Olof Sudmeijer

Chain transfer reactions in propylene polymerization with zirconocene catalysts

The chemical structures of end groups of medium-low molecular weight atactic and isotactic polypropylenes (a-PP and i-PP), produced with zirconocene/methylalumoxane catalysts, have been analyzed and used to infer the chain-transfer reaction mechanisms, which are then correlated with the zirconocene ligand structure and the polymerization conditions. For the chiral, isospecific ansa-zirconocenes such as rac-[ethylenebis(1-indenyl)]ZrCl2/methylalumoxane (1/MAO) and rac-[ethylenebis(4,7-dimethyl-1-indenyl)]ZrCl2/methylalumoxane (2/MAO) catalysts, i-PP molecular weight is dependent on the regiospecificity of the catalyst, as shown by the presence of cis-2-butenyl end groups, formed by chain transfer to the monomer after a secondary propylene insertion. At low monomer concentration, chain-transfer with 2/MAO shifts from predominant transfer to the monomer after a secondary propylene insertion to β-methyl (allyl end groups) and β-hydrogen transfers after a primary insertion (2-propenyl, or vinylidene, end group). Ansa-bis(3-R-indenyl)ZrCl2 (ansa = CH2CH2, Me2Si, Me2C; R = Me, t-Bu, Me3Si) catalysts, which are highly regiospecific, produce polypropylenes with chain transfer via both β-hydrogen transfer after a primary insertion and β-methyl transfer. For example, rac-Me2C(3-t-Bu-Ind)2ZrCl2 (4) exhibits the highest selectivity for β-methyl transfer so far observed in an isospecific zirconocene. As for 2/MAO, the rate of β-methyl transfer in 4/MAO increases by lowering [propylene].

Factors influencing chain transfer with monomer and with hydrogen in propene polymerization using MgCl2-supported Ziegler-Natta catalysts

13C and 1H NMR analysis of polypropylene prepared using various MgCl2-supported catalysts at different polymerization temperatures at different hydrogen concentrations has revealed that chain transfer with monomer, yielding vinylidene-terminated chains, is essentially unaffected by hydrogen but is dependent on catalyst selectivity. Chain transfer with hydrogen and with monomer are therefore processes which operate independently of each other, but in both cases, the incidence of chain transfer increases with decreasing catalyst (site) selectivity. Chain transfer with monomer takes place after primary insertion, while chain transfer with hydrogen takes place after either primary or secondary insertion. Chain initiation via a secondary insertion into a Ti-H species leads either to a 2,3-dimethylbutyl terminal unit, as has recently been observed in metallocene-catalyzed polymerizations, or to a further transfer reaction with hydrogen, generating traces of propane.

Phosphorus-bridged metallocenes: New homogeneous catalysts for the polymerization of propene

Abstract The synthesis of a new class of metallocenes for the syndiospecific, aspecific and isospecific polymerization of propene is reported. This has been achieved by the incorporation of a phosphorus linking the cyclopentadienyl-type rings. The catalyst precursors, syndiospecific PhP(fluorenyl–Cp)ZrCl2 (2), aspecific PhP(fluorenyl)2ZrCl2 (4), and isospecific PhP(indenyl)2ZrCl2 (5), RP(2-Me,4-Ph-indenyl)2ZrCl2 (R=Ph (6); R= i Pr (7)) were prepared. Compound 2, after activation by methylaluminoxane (MAO), in LIPP at 67°C affords syndiotactic polypropene (s-PP) with an activity of 155 kg s-PP/g Zr·h. The physical properties of the s-PP (stereoregularity and molecular weight) are similar to that of conventional carbon-bridged systems. Ab initio calculations on model compounds assisted in rationalizing the high syndiospecificity of 2 in contrast to the much poorer stereoregularity of closely related Me2Si(fluorenyl–Cp)ZrCl2. Aspecific metallocene 4, after activation with MAO, affords high molecular weight atactic-PP, albeit with a low activity. Metallocenes 6 and 7, activated by MAO, afford isotactic polypropene (i-PP) with extremely high stereoregularity (>98% mmmm pentads), melting points 156–160°C and molecular weights tunable in the range 250,000–1,100,000. Activities of up to 580 kg i-PP/g Zr·h for 6/MAO (LIPP, 67°C, 37 000 equiv. MAO) and 1265 kg i-PP/g Zr·h for 7/MAO (LIPP, 50°C, 37 000 equiv. MAO) have been obtained.

Effect of polymerization temperature on the microtacticity of isotactic poly(propylene) prepared using heterogeneous (MgCl2‐supported) Ziegler‐Natta catalysts

A study of the effect of polymerization temperature on the microtacticity of poly(propylene) prepared using various MgCl 2 -supported catalysts has shown that, in almost all cases, an increase in temperature leads not only to higher proportions of isotactic polymer but also to increased stereoregularity of the isotactic fraction. The results indicate a greater relative increase in productivity, with increasing temperature, for highly isospecific as opposed to moderately isospecific centres. It is suggested that easier propagation after the occasional regioirregular (2,1-) insertion may contribute to this effect.

New insight into propene polymerization promoted by heterogeneous Ziegler-Natta catalysts

The configurational analysis of polypropylenes made with coordination catalysts has received a tremendous impulse by the recent application of high-field 13C NMR techniques. In particular, unprecedented determinations of stereosequence distribution at heptad/nonad level have made more realistic and sophisticated models of chain propagation applicable for the first time in a statistically significant manner. In this presentation, the most recent applications to «high-yield» MgCl2-supported Ziegler-Natta catalysts are illustrated