Analysis of the molecular weight distribution (MWD) in polymerization processes : contribution of different families of active sites to the MWD of polyethylene prepared using supported transition metal catalysts

Abstract

The broad molecular weight distribution (MWD) curves obtained by gel-permeation chromatography (GPC) of polyethylenes prepared in the presence of MgCl2 supported monometallic (Ti or Hf) and bimetallic (Ti/Hf) catalysts were analyzed by deconvolution into component curves. The MWD curves of the monometallic catalysts can be accounted for with only three component curves associated with three distinct families of active sites. The term family is used to indicate a group of relatively similar active sites producing a polymer with Mw/Mn < 3. This approach seems valid for both Ti and Hf catalysts in spite of the different characteristic values; indeed we have for the former system 1 < Mw/105 < 2,5 with 6 < Mw/Mn < 10, whereas for the latter Mw ≈ 7 × 105 and Mw/Mn ≈ 18. In the case of the bimetallic Ti/Hf system with 1,9 < Mw/105 < 2,6 and Mw/Mn ≈ 17, the MWD distribution can be better accounted for on the basis of 6 families of active sites, corresponding to the linear combination of the balanced contributions from both Ti and Hf sites.