Influence of Temperature on the Regioselectivity of Highly Isospecific C₂-Symmetric Zirconocenes in Propene Polymerization

Abstract

The influence of polymerization temperature (Tp) on the isoselectivity of the C2‐symmetric zirconocene catalysts rac‐Me2Si(2‐Me‐4‐PhInd)2ZrCl2/MAO (1/MAO), rac‐Me(CyHex)Si[2‐Me‐4‐(4′‐tBuPh)Ind]2ZrCl2/MAO (2/MAO), and rac‐Me2Si(2‐Me‐4‐Ph‐5‐OMe‐6‐tBuInd)2ZrCl2/MAO (3/MAO) in propene polymerization, has been studied in the range 30–85 °C in liquid propene. All three catalysts show similar polymerization activity versus Tp profiles, with a maximum at about 60 °C (1/MAO), 80−85 °C (2/MAO), and 70−80 °C (3/MAO). 3/MAO shows the highest activity at any Tp. The isoselectivity of the three catalysts remains high and approximately constant in the whole Tp range investigated, as indicated by the roughly constant isotactic triad content (mm > 99%) of the produced isotactic poly(propylene)s (iPP). Surprisingly, the melting and crystallization temperatures of iPP increase by increasing TP. This behavior is due to an unexpected increase in catalyst regioselectivity with Tp, as indicated by the decrease in the content of 2,1‐erythro regiodefects (2,1e) at increasing Tp. A mechanism involving a metal cation–2,1‐coordinated propene slow state is proposed to account for this so far unnoticed behavior. The correlations between Tp, Tm, and [2,1e] are defined for these three catalysts.

Influence of 2,1‐erythro regiodefects on the crystallization behavior of isotactic polypropylene

“…The trend in Figure A is approximately linear only when the regiodefects content is above 0.7%‐0.8%. This seemingly linear correlation extrapolates to T c of 118°C, falling quite short from the experimental values of a nearly perfect iPP (PP0), and have thus no predicting power for highly stereoregular and regioregular iPP, as shown earlier on a larger set of iPP samples also for what concerns melting temperature . The full thermograms of the melting step of DSC experiments are reported in the Supporting Information.…”

“…All these iPP samples showed a relatively broad molecular weight distribution curve, which is a consequence of the catalyst and polymerization conditions employed for their preparation . GPC measurements were performed as previously described …”

“…We have recently shown that, by proper choice of the ligand structure and the polymerization conditions, the amount of 2,1e regiodefects in polypropylenes from highly isoselective metallocene catalysts can reach up to 3%, while keeping stereodefects and isomerized units to a very low level and the molecular weight well above 100 000 Da, and that the dependence of melting point on the amount of regiodefects does not follow Flory's law of melting point depression …”

“…[31] We have recently shown that, by proper choice of the ligand structure and the polymerization conditions, the amount of 2,1e regiodefects in polypropylenes from highly isoselective metallocene catalysts can reach up to 3%, while keeping stereodefects and isomerized units to a very low level and the molecular weight well above 100 000 Da, and that the dependence of melting point on the amount of regiodefects does not follow Flory's law of melting point depression. [32] As a follow-up to this previous study, we have produced iPP samples containing 2,1e regiodefects from $0% up to 3.5%, and only traces of stereodefects. In this work, we describe the solidification behavior under (1) isothermal conditions, (2) low cooling rates typical for DSC experiments, (3) mild cooling rates typical for compression molding, and (4) high (extrapolated from DSC) cooling rates typical for fast processing like, for example, film casting or injection molding.…”

“…13 C NMR spectra were acquired as previously described. [32] The GPC results are also listed in Table 1. All these iPP samples showed a relatively broad molecular weight distribution curve, which is a consequence of the catalyst and polymerization conditions employed for their preparation.…”