Polypropylene stereochemistry, control

The hypothesis of stereochemical control linked to catalyst chirality was recently confirmed by Ewen (410) who used a soluble chiral catalyst of known configuration. Ethylenebis(l-indenyl)titanium dichloride exists in two diaste-reoisomeric forms with (meso, 103) and C2 (104) symmetry, both active as catalysts in the presence of methylalumoxanes and trimethylaluminum. Polymerization was carried out with a mixture of the two isomers in a 44/56 ratio. The polymer consists of two fractions, their formation being ascribed to the two catalysts a pentane-soluble fraction, which is atactic and derives from the meso catalyst, and an insoluble crystalline fraction, obtained from the racemic catalyst, which is isotactic and contains a defect distribution analogous to that observed in conventional polypropylenes obtained with heterogeneous catalysts. The failure of the meso catalyst in controlling the polymer stereochemistry was attributed to its mirror symmetry in its turn, the racemic compound is able to exert an asymmetric induction on the growing chains due to its intrinsic chirality. 

The stereochemistry of the products is often controlled through control of the reaction temperature. For instance, use of low temperatures, where the alkyl shift and migration is retarded, favors formation of syndiotactic polypropylene (sPP). Commercial iPP is produced at room temperatures. 

Three stereoisomers are possible in the cholestanylindene-derived zir-conocene complexes illustrated in Scheme 67. Two are racem-like, and the other is meso-like depending on the geometry of the metallocene moiety. The stereochemistry of the reaction is controlled by both the structure of the metallocene skeleton and steroidal substituent. Polymerization of propylene with 0-C activated with MAO gave polypropylene of 240,000, about 40% mmmm approximately 70% is due to enantiomorphic site control and the rest is due to chain-end control. Use of the catalyst derived from a /3-A-B mixture produced a mixture of polymers. The a-A and a-B/MAO catalysts afforded isotactic poly-... 

The revolutionary discoveries by Ziegler and Natta, relating to the low pressure polymerization, respectively, of ethylene and of propylene and other a-olefins onto the previously unknown crystalline polymers, opened a new era in science and technology. Since then, remarkable progress has been made in the fields of coordination catalysis, macromolecular science and stereochemistry. With the discovery and development of the new generation catalytic systems for polyethylene in the late 1960 s, and more recently for polypropylene, enormous progress was made in terms of polymerization process as to economics and product quality Further process simplification and, above all, ever more accurate product quality control by taylor made catalytic systems is the aim of the 1980 s. 

Early metal-metallocene-alkene polymerization catalysts permit the synthesis of highly isotactic polypropylene . They rely on controlling the stereochemistry of alkene insertion by the use of chiral C2 symmetric metallocenes . Late metal systems for alkene polymerization , and copolymerization of alkenes and CO , have also been developed. 

Syndiotactic propagation of propylene is know to be catalyzed by homogeneous vanadium catalyst (1 ). In the polypropylene samples prepared with the homogeneous catalysts, the relative population of iso-, hetero- and syndiotactic triads is in accordance with that predicted from the first order Markov model (25, 26). There is no chiral structure around the homogeneous vanadium species. The stereochemistry of the entering monomer is controlled by the chirality of the growing chain end, in contrast with the isotactic propagation. 

Inoue et al. ( ) found that a porphyrin-Zn alkyl catalyst polymerized methyloxirane to form a polymer having syndio-rich tacticity. The relative population of the triad tacticities suggests that the stereochemistry of the placement of incoming monomer is controlled by the chirality of the terminal and penultimate units in the growing chain. There is no chirality around the Zn-porphyrin complex. Achiral zinc complex forms syndio-rich poly(methyloxirane), while chiral zinc complex, as stated above, forms isotactic-rich poly(methyloxirane). The situation is just the same as that for propylene polymerizations. Achiral vanadium catalyst produces syndiotactic polypropylene, while chiral titanium catalyst produces isotactic polypropylene. 

