体中比单前驱体中的水解速率常数有很大程度的减少.水解动力学表明,TEOS和DDS在双前驱体体系中显示出更平行的水解速率.利用固体29SiMAS NMR,XPS及小角X射线散射（SAXS）手段对双前驱体体系研究得到的信息显示,碱催化条件下原位的TEOS水解中间物与DDS中间产物的原位共缩聚程度很弱.

Liquid-state 29Si NMR Investigation on Ammonia-catalyzed Hydrolysis Kinetics of a Mixture of Tetraethoxysilane and Dimethyldiethoxysilane

In situ liquid-state ~(29)Si NMR was used to follow the ammonia-catalyzed hydrolysis and conden-sation of tetraethoxysilane (TEOS) and dimethyldiethoxysilane (DDS) mixed systems with methanol used as solvent. By varying the molar ratio of TEOS, DDS, water and ammonia in initial solutions, the individual hydrolysis rate constants for TEOS and DDS in mixture were calculated as well as the corresponding reac-tive orders through fitting the concentration curves as functions of time for each intermediate species, which is similar to their individual systems, and under ammonia catalysis, the hydrolysis reaction orders of TEOS and DDS in mixed systems all retained the first-order. The hydrolysis rate constant of TEOS in mixed sys-tems was larger compared with TEOS in single precursor systems. Meanwhile, the hydrolysis rate constant of DDS in mixed system was smaller than the rate constant of DDS in the single DDS system. Another im-portant result was that the reactive orders of both ammonia and water were increased to different extent for TEOS and DDS in mixed systems. Hydrolysis and condensation kinetics show more compatible hydroly-sis-condensation relative rates between TEOS and DDS. Assisted by the solid ~(29)Si MAS NMR, XPS and SAXS, the obtained hybrid gel shows weak cross-condensation.