Syndiospecific catalysts

In contrast to the case of Cp2ZrX2/MAO giving atactic poly(alkene)s, Cp MCl2/MAO, M = Zr (139) and Hf (140), are the catalyst precursors of the syndiotactic polymerization of 1-butene and propylene [176]. Triad distribution indicated that this is chain-end controlled syndiospecific polymerization. The syndiospecificity is attributed to the increase of steric encumbrance around the metal center. Thus, Cp HfX2 is the most effective syndiospecific catalyst component in this system. 

As regards the polymerisation of higher linear oc-olefins, e.g. 1-butene, with homogeneous vanadium-based syndiospecific catalysts, it rarely occurs and affords only trace amounts of a low molecular weight syndiotactic polymer [394]. 

The same conclusion as in the case of propylene homopolymerisation has been drawn considering IR [396] and NMR [389,395] spectra of ethylene/propylene copolymers obtained with vanadium-based syndiospecific catalysts. The type of propylene insertion depends on the kind of last inserted monomer unit secondary insertion [scheme (40)] occurs more frequently when the last monomeric unit of the growing chain is propylene, while primary propylene insertion [scheme (39)] is more frequent when the last monomeric unit of the growing chain is ethylene [2]. The above explains the microstructure of ethylene/propylene copolymers obtained with vanadium-based Ziegler-Natta catalysts. These copolymers contain both m and r diads when the sequence of propylene units is interrupted by isolated ethylene units i.e. a propylene insertion after an ethylene insertion is substantially non-stereospecific [327,390,397], The existence of a steric interaction between the incoming monomer molecule and the last added monomer unit is also confirmed by the fact that the propagation rate for the secondary insertion of propylene in syndiospecific polymerisation is lower than for primary insertion in non-stereospecific polymerisation [398],... 

Figure 3.33 Possible diastereoisomeric catalytic complexes for the soluble vanadium-based syndiospecific catalysts, with a si chain, presenting suitable orientation of the chain and of the propylene molecule. The corresponding minimum energies are indicated below the models, o - V or Al O Cl C O H - CH3 or CH2. Reproduced by permission from Ref. 328. Copyright 1985 American Chemical Society...

Zirconium-based cationic complexes derived from ZrBz4 and B(CeF5)3 or [Me2N(Ph)H]+[B(C6F5)4]- do not provide active syndiospecific catalysts for styrene polymerisation [70],... 

Similarly, the same catalysts that promote the syndiospecific polymerisation of styrene also polymerise ethylene and a-olefins [106,107], ring-substituted styrenes [6] and conjugated dienes [44,74,108-110], These monomers can also be copolymerised with each other [111-114], Substituted styrenes, which yield syndiotactic polymers by polymerisation run with syndiospecific catalysts, form copolymers with styrene the polymerisation rate increases with increasing nucleophilicity of the comonomer. The random copolymers formed are co-syndiotactic [6,111,112]. 

The different orientation of the monomer in systems with Ziegler-Natta catalysts based on Ti or Nd and Co or Ni precursors has been suggested [41] to be determined by the presence in the isospecific catalysts (Ti- and Nd-based ones) of anionic ligands bound to the transition metal and the absence of any anionic ligand in the syndiospecific catalysts (Co- and Ni-based ones). In the... 

Zam belli and Tosi have extensively studied the stereochemistry of the propagation step in propylene polymerization on Ziegler-Natta catalysts. Specific features of this process are shown in Table 4. Cis-addition of the olefin to the active metal-carbon bond has been observed both in isospecific and syndiospecific polymerization. The olefin addition to the active bond proceeds with the participation of the primary (L,(Mt—CH2—CHR—P) and secondary (L,Mt—CHR—CH2—P) carbon atoms of the growing polymer chain using isospecific and syndiospecific catalysts, respectively. 

Finally, IR spectroscopy contributed much to the finding of Zam-belli et al. (96) that C2-C3 copolymers tend to have an alternating sequence distribution if prepared by syndiospecific catalysts (i.e. catalysts that polymerize propylene to syndiotactic polymer) and a random distribution when prepared by isospecific or non-stereospecific catalysts. 

Table 9. Liquid Propene Pol5Tnerization with Syndiospecific Catalysts ...

The first studies on the dependence of catalyst performance on propene concentration with zir-conocene catalysts have been reported by Ewen, for the syndiospecific catalyst and by Rieger, for... 

Among group 4 metal complexes of formula MXn, tetrabenzyltitanium activated with MAO is the most active and syndiospecific catalyst for styrene polymerization ([r] = 1, sPS% = 93% where sPS% is the percentage of acetone or 2-butanone insoluble fraction in the obtained polymer). The... 

Longo, R Grassi, A. Oliva, L. Copolymerization of styrene and ethylene in the presence of different syndiospecific catalysts. Makromol. Chem. 1990,191, 2387-2396. 

It should be pointed out that, in contrast to the case with the isospecific catalysts, the sense of the enantioface selectivity for olefin insertion has not been determined for the syndiospecific catalysts IV. The model shown below is based on that offered by Corradini and coworkers that the orientation of the growing polymer chain is the primary stereodifferentiating element. ... 

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