Polypropylene epimerization

The Suter-Flory model was successfully used to interpret the results of the epimerization reaction carried out on propylene oligomers (204) and on polypropylene itself (106, 205). In both cases a slight prevalence of the r dyad over the m (52/48) is observed. The epimerized polypropylene has a microstmcmre almost coincident with a Bernoulli distribution and represents the polymer sample closest to an ideal atactic polymer so far obtained. 

The dependence the polymer microstructure on the propylene concentration in the reactor is shown in Table XVI. By varying the monomer concentration in operation with the supported catalyst, one can obtain polypropylenes with relatively low concentration of rrrr pentads. At low propylene concentrations, it is possible that m insertions are formed by epimerization. The yield of these isolated m diads is significantly increased when the catalyst is supported. Similar effects were shown by catalysts producing isotactic polymer. 

Abstract The synthesis and X-ray structure of various octahedral zirconium complexes and their catalytic properties in the polymerization of a-olefins are described. Benzamidinate, amido, allylic, and phosphinoamide moieties comprise the study ligations. For the benzamidinate complexes, a comparison study between homogeneous and heterogeneous complexes is presented. For the phosphinoamide complex, we show that the dynamic symmetry change of the complex from C2 to C2v allows the formation of elastomeric polymers. By controlling the reaction conditions of the polymerization process, highly stereoregular, elastomeric, or atactic polypropylenes can be produced. The formation of the elastomeric polymers was found to be the result of the epimerization of the last inserted monomer to the polymer chain. 

The atactic polymer obtained with MAO can be rationalized by an intramolecular epimerization reaction of the growing polypropylene chain at the last inserted monomeric unit [50-57], which at low monomer concentration was found to be faster than the stereoregular insertion of propylene (Scheme 2a). 

Scheme 2a, b Proposed mechanism for the intramolecular epimerization of the growing polypropylene chain (a), and plausible mechanism for the expected isomerization of a-olefins (b)... 

Investigations of the bis(benzamidinate) dichloride or dialkyl complexes of Group 4 metals show that these complexes, obtained as a racemic mixture of c/s-octahedral compounds with C2 symmetry, are active catalysts for the polymerization of a-olefins when activated with MAO or perfluoroborane cocatalysts [29-41]. As was demonstrated above, polymerization of propylene with these complexes at atmospheric pressure results in the formation of an oily atactic product, instead of the expected isotactic polymer. The isotactic polypropylene (mmmm>95%, m.p.=153 °C) is formed when the polymerization is carried out at high concentration of olefin (in liquid propylene), which allows faster insertion of the monomer and almost completely suppresses the epimerization reaction. 

The polymerization of propylene using complex 14 activated by MAO (Al Zr ratio=500, solvent toluene, 25 °C) yielded 80 g polymer-mol Zrl-hrl with a molecular weight Mw= 115,000 and polydispersity=2.4 [119]. The reaction was carried out in liquid propylene to avoid, as much as possible, the epimerization of the last inserted monomer unit and to allow rational design of the elastomeric polymer. The formation of elastomeric polypropylene is consistent with the proposed equilibrium between ds-octahedral cationic complexes with C2 symmetry inducing the formation of the isotactic domain, and tetrahedral complexes with C2v symmetry responsible for the formation of the atactic domain (Scheme 7). The narrow polydispersity of the polypropylene obtained supports the polymerization mechanism in which the single-site catalyst is responsible for the formation of the elastomeric polymer. 

To corroborate that the epimerization reaction is responsible for the stereoerrors in the polypropylene chain, we treated 1-octene with complex 14 activated with MAO. The reaction resulted in the quantitative formation of frans-2-octene. In addition, the reaction of allylbenzene with the system 11/MAO at room temperature produced 100% conversion of the former to trans-methylstyrene. The isomerization results indicate that during the polymerization, the metal center in 14 is also able to migrate through the growing polymer chain, inducing branching and additional environments for the methyl ligands (Scheme 2) [124]. 

Three types of elastomeric polypropylene are known (1) a large molecular weight atactic polymer [125], (2) polymers with alternating isotactic and atactic blocks [108,109,126-129], and (3) polymers formed by the dual-site mechanism [110]. The structure of the obtained elastomeric polypropylene was elucidated by comparison of the 13C-NMR spectra with those of an atactic oily polymer and isotactic polypropylene prepared by the zirconium complex 4. On the basis of the NMR data, for all the samples the statistical lengths of the isotactic blocks between two neighboring epimerization stereodefects were calculated [41]. It was found that for the isotactic polymer 90%), the... 

