Selectivity Control By Temperature Variation in the Formation of Isotactic vs. Syndiotactic Polypropylene with a Titanocene/Alumoxane Catalyst

Abstract

The opposing effects of activation enthalpy and entropy most likely cause the transition from isotactic via atactic to syndiotactic polypropylene in polymerizations with the soluble metallocene catalyst 1 (X  Ph) and methylalumoxane when the reaction temperature is increased from −50 to +10°C. This proves how important temperature considerations are for structure–reactivity discussions.

High molecular weight atactic polypropene synthesized with (?5-pentamethylcyclopentadienyl)tribenzyl titanium activated with MAO

“…The triad sequence distribution was measured and found to be as follows: mm ϭ 6.15%, mr ϭ 40.87%, and rr ϭ 52.98% (Bernoullian factor B ϭ 1.03); this indicates that the insertion of propene with the catalyst system followed a chain-end control model. 22 Furthermore, PP2 was relatively syndiotactoid (r : m ϭ 0.734 : 0.266), a finding similar to the results previously reported by our laboratory for moderate atactic polypropene obtained from the CpTi(OBz) 3 /MAO catalyst. 16 An obvious amount of head-to-head and tail-to-tail propene units could be detected at 31.1 ppm (T ␤␥ ), 35.7 ppm (T ␣␤ ), and T ϭ 38.3 ppm (T ␣␥ ).…”

Synthesis of high-molecular-weight elastomeric poly(propylene) with half-titanocene/MAO catalyst

“…The 13 C NMR analysis confirms that the half-titanocene/MAO catalyst affords atactic poly(propylenes) and the polymerization mechanism is Bernoullian in all cases. The triad sequence distribution of the sample was measured to be mm = 11%, mr = 44%, and rr = 45% with a Bernoullian factor B = 0.99, which indicates that a chain end control 26) is predominant with this catalyst. The atactic poly(propylenes) obtained here are slightly syndiotactoid (r : m = 67 : 33).…”

Stereoregular Polymers

“…Thus, the produced polymer is atactic, as a result of random coordination of re-and si-face of propylene. Cp 2 TiR 2 /MAO systems afford isotactic polypropylene via the chain-end control mechanism at low temperature (−45 • C) (18)(19)(20).…”