Stereochemistry of the Styrene Insertion

It was concluded that the double-bond opening mechanism in syndiospe-cific polymerizations of styrene is the cis opening. This is the same opening mechanism as for the syndiospecific polymerization of propylene using the VCl4/Al(C2H5)2Cl catalyst system at low temperatures [8], 

The regiochemistry of the insertion can, in principle, proceed in two ways, either primary (Eq. 4.1) or secondary (Eq. 4.2)  

The polymerization of propylene and/or higher a-olefins with heterogeneous Ziegler-Natta catalysts proceeds by a primary insertion with occasional errors [9]. Vanadium complexes produce syndiotactic polypropylene by secondary insertion [10]. 

The phenyl group of styrene is a bulky group and is an electron-withdrawing substituent. Styrene behaves differently from an alkyl-olefin with respect to its polymerization reaction. The possible insertion process for styrene into a metal-methyl bond, which is presumed to arise from the methylation of the titanium by MAO in syndiospecific catalyst systems, may be formulated as a primary insertion (Eq. 4.3) or as a secondary insertion (Eq. 4.4)  

The C-NMR spectrum of the low-molecular-weight SPS formed in the reaction shows additional resonances in the region of the saturated carbons at 5 = 37.0 and 21.0ppm. These resonances can be attributed to C(l), C(2) of C(1)H(C6H6)C(2)H3 (Fig. 4.4). 

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STEREOCHEMISTRY OF THE STYRENE INSERTION 45 TABLE 4.1 Chemical Shifts and Coupling Constants of Copolymers A and B... 

Zambelli et al. [12] independently studied the stereochemistry of the styrene insertion catalyzed by the Ti(benzyl)4/MAO system. The C-NMR spectrum of the polymer obtained showed that the chain end of the polymer was consistent with a secondary insertion mechanism both in the initiation steps and in the propagation steps even when the former occurs via M-H bonds. They also showed that most of the polymer chains were initiated by M-H bonds. 

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