Aregic PVF, regiosequence distributions, 160-63 Aromatic C-1 resonance, epimerized isotactic PS, 202-11 Atactic polypropylene, 3C spectra, 7 Attached proton test (APT), optimizing sensitivity, 99... 

Collins has reported the synthesis of a related class of metallocenes (43), some of which form elastomeric, stereoblock polypropylene when activated by The elastomeric properties of the polymer formed using 43 (M = Hf, X = SiMe2 7)xn = 25 °C) were far superior to those formed by the other metallocenes in the study. The polymers made using 42 and 43 have similar microstructures, as well as physical and mechanical properties. However, after detailed microstructural analysis of the polymer the authors proposed an alternate mechanism of stereocontrol to Chien s site epimerization model (Scheme... 

Busico et al., on the other hand, came to a conclusion [299] that the stereoregularity of polypropylene produced with C2-symmetric group 4 ansa-metallocene catalysts is a result of the interplay of two competing reactions. These are isotactic monomer polyinsertion and a side process of epimerization of the polymer chain at its active end. That makes this class of homogenous catalysts different from the typical Ziegler-Natta catalyst, because with these catalysts enantioselectivity and stereoselectivity are not necessarily coincidental [96]. 

Low monomer concentration can also decrease the isotacticity of polypropylene. The three-coordinate metallocene alkyl species that exists in the absence of a n-coordinated monomer is capable of racemizing the methyl group of the last-inserted monomer unit. Evidence for the occurrence of chain-end epimerization has come from studies using deuterium-labeled propylene " the relevant mechanistic steps are believed to be a series of P-hydride eliminations and subsequent isomerizations with a tertiary alkyl species as an intermediate (Scheme 1.8). 

Of all the hafnium compounds listed in Figure 2.7, none excels their zirconium counterparts in syndioselectivity. This observation is sometimes attributed to hafnium s decreased rate of bimolecular propagation relative to unimolecular site epimerization. Lower overall activities with hafnium catalysts corroborate this argument. The most syndioselective hafnium catalyst appears to be Cl-symmetric i-92-Hf, which produces syndiotactic polypropylene with [r] = 92.2% and... 

The zirconium-based catalyst 5 -136/MAO is highly syndioselective ([r] > 99%, Tp = 0 C) and apparently provides the highest-melting syndiotactic polypropylene known The unannealed Tm for the polypropylene is 164 °C, and the annealed Tm is 174 °C. Stereoerrors have not been detected in this polymer by NMR, but pentads presumably attributable to site epimerization (single m mistakes) can be found if polymerizations are conducted in dilute propylene at elevated temperatures. 

The stereochemical mechanism responsible for the isoselectivity of Ci-symmetric metallocene catalysts has been a topic of considerable debate. There are two limiting mechanisms possible for the formation of isotactic polypropylene with such Ci-symmetric catalysts having diastereotopic coordination sites. These are the site epimerization mechanism and the alternating mechanism, as shown in Scheme 2.3. 

The vast majority of published reports for isotactic polypropylene formation with metallocenes based on i-1 invoke the site epimerization mechanism to account for the observed isoselectivity.When the growing polymer chain occupies the coordination site distal to the terf-butyl group (Scheme 2.3, site a), it is directed away from the benzo substituent of the fluorenyl ligand in the transition state for monomer insertion. The methyl group of the incoming monomer is... 

FIGURE2.12 A statistical analysis of isotactic polypropylene from i-l/MAO cannot conclusively differentiate between the site epimerization model and the alternating model. (Reprinted with permission from Miller, S. A. Bercaw, J. E. OrganometalUcs 2006, 25, 3576-3592. Copyright 2006 American Chemical Society.)... 

Naturally, this catalyst is not perfect and stereoerrors are invariably present in the formed polymer. The principal stereoerrors arise from enantiofacial misinsertion at the more stereoselective site and site epimerization. Such mistakes give rise to mr and rm triads and m and r dyads in an otherwise consistent procession of mm and rr triads (Figure 2.17). Thus, perfect hemiisotactic polypropylene... 

FIGURE 2.17 Enantiofacial misinsertion and site epimerization stereoerrors give rise to pentads containing isolated m and r dyads (in bold) such pentads are forbidden in perfectly hemiisotactic polypropylene. 

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