Copolymerization of ethene with styrene using CGC catalysts: the effect of the cyclopentadienyl ligand substitution on the catalyst activity and copolymer structure

Abstract

The ethene–styrene copolymerization has been investigated using the dimethylsilylene-bridged (amidocyclopentadienyl)dichlorotitanium(IV) complexes [TiCl₂{η⁵-1-(SiMe₂Nt-Bu-κN)-2,3,4-Me₃-5-R-C₅}], where R = Me (1), H (2), Bu (3), Ph (4), 4-fluorophenyl (5), and but-2-en-2-yl (6) in combination with methylalumoxane (MAO) as catalysts. The nature of the substituent R strongly influenced the catalyst activity and selectivity and the copolymer microstructure and molecular weight. The catalysts derived from 1 to 3 were by about one order more active than those derived from 4 to 6. At the optimum Al/Ti molar ratio of 900, the highly active catalysts produced a pseudo-random copolymer (95–97 wt.%) containing up to 47.8 mol% of incorporated styrene. The low-active catalysts gave mixtures of a pseudo-random copolymer (76–85 wt.%) with polyethene (10 wt.%) and polystyrene sequences (3–7 wt.%). The X-ray diffraction crystal structures of 2 and 4 were determined. Comparison of crystal structures of 1 and 2 versus 4 and 5 revealed a slightly shorter distances Ti–Cg (Cg – centroid of the cyclopentadienyl ring) and slightly larger Cl–Ti–Cl angles in 1 and 2, indicating a higher electron density at the titanium atom. An electron attracting effect of phenyl or alkenyl substituents as well as their steric hindrance can account for a low catalytic performance of 4–6/MAO catalysts.

Introduction

A new class of highly active catalysts for polyalkene production emerged with the synthesis of (tert-butylamido)dimethyl(2,3,4,5-tetramethylcyclopentadienyl)silane which affords the η⁵:η¹(N)-dianionic ligand [1] and, subsequently, ansa-{η⁵:η¹(N)-1-[(tert-butylamido)dimethylsilyl]-2,3,4,5-tetramethylcyclopentadienyl}dichlorotitanium(IV) (1) [2]). These catalysts, called “constrained geometry catalysts” (CGC), arise from mixing of the ansa-amidosilylcyclopentadienyl-titanium or -zirconium dichlorides with excess methylalumoxane (MAO) [3]. Compared to titanocene or zirconocene-based single-site catalysts they provide more acidic and less sterically encumbered cationic centres [4] and, consequently, display very high activities in the polymerization of ethene, propene and in copolymerizations of ethene with terminal alkenes, cycloalkenes and styrene [5]. In attempts to tune the polymerization properties the parent CGC complex 1 was modified by replacing the cyclopentadienyl ligand by indenyl or fluorenyl ligands [6], and their rings [7] as well as the amide groups were variously substituted [8]. Recently, an effective route to CGC complexes substituted on otherwise fully methyl-substituted cyclopentadienyl ring in vicinal position to the ansa-bridge was developed affording a series of racemic complexes [TiCl₂{η⁵:η¹(N)-C₅(1-SiMe₂Nt-Bu-2,3,4-Me₃-5-R)}], where R = H (2), Bu (3), Ph (4), 4-fluorophenyl (5), and but-2-en-2-yl) (6) [9], [10]. The substituent R has been shown to considerably modify the activity of CGC catalysts in the polymerization of ethene [11] and propene [10], and to modify molecular weights of the polymers. Polydispersities of both the polymers were close to 2, indicating a single-site character of these catalysts. NMR analyses of these polymers revealed a low branching in polyethene [11] and a low content of syndiotactic attachments in largely atactic polypropene [10].

The catalytic copolymerization of ethene and styrene which is only poorly catalyzed by titanocene-based catalysts [12] has been successfully conducted by CGC catalysts achieving a high styrene incorporation (up to 37%) [3], [13]. Particularly, the copolymerization using the parent 1/MAO catalyst was investigated in detail and the copolymers were fully characterized [14]. The 1/MAO catalyst appeared to be more active than the catalysts based on indenyl, fluorenyl or benzylamido derivatives presumably due to a higher electron density induced by fully methyl-substituted cyclopentadienyl ligand in 1 [15].

Here we report the catalytic copolymerization of ethene and styrene as depending on the substituent R in the series of catalysts 1–6/MAO under optimized conditions.