Reactivity of Secondary Metal−Alkyls in Catalytic Propene Polymerization: How Dormant Are “Dormant Chains”?

In this communication, we report on the direct measurement of dormant chain concentration and of the relative reactivity of authentic primary and secondary Zr−polymeryls toward propene, ethene, and H2 under practical conditions for a new highly regio- and stereoselective postmetallocene polymerization catalyst with controlled kinetic behavior. The results, in particular, confirm the poor reactivity toward propene of secondary M−polymeryls and the possible accumulation of dormant chains in propene homopolymerization

Selectivity of Metallocene-Catalyzed Olefin Polymerization: A Combined Experimental and Quantum Mechanical Study. 1. Nonchiral Bis(cyclopentadienyl) Systems

Chemo-, regio- and enantioselectivities in the (co-)polymerization of propene and ethene have been measured (in some cases for the first time) for a number of prototypical metallocene catalysts (in combination with methylalumoxane), and compared with those calculated at the DFT level for gas-phase cationic systems. In this paper, the first part of the study, we discuss the achiral catalysts Cp2TiCl2, Cp2ZrCl2, and Me2SiCp2ZrCl2. All three catalysts were confirmed to be highly regioselective for propene, in favor of 1,2-insertion. The performance of the titanocene in this respect is truly remarkable (only two misinsertions in 10 000 at −15 °C), whereas that of the two zirconocenes turned out to be worse than generally assumed (two to three misinsertions in 1000), andquite unexpectedlynearly identical (both in experiment and calculations), despite the decidedly more open structure of the Si-bridged system. Understandably, in all cases ethene was found to insert faster than propene, but the difference in relative reactivity of the two monomers is particularly dramatic when the growing chain is secondary, which confirms the “dormant” character of the latter for propene. The fair correlation between observed and calculated selectivities suggests that, at least for the systems considered here, solvent and counterion effects, though undoubtedly important, are rather indiscriminate and that olefin insertion is indeed the rate-determining step

Unprecedented High-Modulus High-Strength Tapes and Films of Ultrahigh Molecular Weight Polyethylene via Solvent-Free Route

Unprecedented High-Modulus High-Strength Tapes and Films of Ultrahigh Molecular Weight Polyethylene via Solvent-Free Rout

Improving the Performance of Methylalumoxane: A Facile and Efficient Method to Trap “Free” Trimethylaluminum

The presence of “free” trimethylaluminum (TMA) in methylalumoxane (MAO) solutions can be highly detrimental to the performance of metallocene and “post-metallocene” olefin polymerization catalysts. The most used strategy to remove “free” TMA is to evaporate MAO solutions to dryness, until a free-flowing white powder (“solid MAO”) is left. This procedure is tedious and potentially hazardous, because in some cases the distillate is a concentrated hydrocarbon solution of TMA. Moreover, “solid MAO” is poorly soluble in common polymerization media, and once in solution it can regenerate TMA to some extent. This communication reports on a facile alternative, which consists in the controlled addition of a sterically hindered phenol, such as 2,6-di-tert-butylphenol, effectively trapping “free” TMA. We show here that 2,6-di-tert-butylphenol/MAO solutions activate equally well the dichloro-precursors of well-known zirconocene and bis(phenoxyimine)Ti catalysts, and that their use in propene polymerization results in a substantially higher productivity, polymer stereoregularity, and/or average molecular mass compared with activation by MAO alone

Selectivity of Metallocene-Catalyzed Olefin Polymerization: A Combined Experimental and Quantum Mechanical Study. The <i>a</i><i>nsa</i>-Me₂Si(Ind)₂Zr and <i>a</i><i>nsa</i>-Me₂C(Cp)(Flu)Zr Systems

DFT calculations are reported for all possible insertions of ethene and propene in rac-Me2SiInd2Zr−R⎤+ and Me2C(Cp)(Flu)Zr−R⎤+ (R = Et and iPr). The results confirm the basic stereoregulation mechanism of Corradini. In addition, they provide an ordering of the possible sources of stereoerrors:  chain misorientation is the main mechanism for unbranched (Et, nPr) chains, whereas for the β-branched iBu chain errors due to chain misorientation and chain−olefin syn orientation are equally likely

Influence of Polymerization Conditions on Melting Kinetics of Low Entangled UHMWPE and Its Implications on Mechanical Properties

The synthesis of linear ultra high molecular weight polyethylene, with a “pseudoliving” catalyst in various conditions results in samples of different molecular weights (<i>M</i>_w ranging between 2 to 35 million g/mol), all having a reduced number of entanglements to an extent that allows the solid-state uniaxial deformation of such high molar masses without melting. The solid-state processing of these materials shows a clear relationship between mechanical properties and molecular weight. For the adopted polymerization conditions, stretching forces required for the uniaxial deformation increase with the increasing molar mass, ultimately limiting the achievable maximum draw ratio in the polymers having <i>M</i>_w > 10 million g/mol. The increase in the stretching force is attributed to the increasing number of entanglements between the crystals with the molar mass. The estimation of entanglements is established with the help of melting kinetics involved in the “disentangled” crystals, and rheological response of the polymer melt obtained just after melting of the crystals. In spite of the increase in the stretching forces with the increasing molar mass, tensile modulus increases with the increasing draw ratio and the molecular weight. However, above the molar mass of 10 million g/mol, the stretching force required increases to the level that the uniaxial deformation becomes difficult–thus limiting the tensile strength

Block Copolymers of Highly Isotactic Polypropylene via Controlled Ziegler−Natta Polymerization

Block Copolymers of Highly Isotactic Polypropylene via Controlled Ziegler−Natta Polymerizatio