Propene polymerization mechanisms by using C1-symmetric catalysts

A set of propene homopolymerizations have been carried out with three different C1-symmetric catalysts, in order to evaluate the polymerization performances. A comparison among the polymer microstructures on the basis of statistical analysis has been performed to have a deeper investigation about the polymerization mechanism. The development of a Cs-symmetric class of catalysts with two enantiomorphic sites, by Ewen and Razavi1, has opened the possibility to synthesize syndiotactic polypropylene: the two enantiomorphic sites of these catalysts enable an alternating orientation of propene insertion, thus forming syndiotactic propylene (sPP) chain. sPP is a thermoplastic material with high melting point and high crystallinity. Concerning the propene homopolymerisation using C1-symmetric catalysts, many data in literature report that the microstructure strongly depends on the hindrance of the R- substituent on Cp group. When the substituent is a Me group or i-propyl, a hemi-isotactic or atactic polypropylene is obtained, when the substituent is the bulkier ter-butyl substituent, isotactic polypropylene is obtained. This behaviour is often explained with the site epimerisation mechanism; according to this mechanism, the ter-butyl side of Cp group is too sterically hindered to accommodate the growing polymer chain, that needs a site epimerisation before every monomer insertion. Also the molecular masses could be influenced by the substituents on the Cp or Flu moieties. In order to investigate all these factors a study of propene homopolymerizations by using different C1-symmetric catalysts was performed. All are ansa-Cp-fluorenyl metallocenes, 1 and 2 have one ter-butyl substituent on the fluorenyl ligand and differ in the bridge, 4 has one ter-butyl substituent on the Cp, 3 has two substituents on the Cp and on the fluorenyl. Results were compared to those obtained with the Cs catalyst Me2C-(C5H4)(C13H8)ZrCl2. Due to the presence of diphenyl bridge the catalyst 1 is the most active, whereas the bulkier ter-butyl group on Cp side of the catalyst 3 leads to the lowest activity; moreover the activity strongly decreases with the increase of the temperature. The polymerization temperature influences the activities; for all the catalysts the higher the temperature the lower the activity. Catalyst structure influences molecular weight too. Indeed diphenyl-bridged system yields higher Mw than isopropyl-bridged catalyst. In addition, Mw linearly depends on temperature and the higher the temperature the lower the Mw. It has to be taken into account that, because the propylene concentration in the polymerization medium is quite the same for all the reactions, an increase of temperature leads to an increase of pressure, which behaves oppositely to the temperature. All the homopolymers obtained were characterized by 13C-NMR. The catalyst 1 gives mainly syndiotactic polypropylene; the syndiotactic pentad percentage decreases with the increasing of the temperature; the monomer concentration seems to have an influence on tacticity, in fact when the initial propylene concentration raises from 5.3 M to 12.0 M, the tactic pentad percentage strongly decreases. The catalyst 2 gives highly syndiotactic polypropylene; the rrrr pentad percentage decreases with the increasing of polymerization temperature similarly to the analogous syndiospecific catalyst 1. The changes in the microstructure of polymers obtained at 50 °C at different monomer concentration confirm that the monomer concentration has the critical effect to reduce the syndiospecificity. Indeed, increasing the monomer concentration to 17.7 M leads to a strong decrease of the syndiotactic pentad percentage. On the contrary, the catalyst 3 gives highly isotactic polypropylene and both the monomer concentration and the polymerization temperature have a small influence on the polymer microstructure. Observed polymer tacticity was compared to that predicted by three statistical models. The first is enantiomorphic site control, which is predicted by the site epimerization mechanism, since it employs a single site with enantioselectivity alfa. The second is an alternating model that is applicable to a catalyst that regularly alternates insertions between a perfectly stereoselective site (alfa = 1) and a site having a variable stereoselectivity equal to beta. The third is an alternating model that is applicable to a catalyst that regularly alternates insertions between two sites of variable stereoselectivity (alfa and beta). Both alternating models assume that no site epimerization is occurring. The polymer tacticities predicted for the polypropylene obtained by catalysts 1, 2, and 3 as well as from 4 and 5 were compared: for catalyst 4, which has one ter-butyl on the Cp, the r.m.s. errors provided by the fits from the site epimerization mechanism and the alternating one are very different, indicating that the alternating two site mechanism better predicts the microstructure of polypropylene produced by this catalyst. On the contrary, for catalyst 5 the r.m.s. errors provided for the three predictive mechanisms are very similar to draw definitive conclusions concerning which mechanism, site epimerization or alternating, predicts the microstructure of the polypropylene produced. The r.m.s. errors provided for catalyst 3 are low and very similar, as to indicate that all mechanisms are possible, that is as catalyst 4 the mechanism should be alternating, but the presence of the second ter-butyl on the same half-space makes the epimerization necessary. This is confirmed from the slight changes in the microstructure with temperature or monomer concentration. Indeed, as described by Bercaw2, when the alternating mechanism is operating and the site epimerization mechanism is accessible, it is possible to detect an increased isotacticity with the polymerization temperature, at the same propene concentration, and a decreasing in isotacticity by increasing the propene concentration. Regarding catalyst 2, with one ter-butyl on the fluorenyl, results are rather similar to those of catalyst 4 with one substituent on the Cp even though 4 gives an isotactic polypropene and 2 a mainly syndiotactic one. Indeed for all the three polymerization temperatures, the epimerization mechanism could not predict the pentad distribution, while both the alternating mechanism well describe the observed microstructure. It is worth noting that also when the alternating mechanism is preferred, an increase in polymerization temperature slightly reduces the stereospecificity. Statistical analysis of polypropene by catalyst 1 reveals results similar to those from 2. The statistical analysis of propene microstructure confirms that the stereospecificity can increase with decreasing the monomer concentration for both the catalysts 1 and 2 and due to the absence of hindered substituent on Cp side the alternating mechanism is preferred with respect the site epimerization one. For the catalyst 3, the data collected by the analysis seem that the alternating mechanism is possible, but the site epimerization becomes necessary due to the presence of the two substituents on the Cp and on the fluorenyl groups.