Polymerisation isospecific

Y. V. Kissin, Isospecific Polymerisation of Olefins with Heterogeneous Ziegler-Natta Catalysts, Springer-Vedag, New York, 1985. 

It is worth noting that, to obtain efficient supported Ziegler-Natta catalysts of the third and fourth generation, it is TiCU rather than preformed TiCL that is supported. When TiCl3 was supported on MgCl2 (e.g. by deposition from the solution of a TiCl3.3Py complex) and then activated with an alkylaluminium compound, the obtained catalyst appeared to be aspecific for propylene polymerisation [69], However, it could be converted into an isospecific catalyst by adding a Lewis base such as ethyl benzoate [70],... 

Dimeric homochirotopic [rac.-Me2Si(Me3Si, t-BuCp)2YH]2 is the first singlecomponent isospecific catalyst it is suitable to study the subtle steric factors that govern the remarkably high stereospecificities exhibited in the polymerisation of a-olefins by this and related two-component class III catalysts of C2 symmetry based on group 4 metallocenes. 

Cationic [ZrBz3]+ species are capable of polymerising oc-olefins such as propylene, 1-butene and 1-pentene at elevated temperature. It is worth emphasising that the non-metallocene [Zr(CH2Ph)3] + [PhCH2B(C6F5)3] catalyst, which is a cationic arene zirconium complex [189], is capable, at least partially, of isospecific a-olefin polymerisation at relatively high temperature [162,189,190],... 

A number of models for catalytic active sites have been proposed to explain the isospecific polymerisation of propylene in the presence of TiC -based catalysts [68]. It was Natta who proposed [366], for the first time, that the steric control in isospecific propylene polymerisation is caused by the structure of the active species located on the borders of crystal layers of violet titanium trichloride. Arlman and Cossee [277] suggested that the isospecific active sites are located on lateral crystal surfaces, which in violet titanium trichloride such as oc-TiCh correspond to (110) planes. Titanium atoms present on the above-defined fracture surface have a vacant octahedral site bonded to five chlorine atoms (Figure 3.26) [1,68]. 

Remember that the polymerisation mechanism devised by Cossee [268] implies two main steps coordination of the monomer at the titanium vacant site with the double bond parallel to the Ti-C bond, and chain migratory insertion of the coordinating monomer molecule (with migration of the growing polymer chain to the position previously occupied by the coordinating monomer molecule) isospecificity of the active site is assumed only if the polymer chain skips back to the original position before further insertion [scheme (50)]. 

Figure 3.37 Models for the primary insertion of propylene into a polypropylene growing chain in isospecific polymerisation with the (R, R)-rac.-(IndCH2)2ZrX2-based catalyst. The growing alkyl chain occupies an open sector of the ligand framework propylene enters the reaction complex with its methyl substituent away from the Cp atom of the last monomeric unit in the chain (the monomer methyl group is directed towards another open sector of the ligand framework). For the sake of clarity, only the C C bonds are sketched for the n ligands. O - Zr O - C or CH3 o - H. Reproduced by permission from Ref. 30. Copyright 1995 Wiley-YCH Weinheim...

Figure 3.38 Possible model site for isospecific propylene polymerisation with the (R, A>)-/Y/r.-(ThindCH2)2 ligand-containing catalyst. The broken lines indicate the forbidden conformation of the growing chain and the favoured coordination of propylene corresponding to this forbidden chain conformation. For the sake of clarity, hydrogen atoms are omitted. O Zr O C, CH, CFF or CH3. Reproduced (by permission from Elsevier Science) from Ref. 1. Copyright 1989 Pergamon Press...

A detailed molecular mechanics analysis of model catalytic sites has rationalised these observed behaviours for homogeneous propylene isospecific polymerisation catalysts based on bridged rac.-metallocene [357], It is interesting that, in contrast to the discussed case of enantioselectivity of the models for the primary insertion of propylene, the enantioselectivity of the models for the secondary insertion of propylene is due to direct interactions of the... 

At the end of considerations of the stereoregulation mechanism in the isospecific polymerisation of a-olefins with homogeneous catalysts obtained from chiral stereorigid metallocenes of class III with C2 molecular symmetry, let us address isospecific propylene polymerisation catalysts, which are methy-laluminoxane-activated non-bridged metallocenes (Table 3.2). 

Activities of a-olefins in insertion polymerisation with coordination catalysts are generally related to the monomer structure both electronic and steric factors influence the activity of a-olefins in stereospecific polymerisation, but the steric factor primarily influences the rate of polymerisation, which has been especially shown for isospecific polymerisation with heterogeneous catalysts [46, 250],... 

The symmetry properties of cycloaliphatic polymers are such that polymers with certain microstructures, e.g. tram-isotactic poly (methylene-1,3-cyclopen-tane), are chiral therefore, the cyclopolymerisation of a, trans selective catalysts of C2 symmetry, such as methylaluminoxane-activated resolved (li )-(Thind CH2)2Zr l,l -bi-2-naphtholate, yielded optically active tram-isotactic poly(-methylene-1,3-cyclopentane). The cyclopolymerisation with the (15) enantiomer of the catalyst gave an enantiomeric polymer [505], On the basis of analysis of 13C NMR spectra, the degree of enantioface selectivity for this cyclopolymerisation was estimated to be of 91% [503,505]. 

Isospecific Polymerisation of Vinylaromatic Monomers with Coordination Catalysts... 

The isospecific polymerisation of styrene was first reported by Natta et al. [1,2], They obtained isotactic polystyrene, employing for the polymerisation a heterogeneous catalyst derived from titanium tetrachloride and an alkylaluminium compound. 

Subsequent investigations revealed that, in principle, styrene undergoes isospecific polymerisation in the presence of heterogeneous Ziegler Natta catalysts [1-4], Although polystyrene of isotactic structure was also prepared with the use of homogeneous nickel-based coordination catalysts, it appeared to be of low molecular weight [22,23]. 

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