Styrene polymerization by nickel and titanium catalysts based on tris(pyrazolyl)borate ligands

Abstract

A polimerização do estireno com uma série de complexos de níquel e titânio contendo ligantes tris(pirazolil)borato estericamente impedidos, Tp Ms = HB(3-mesitila-pirazolil) 3 e Tp Ms* = HB(3-mesitila-pirazolil) 2 (5-mesitila-pirazolil) foi estudada na presença do cocatalisador MAO sob várias condições de polimerização. Todos complexos mostraram-se ativos na polimerização do estireno produzindo quase exclusivamente poliestireno atático. As atividades catalíticas bem como as conversões com os complexos de níquel foram maiores em relação àquelas obtidas com os complexos de titânio. Maiores atividades foram encontradas a 80 °C. As propriedades dos poliestirenos são substancialmente afetadas pelas condições de reação tais como temperatura de polimerização, tempo e razão molar [Al]/ [M]. Os resultados de GPC mostram que o peso molecular (M w ) dos poliestirenos é sensível à temperatura de polimerização variando de 16.300 a 138.300 g mol -1 .Styrene polymerization with a series of nickel and titanium complexes based on sterically hindered tris(pyrazolyl)borate ligands, Tp Ms = HB(3-mesityl-pyrazolyl) 3 and Tp Ms* = HB(3-mesitylpyrazolyl) 2 (5-mesityl-pyrazolyl) have been studied in the presence of MAO cocatalyst under various polymerization conditions. All complexes showed to be active for styrene polymerization producing almost exclusively atactic polystyrene. The catalytic activities as well as the styrene conversions with nickel complexes were higher than those with the titanium analogues. Higher activities were found at 80 °C. Polystyrene properties are substantially affected by the reaction conditions such as temperature of polymerization, time, and [Al]/[M] molar ratio. The GPC results showed that the molecular weight (M w ) of the polystyrene is sensitive to the temperature of polymerization varying from 16,300 to 138,300 g mol -1 .Keywords: nickel, titanium, tris(pyrazolyl)borate ligand, polystyrene IntroductionThe chemistry of the non-metallocene catalysts has grown considerably over the past two decade, largely due to the remarkable variety of non-cyclopentadienyl ligands available and their high-performance in olefin polymerization. 1 Many contributions have thus been made concerning this topic, especially using Group 4 and Group 10 metals for polymerization of ethylene, 2 propylene, 3 and in copolymerization reactions. 4 Among linear α-olefins, special attention has been devoted to the use of styrene as monomer for production of polystyrene considering their interesting chemical and physical properties for industrial applications. The polymerization process of styrene can lead to three tactic structures which are depend on the nature of the catalyst precursor, including isotactic (iPS), syndiotactic (sPS) and atactic (aPS) polystyrenes. 5 In contrast to the well-known isotactic polystyrene, which has a very low crystallization rate and is therefore useless for most industrial purposes, 6 the syndiotactic polystyrene (sPS) shows a fast crystallization rate (more than an order of magnitude higher than that of...