Syndiospecific catalysts/polymerization

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. 

We and others have revealed that syndiospecific propylene polymerization is exclusively initiated by 1,2-insertion followed by 2,1-insertion as the principal mode of polymerization [64]. This is the first example of a predominant 2,1-insertion mechanism for chain propagation exhibited by a group 4 metal-based catalyst. The unusual preference for 2,1-regiochemistry displayed by the Ti-FI catalysts compared with the Zr- and Hf-FI catalysts is apparently inconsistent with the crys-tallographically characterized structures, which indicate that the Ti is shielded more by the phenoxy-imine ligands and thus possesses higher steric compression. The reason for the unusual preference in the regiochemistry of Ti-FI catalysts is unclear at the present time. 

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. 

Substituted styrenes with linear alkyl substituents (C6 to Ci2) at the para-pos i on on the phenyl ring have been polymerized using Gp Ti(OMe)3/MAO as catalyst,953 and the kinetics of the syndiospecific slurry polymerization of styrene in heptane with this catalyst has been investigated.954... 

Table 19 Syndiospecific propylene polymerization catalyzed by bis(phenoxy-imine) catalysts... 

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. 

The pooled library of salicylaldimine-titanium products was combined with MAO and 150 ml toluene in a pressure vessel, and screened at 20 °G under 2.7 atm of propylene for 6h. Although 90% of the polymers produced were determined to be atactic through extraction experiments with diethylether, the remaining 10% of the polymers produced were found to contain insoluble syndiotactic polypropylene. Small subsets of the complex libraries were then resynthesized and screened in order to ascertain the particular metal-ligand combination responsible for the stereocontrol. The authors concluded the study by identifying and isolating a new syndiospecific propylene polymerization catalyst 2. 

J.E. Kemnitzer, S.P. McCarthy, R.A. Gross, Syndiospecific ringopening polymerization of beta-butyrolactone to form predominantly syndiotactic poly (beta-hydroxybutyrate) using tin (TV) catalysts. Macromolecules 26 (23) (1993) 6143-6150. 

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... 

Kaminsky, W. Lenk, S. Scholz, V. Roesky, H. W. Herzog, A. Fluorinated half-sandwich complexes as catalysts in syndiospecific styrene polymerization. Macwmolecules 1997, 30, 7647-7650. 

Grassi, A. Pellecchia, C. Oliva, L. Laschi, F. A combined NMR and electron spin resonance investigation of the (C5(CH3)5)Ti(CH2C6H5)3/B(C6F5)3 catalyst system active in the syndiospecific styrene polymerization. Macromol. Chem. Phys. 1995,196, 1093-1100. 

Grassi, A. Lamberti, C. Zambelli, A. Mingozzi, I. Syndiospecific styrene polymerization promoted by half-titanocene catalysts A kinetic investigation providing a closer insight to the active species. Macromolecules 1997, 30,1884-1889. 

Minieri, G Corradini, R Guerra, G ZambeUi, A. Cavallo, L. A theoretical study of syndiospecific styrene polymerization with Cp-based and Cp-free titanium catalysts. 2. Mechanism of chain-end stereocontrol. Macromolecules 2001, 34,5379-5385. 

Syndiospecific catalyst systems for styrene polymerization which are composed of several titanium or zirconium compounds and methylalumoxane (MAO) as a cocatalyst have been reported by several authors (i.e., Ti(0R), >2) zr(0R)4, TiCl4,2) Cp2TiCl2 (Cp=cyclopentadienyl), CpTiCl3,2) TiBz " (Bz=benzyl), or Zr(Bz)4. " However, as far as we know, the copolymerization of styrene and olefin in the presence of these catalyst systems has not yet been reported. 

Catalysts which are isospecific for the polymerization of propylene, butene-1, etc., can also be used for the preparation of copolymers of these olefins with ethylene and with each other. There is much evidence to indicate that the chiral specificity of the catalyst is retained during these reactions [72-76], Thus, the spectra of ethylene(e)-propylene(P) copolymers prepared with an isospecific catalyst system are much simpler than those prepared with a nonspecific catalyst [75]. Studies on copolymers of propylene with c-enriched ethylene prepared with an isospecific catalyst indicate that the propylene-ethylene-propylene triads have a single type of configuration, which is probably meso. This is indicated by the fact that only two resonances are observed for ethylene units in P-E-P triads. Furthermore, there is no evidence for the presence of ethylene units flanked by two tertiary carbon atoms (due to inversions of propylene units) in the copolymers. In contrast, copolymers prepared from nonspecific or syndiospecific catalysts have been shown to contain ethylene units centered in both meso- and racemic- PEP... 

Saito, J., Mitani, M., Onda, M. etal. (2003) Syndiospecific living polymerization of propylene with flutnine-containing titanium FI catalyst. Science and Technology in Catalysis, 145,515 516. 

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) ... 

Half-titanocenes such as Cp TIF3, Cp Ti(OMe)3, and IndTlCb are efficient catalyst precursors for syndiospecific styrene polymerization, as described above [9-11]. However, these catalyst precursors showed low catalytic activities and the resultant polymers in the ethylene/styrene copolymerization afforded a mixture of PE, syndiotactic polystyrene (SPS), and the copolymer... 

Linked half-titanocenes (so-called constrained geometry-type titanium complexes) like [Me2Si(C5Me4)(N Bu)]TiCl2 have also shown to be efficient catalyst precursors [2,16-19], although they have shown extremely low activity for syndiospecific styrene polymerization [16a,19a]. It has also been reported that styrene incorporation by linked Cp-amide Ti catalyst (constrained geometry... 

Based on the above results, modified half-titanocenes, Cp"Ii(L)X2, are better catalyst precursors for the synthesis of ethylene/styrene copolymers with high yields, and both the activity and the styrene incorporation are highly dependent upon the kind of ligands used (both cyclopentadienyl and anionic donor). These complexes are also effective not only for ethylene polymerization but also for syndiospecific styrene polymerization, and the results stand in unique contrast to those with ordinary half-titanocenes, Cp TiXs [9-12], These catalysts may be suitable for mechanistic study on syndiospecific styrene polymerization as well as copolymerization. 

The exclusive formation of copolymers without formation of SPS as a byproduct was observed with the introduction of ethylene into a solution of syndiospecific styrene polymerization using Cp TiCl2(0-2,6- Pr2C6H3)-MA0 catalysts (Scheme 5.7). Note that the catalytic activities and the M values as well as the styrene contents in the latter copolymerizations were identical to those in independent runs, and the results clearly indicate that the catalyticaUy active species for the syndiospecific styrene polymerization can be tuned to the active species for copolymerization [41]. In contrast, styrene polymeriza-... 

Rare-Earth Metal Complexes as Catalysts for Syndiospecific Styrene Polymerization... 

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