Stereochemical control mechanisms polymerization

Ewen, J.A. Elder, M.J. Jones, R.L. Haspeslagh, L. Atwood, J.L. Bott, S.G. Robinson, K. Metallocene/polypropylene structural relationships implications on polymerization and stereochemical control mechanisms. Macromol. Symp. 1991, 48, 253. 

In this contribution we present the crystal structure of iPr[CpFlu]Zr(CH2>2> bulk and slurry propylene polymerization results with MAO as a cocatalyst, polymer C-13 NHR data, and kinetic or statistical models that relate the spp polymer microstructures to the structure of the metallocene and generally accepted mechanisms of stereochemical control and polymerization. 

A model assuming that Cp substituents distal to the bridge experience steric non-bonded contacts with the monomer methyl group, perhaps mediated by the chain end, accounts for the specificity of the chiral metallocenes that produce isotactic, atactic, syndiotactic, hemiisotactic, and random or block cotactic polypropylenes. The tacticities as well as the microstructures of these polymers are accomodated by these simple concepts, the geometry of the metallocene ligands, and by generally accepted fundamental aspects of the polymerization and stereochemical control mechanisms. 

Analysis of the poly(methyl methacrylate) sequences obtained by anionic polymerization was undertaken at the tetrad level in terms of two different schemes (10) one, a second-order Markov distribution (with four independent conditional probabilities, Pmmr Pmrr, Pmr Prrr) (44), the other, a two-state mechanism proposed by Coleman and Fox (122). In this latter scheme one supposes that the chain end may exist in two (or more) different states, depending on the different solvation of the ion pair, each state exerting a specific stereochemical control. A dynamic equilibrium exists between the different states so that the growing chain shows the effects of one or the other mechanism in successive segments. The deviation of the experimental data from the distribution calculated using either model is, however, very small, below experimental error, and, therefore, it is not possible to make a choice between the two models on the basis of statistical criteria only. 

Anionic ring-opening polymerization of l,2,3,4-tetramethyl-l,2,3,4-tetraphenylcyclo-tetrasilane is quite effectively initiated by butyllithium or silyl potassium initiators. The process resembles the anionic polymerization of other monomers where solvent effects play an important role. In THF, the reaction takes place very rapidly but mainly cyclic live- and six-membered oligomers are formed. Polymerization is very slow in nonpolar media (toluene, benzene) however, reactions are accelerated by the addition of small amounts of THF or crown ethers. The stereochemical control leading to the formation of syndiotactic, heterotactic or isotactic polymers is poor in all cases. In order to improve the stereoselectivity of the polymerization reaction, more sluggish initiators like silyl cuprates are very effective. A possible reaction mechanism is discussed elsewhere49,52. 

When the stereocontrol occurs by a chain end control mechanism, a stereochemical defect results in the stereochemistry of the defect being propagated along the chain until the next defect occurs (polymer 1 in Fig. 7). If a stereochemical defect occurs in a polymerization using an initiator that exhibits enantiomeric site control, the mistake will be rectified with the next incoming lactide unit (polymer 2 in Fig. 7). This is because it is the chirality of the metal centre which determines the PLA tacticity and not that of the last inserted lactide unit. 

J. A. Ewen, Mechanisms of Stereochemical Control in Propylene Polymerizations with Soluble Group 4B Metallocene/Methylalumoxane Catalysts, J. Am. Chem. Soc. 106, 6355-6364 (1984). 

It is infonnative to consider how tacticity arises in terms of the mechanism for propagation. The radical center on the propagating species will usually have a planar sp configuration. As such it is achiral and it will only be locked into a specific configuration after the next monomer addition. This situation should be contrasted with that which pertains in anionic or coordination polymerizations where the active center is pyramidal and therefore has chirality. I his explains why stereochemical control is more easily achieved in these polymerizations. 

