SUMMARY Investigations of the effects of polymerization conditions on the molecular weight distribution (MWD) of polyethylene synthesized with soluble metallocene/methylaluminoxane (MAO) catalysts have been performed. The following variables were investigated in this study: catalyst type, polymerization temperature, catalyst concentration, MA0 concentration, chain transfer agent, ethylene partial pressure, as well as the substitution of MA0 with trimethylaluminium (TMA), and of different catalyst activities of polyethylene. Similarities and differences with other published results are highlighted. In all cases, an effort was made to illustrate the significance of the effects by presenting replicate measurements. Catalysts investigated were bis(cyclopentadieny1)zirconium dichloride (Cp2ZrClz (l)), its titanium and hafnium analogues (Cp2TiC12 (2) and CpzHfC12 (3)), as well as rac-ethylenebis(indeny1)zirconium dichloride (Et(Ind)ZrCl2 (4)) and rac-ethylenebis(4,5,6,7-tetrahydroindenyl)zirconium dichloride (Et(I141nd)2ZrC12 (5)). According to a 2' factorial experiment, independent increases in the concentrations of catalyst or MA0 cause a decrease in average molecular weight, with no interaction between these two factors. Replacing MA0 with TMA at constant overall aluminium concentration causes a drastic decrease in average molecular weight. Extremely high polymerization rates were observed to impart only a slight increase in the breadth of the MWD. The effects of ethylene partial pressure suggest that for the zirconium catalysts, transfer to monomer is the main chain transfer mechanism, while for hafnium catalysts, this is not the case.

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The results of an interlaboratory or round‐robin experiment in high‐temperature gel permeation chromatography (HT‐GPC) analysis are presented. The intention was to determine and raise awareness of interlaboratory reproducibility of HT‐GPC techniques. Fifteen laboratories performed analyses of five polyethylene samples and standards SRM 1475 and 1476. Reproducibility, as measured by the interlaboratory standard deviation (sLAB), was greatly influenced by the breadth of the molecular weight distribution (MWD) and branching. The sLAB values for the weight‐average molecular weight (Mw) of linear polyethylenes of narrow and broad MWDs were 4 and 14%, respectively. For branched polymers, GPC viscometry methods are shown to measure significantly higher molecular weights than the noncoupled GPC method, with higher variance. For branched polyethylenes measured with GPC viscometry, the reproducibility of Mw was characterized by sLAB = 18%. Reproducibility of the SRM 1475 standard was better than for unknowns. The results for branched standard SRM 1476 emphasize the important role of the detection method in GPC but call into question the use of this material as a molecular weight standard. For single‐site polyethylene, only a handful of labs measured an MWD that closely matched the Flory distribution. Qualitatively, the responses indicate that many variations in instrument and analytical methods exist among laboratories; this is partly a reflection of the development and refinements that this technique must yet undergo before a truly standard method is widely accepted and practiced. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 905–921, 2002

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Copolymers based on ethylene, propylene, and dienes (EPDM) are often commercially produced with a vanadium catalyst, an alkylaluminum cocatalyst and optionally a halocarbon promoter. It is widely accepted that the catalytic species in vanadium polymerization is in the trivalent oxidation state V(III) and that the role of organohalide promoters is to oxidize the vanadium species, reduced by the alkylaluminum cocatalyst to divalent V(II), back to the trivalent state [1][2][3] .Mechanisms for polymerizations with vanadium have been proposed and kinetic modelling has been attempted in the past [4][5][6][7][8][9] , but monomer-coordinated species and two-site behaviour, occasionally alluded to but not characterized, were not considered. In the present study, EPM and EPDM polymers were made in solution with a vanadium catalyst in a continuous stirred-tank reactor.Observations about this system include [10] :• the catalyst system without promoter and without hydrogen produces polymer with bimodal MWDs, suggesting two-site catalyst behaviour.• the addition of catalyst promoter causes an order of magnitude increase in catalyst productivity, and eliminates the higher-MW component in the MWD.• the addition of hydrogen also precludes bimodal MWDs, regardless of the presence of promoter.• except at very low monomer concentrations, the polymerization rate has a zero-order rather than a first-order response to increasing monomer concentration.These seldom-discussed features of vanadium catalysis are illustrated by a selection of EP copolymerizations, and can be explained with the aid of a polymerization model. The polymerization rate, which is almost zero-order with respect to monomer at higher monomer concentrations (with or without halocarbon promoter) is described by a mechanism whereby propylene forms a stable propylene-active site complex prior to insertion in the growing polymer chain. Model results suggest that the second active site type, which makes high molecular weight polymer, is formed from the monomer-coordinated species. Although polymerisation rate and MWD can be described by the kinetic model, many fundamental questions require discussion and further investigations.