Atom Transfer Reactions: Implications for Polymerization Catalysis

斯坦福大学化学系罗伯特·M·韦茅斯博士在化学部无机化学、生物无机化学和金属有机化学计划的支持下，发展了钛(III)氧化态下明确定义的钛配合物的化学，研究在非极性介质中介导钛(III)和钛(IV)可逆相互转化的原子转移反应，并开发基于钛配合物的在催化过程中改变氧化状态的新催化体系。这些研究针对的是苯乙烯间规聚合的催化剂，以及用于生产苯乙烯和非共轭烯烃嵌段共聚物的新催化剂体系的开发。间规苯乙烯聚合的机理研究将集中在明确定义的钛(III)前驱体对苯乙烯聚合的反应性。将进行苯乙烯和乙烯均聚合实验和苯乙烯/乙烯共聚实验，以测试钛(III)和钛(IV)络合物对苯乙烯和乙烯聚合活性。钛与稳定的氮氧化物如2，2，6，6-四甲基哌啶-N-氧基(TEMPO)的配位化学将作为一种在非极性溶剂中实现钛(III)和钛(IV)可逆相互转化的策略。钛-TEMPO络合物的合成和钛-氮氧键的键能将通过热分解和氮氧化物交换平衡来研究。将研究钛配合物与其他金属卤化物之间原子转移平衡的动力学和热力学，以提供金属-卤素键能量学的基本信息。这些研究将指导选择合适的原子转移氧化还原伙伴来介导钛(III)和钛(IV)配合物的相互转化。聚烯烃和聚苯乙烯是两类重要的聚合物。这项研究旨在开发一种催化剂体系，可以产生同时含有聚烯烃和聚苯乙烯碎片的聚合物。如果每个单独成分的“嵌段”的组成和大小可以控制，这种“嵌段共聚”的性质就可以被精确地设计出来。在这个项目中，将开发新型的钛催化剂，当钛带有3+电荷时，将聚合苯乙烯，而当钛带有4+电荷时，将使烯烃聚合。通过改变金属电荷，可以控制嵌段共聚物的组成。

Dr. Robert M. Waymouth, Chemistry Department, Stanford University, is supported by the Inorganic, Bioinorganic, and Organometallic Chemistry Program of the Chemistry Division to develop the chemistry of well-defined titanium complexes in the Ti(III) oxidation state, to investigate atom transfer reactions that mediate the reversible interconversion of Ti(III) and Ti(IV)in non-polar media, and to develop new catalytic systems based on titanium complexes that change oxidation state during catalysis. These studies are directed at catalysts for the syndiospecific polymerization of styrene as well as the development of new catalyst systems for the production of block copolymers of styrene and non-conjugated olefins. Mechanistic studies of syndiospecific styrene polymerization will focus on the reactivity of well defined Ti(III) precursors for styrene polymerization. Styrene and ethylene homopolymerization experiments and styrene/ethylene copolymerization experiments will be carried out to test the competence of both Ti(III) and Ti(IV) complexes for styrene and ethylene polymerization activity. The coordination chemistry of titanium complexes with stable nitroxides such as 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) will be carried out as a strategy for reversibly interconverting Ti(III) and Ti(IV) in non-polar solvents. The synthesis of Ti-TEMPO complexes and the bond energies of the Ti-nitroxide bonds will be investigated by thermolysis and nitroxide exchange equilibria. The kinetics and thermodynamics of atom-transfer equilibria between titanium complexes and other metal halides will be investigated to provide fundamental information on metal-halogen bond energetics. These studies will guide the choice of appropriate atom-transfer redox partners to mediate the interconversion of Ti(III) and Ti(IV) complexes. Polyolefins and polystyrenes are two important classes of polymers. This research seeks to develop a catalyst system that can produce a polymer that contains both polyolefin and polystyrene fragments. The properties of such "block co-polymers" can be precisely engineered if the composition and the size of the "blocks" of each individual component can be controlled. In this project, new titanium catalysts that will polymerize styrene when the titanium has a 3+ charge and olefins when it bears a 4+ charge will be developed. By altering the metal charge the composition of the block copolymer can be controlled.