When the counterion is strongly coordinating with the active center of polymer s terminal unit and the incoming monomer, isotactic polymer wiU be more favored. This task is hard to achieve when nonpolar monomers are incorporated. Polarity is attained when cationic or anionic polymerization mechanism is adopted. It is also useful to employ nonpolar solvent at low-temperamre conditions. Polar solvents would disrupt coordination and consequentiy lose stereochemical control leading to syndiotactic or atactic polymer. 

The catalytic systems for syndiotactic polymerization of meso- K proposed and investigated in recent years are summarized in Table 6.2. Although the approach has only been applied successfully in olefin s syndiotactic polymerization, the SCM route can be used in some other monomers syndiotactic polymerizations with a high degree of stereochemical integrity. Since the chain-end control mechanism cannot accurately explain stereochemistry in metal alkoxide initiating LA polymerizations [18, 19], many studies have focused on the initiators related to the site control mecha-nism[13,14,16,17,29-31]as shown in Table 6.2. The chain-end control mechanism has also been reported [15]. 

CEM a route of syndiotactic polymerization that is based on the chain-end control mechanism, where the last stereocenter of the growing polymer dominates the stereochemical outcome during monomer addition process SCM a recent strategy related to application of Cs-symmetric catalysts, where regularly alternating monomer insertion on enantiotopic coordination sites forms syndiotactic polymers depending on the site control mechanism. 

Prior to the mid-1980 s, catalysts formed using achiral CpaMCb precursors were found to produce only atactic polypropylene (which, incidentally cannot be obtained in the pure form directly from heterogeneous catalysts). In 1984, Ewen reported the use of metallocene-based catalysts for the isospecific polymerization of propylene.38 The polymerization of propylene at -45°C using a Cp2TiPh2 (I,Fig.4) / MAO catalyst system produced a partially isotactic polymer with an mmmm pentad content of 52% (versus 6.25% for a purely atactic polymer). NMR analysis of the polymer revealed the stereochemical errors mmmr and mmrm in the ratio of 1 1, which is indicative of a stereoblock microstructure (Fig.5). Such a structure is consistent with a chain-end control mechanism,39 where the stereocenter of the last inserted monomer unit provides... 

At the same time, the fact that the homogeneous catalyst precursors are structurally well-defined has provided an extraordinary opportunity to investigate the origin of stereospecificity in olefin polymerization at a level of detail that was difficult if not impossible with the conventional heterogeneous catalysts. For example, NMR analysis of the isotactic polymer produced with HI revealed the stereochemical errors mmmr, mmrr, and mrrm in the ratios of 2 2 1 (Fig.5). This observation is consistent with an enantiomorphic site control mechanism, where the geometry of the catalyst framework controls the stereochemistry of olefin insertion.6 30,31 These results established unambiguously a clear experimental correlation between the chirality of the active site, which could be established by x-ray crystallography of the metallocene catalyst precursor, and the isotacticity of the polymer produced. 

The NMR spectrum for polypropylene produced at 50°C with this catalyst reveals a high degree of stereocontrol (81% rrrr pentads). Moreover, analysis of the stereochemical defects (predominantly rmmr pentads) were indicative of a site control mechanism. For a site control mechanism to operate in syndiospecific polymerization, the olefin must alternately bind to coordination sites with opposite enantioface selectivity. The model for this polymerization is shown in Scheme HI. 

Ewen JA (1984) Mechanisms of stereochemical control in propylene polymerizations with soluble group 4B metallocene/methylalumoxane catalysts. J Am Chem Soc 106 6355... 

Syndiotactic polypropylene (sPP) with a. ..rrrrrmrrrrrmmrrrrr... mixed microstructure is obtained with the iPr[CpFlu]ZrCl2/HAO (MAO methylaluminoxane Cp cyclopentadienyl anion Flu - fluorenyl anion). The structures of the metallocene and the polymers are in accord with chain migratory insertion being the predominant mechanism of chain growth and with stereochemical control being provided by the alternating handedness of polymerization active, cationic Zr monoalkyls. 

These considerations concerning the mechanism of stereochemical control in ionic polymerization reactions, therefore, can account for the tacticities obtained in most homogeneous anionic and cationic polymerizations as shown in Tables 3 and 6, respectively (3). 

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