Styrene polymerization syndiospecific

The mechanisms of stereoselectivity which have been proposed for chain-end stereocontrolled polymerizations involving secondary monomer insertion also present a general pattern of similarity. In fact, molecular modeling studies suggest that for olefin polymerizations (both syndiospecific and isospecific, Section 4.1.2) as well as for styrene polymerization (syndiospecific, Section 4.2), the chirality of the growing chain would determine the chirality of a fluxional site, which in turn would discriminates between the two monomer enantiofaces. 

Luo, Y.J., Baldamus, J., and Hou, Z.M. (2004) Scanditrm Ualf-metallocene-catalyzed syndiospecific styrene polymerization and styrene-etUylene copolymerization unprecedented incorporation of syndiotactic styrene-styrene sequences in styrene-etUylene copolymers. Journal of the American Chemical Society, 126, 13910. 

Zambelli et al. reported on the mechanism of styrene polymerization [36]. They showed that the main chain of the syndiotactic polymer has a statistically trans-trans conformation. It was established then the double-bond opening mechanism in the syndiospecific polymerization of styrene involves a cis opening. The details in the control of the monomer coordination for this polymerization mechanism were examined by Newman and Malanga using detailed, 3C NMR. It was shown through the analysis of tacticity error (rmrr) that the tacticity in the polymer is chain-end controlled and that the last monomer added directs the orientation and coordination of the incoming monomer unit prior to insertion [37]. 

Soon after syndiospecific styrene polymerization, attention was directed to the homopolymerization of substituted styrenes as well as to their co-polymerization with styrene.956,957,964,1027-1029 Mono-Cp-based Ti systems are capable of homopolymerizing methyl-substituted styrenes and />-chlorostyrene, as well as co-polymerizing them with styrene. The general trend that emerged is that electron-withdrawing Cl substituents decrease the reactivity relative to styrene, whereas electron-releasing Me groups increase it. In both cases, syndiotactic co-polymers were obtained. 

CpTiCl3/MAO exhibits high catalytic activity not only for syndiospecific styrene polymerization but also for the polymerization of conjugated dienes,... 

Mono-Cp (single-ring)-type titanium complexes Cp TiXs (X = Cl, CH3, CH2Ph, BuO) when activated with MAO, B(C6F5)3, or Ph3C+B(C6F5)4 catalyze rapid syndiospecific styrene polymerization to produce highly syndiotactic polystyrene.2 3-248 gg. 

Maron and coworkers [180] reported that theoretical methods were used to investigate the syndiospecificity of the styrene polymerization catalyzed by single-site, single-component allyl ansa-lanthanidocenes ... 

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... 

Very recently, Hou and coworkers have reported that rare earth half-metallocene complexes (Figure 14.16, 85-88) activated with [Ph3C] [B(C6F5)4] afford highly active systems for syndio-specific styrene polymerization, producing sPS with high syndiotacticities rrrr > 99%) and rather narrow polydispersities (Mw/Mn = 1.29-1.55). The activity of scandium complex 85 is comparable with that for the most active titanium catalysts (1.36 x 10 g sPS/(mol Sc-h)). The neutral allyl lanthanide complexes 89-92 (Figure 14.16) in the absence of a cocatalyst are also active for the syndiospecific polymerization of styrene (rrrr > 99%), but with lower activities that are in the order... 

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. Zambelli, A. Reductive decomposition of cationic half-titanocene(IV) complexes, precursors of the active species in syndiospecific styrene polymerization. Organometallics 1996, 15, 480-482. 

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. 

Grassi, A. Saccheo, S. ZambeUi, A. Reactivity of the [(r -C5Me5)TiCH3][RB(C6F5)3] complexes identified as active species in syndiospecific styrene polymerization. Macromolecules 1998, 31, 5588-5891. 

Mahanthappa, M. K. Waymouth, R. M. Titanium-mediated syndiospecific styrene polymerizations Role of oxidation state. J. Am. Chem. Soc. 2001,123, 12093-12094. 

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. 

The electron spin resonance (ESR) spectra of the toluene mixtures of the different titanium compounds, MAO, and TIBA are the same (Rg. 2.5). The active site of the syndiospecific styrene polymerization is the same after the reaction of the titanium compounds with TIBA. 

The different transition metals for the syndiospecific polymerization of styrene were summarized. Compounds of titanium with one cyclopentadienyl ligand show a high performance for the SPS production. Transition metals are stabilized by cyclopentadienyl ligands with electron-releasing substituents, and bulky substituents decrease the catalytic activities. o-Bonded groups at the transition metal complex are substituted by other groups by MAO or TIBA and showed comparable catalytic activities in the syndiospecific styrene polymerization [31]. 

In case of borate as cocatalyst, the catalytic activity of the titanium complex with a pentamethylcyclopentadienyl hgand is high, but a titanium complex with a cyclopentadienyl ligand without any substituents is not active for the syndiospecific styrene polymerization. The reason is that the reaction product of the borate and the cycopentadienyltitanium compound is unstable. The stability of the active site with the borate compound is lower in comparison to that with MAO. The reaction of CH2(Cp)2Ti(Me)2 with dimethylanilinium tetrakis(pentafiuorophenyl)borate or tris(pentafluorophenyl)borane in an equimolar mixture has been examined by Miyashita, Nabika, and Suzuki [11]. Two types of methylene bis(cyclopentadienyl)titanium ion complexes were isolated (see Fig. 3.6). These complexes were active in the polymerization of styrene, but only atactic polystyrene was formed. 

The cocatalysts for the syndiospecific polymerization of styrene were summarized. MAO and borate or borane compounds are useful cocatalysts for the syndiotactic styrene polymerization. There is an optimum molecular weight of MAO with regard to the polymerization activity of the transition metal complex, whereas TMA as an impurity in MAO reduces the activity of the catalyst complex [18]. The performance of MAO and borane compounds as cocatalysts can successfully be enhanced by the addition of selected new chemicals. 

The analysis of experimental data and the theoretical and kinetic analyses are summarized and the mechanisms of the syndiospecific styrene polymerization are clarified. 

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... 

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