Effects of supported (ⁿBuCp)₂ZrCl₂ catalyst active center multiplicity on crystallization kinetics of ethylene homo- and copolymers

Two different supported zirconocene, that is, bis(n-butylcyclopentadienyl) zirconium dichloride (ⁿBuCp)₂ZrCl₂, catalysts were synthesized. Each catalyst was used to prepare one ethylene homopolymer and one ethylene-1-hexene copolymer. Catalyst active center multiplicity and polymer crystallization kinetics were modeled. Five separate active center types were predicted, which matched the successive self-nucleation and annealing (SSA) peak temperatures. The predicted crystallinity well matched the differential scanning calorimetric (DSC) values for a single Avrami-Erofeev index, which ranged between 2 and 3 for the polymers experimented. The estimated apparent crystallization activation energy E_a did not vary with cooling rates, relative crystallinity α, and crystallization time or temperature. Therefore, the concept of variable/instantaneous activation energy was not found to hold. E_a linearly increased with the weight average lamellar thickness L_{wav DSC-GT}; and for each homopolymer, it exceeded that of the corresponding copolymer. Higher E_a, hence slower crystallization, was identified as a pre-requisite to attain higher crystallinity. Crystallization parameters were correlated to polymer backbone parameters, which are influenced by catalyst active center multiplicity.