In 1962. Natta and Zambelli reported a heterogeneous. vanadium-based catalyst mixture which produced partially syndiotactic polypropylene at low polymerization temperatures. " The regiochemistry of the insertion was determined to be a 2.1-insertion of propylene, and a chain-end control mechanism determined the s mdiospecificity of monomer insertion. This catalyst system suffered from both low activity and low stereoselectivity. Highly active single-site olefin polymerization catalysts have now been discovered that make syndiotactic polypropylene with nearly perfect stereochemistry. Catalysts of two different symmetry classes have been used to make the polymer, with Cs-symmetric catalysts typically outperforming their Q -symmetric counterparts due to different mechanisms of stereocontrol (Figure 10). 

Also, the chain end of the growing polymer can control the stereochemistry of the polymerization, as demonstrated by Ewen Cp2TiPh2/MAO as a catalyst produces isotactic polypropylene. ... 

Much research has been conducted to generate catalysts that form polypropylenes containing microstructures and architectures that can be rationally controlled. This catalyst development has involved some of the most elegant mechanism-based design of catalysts in any area of organometallic chemistry. This section describes some of the basic principles that explain how the stereochemistry of polyolefins is controlled. 

As far as polymer stereochemistry is concerned, a controversial issue is what should be defined as stereoblock-isotactic Isotactic polypropylene is usually obtained as a result of site control (i.e., the preference of an intrinsically chiral transition metal active species to react with one of the two enantiofaces of the prochiral monomer). In the case of a simple C2-symmetric single-center catalyst with homotopic active sites, if we denote as o the probability that the monomer inserts with a given enantioface at an active site of given chirotopicity, the fractions [m] and [r] of meso and racemo diads in the polymer are given by Equations 8.1 and 8.2... 

Zambelli et al. have studied the effect of incorporating small amounts of ethylene on the stereochemistry of polypropylene prepared with stereoregular catalysts, concluding that for an isotactic catalyst, insertion of ethylene has no effect on the propylene stereochemistry, whereas for a syndiotactic catalyst, the stereochemistry alters. For the isotactic case therefore, the stereochemistry is controlled by the catalyst, a conclusion also reached by Sanders and Komoroski. However Tonelli has questioned the deduction of Zambelli et al., on the basis that chemical shift calculations suggest that the chemical shift of the central ethylene unit in a PPEPP sequence used by Zambelli et al. is in fact independent of the adjacent propylene stereochemistry, and cannot give information on the mechanism. 

Control of polymer stereochemistry is a major research area in academic and industrial laboratories. This is because polymers with different stereochemistries often have very different properties. For example, atactic polypropylene is a gummy, sticky paste sometimes used as a binder, while isotactic polypropylene is a rugged plastic used for bottle caps. Recent advances (see the Going Deeper highlight on the next page and Chapter 13) have greatly improved the ability to control polymer stereochemistry, leading to commercial production of new families of polymers with unprecedented properties. 

This process is often used, e.g. in Ziegler-Natta type polymerisations of ethylene and propylene where the catalyst is supported on inert silica particles so the reaction therefore takes place at the surface. This helps control the stereochemistry (especially for isotactic polypropylene). 

The focus of this section is the polymerization stereocontrol made possible by ligand modification of metallocenes. Thus, propylene will be the monomer of focus since the stereochemistry of polypropylene is the best tmderstood of any polyolefin," if not of any synthetic polymer ever studied. The observed correlation between a catalyst s stmcture/symme-try and a catalyst s stereoselectivity is often referred to as Ewen s Symmetry Rules.Metallocenes have been manipulated to a remarkable degree to direct the enantiomorphic site control mechanism for polymerization stereoselectivity. ... 

Syndiotactic polypropylene is formed by the nickel(II)-diimine complex 67 (M=Ni) at low temperature ([rrrr]=0.80 at -78 °C,0.65 at 0 °C) [204,205]. Polymerization proceeds by 1,2-insertion and the stereochemistry is regulated under chain-end control. On the other hand, isotactic polypropylene can be prepared using the iron complexes 68 (M=Fe [mmmm]=0.55-0.67 at -20 °C) despite the low molecular weight of the polymer [206]. Polymerization proceeds via a 2,1-insertion mechanism by chain-end control. 

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