Effects of Supported (ⁿBuCp)₂ZrCl₂Catalyst Active-Center Distribution on Ethylene–1-Hexene Copolymer Backbone Heterogeneity and Thermal Behaviors

Abstract

Two catalysts, denoted as catalyst 1 [silica/MAO/( n BuCp)2ZrCl2] and catalyst 2 [silica/ n BuSnCl3/MAO/( n BuCp)2ZrCl2] were synthesized and subsequently used to prepare, without separate feeding of methylaluminoxane (MAO), ethylene homopolymer 1 and homopolymer 2, respectively, and ethylene–1-hexene copolymer 1 and copolymer 2, respectively. Gel permeation chromatography (GPC), Crystaf, differential scanning calorimetry (DSC) [conventional and successive self-nucleation and annealing (SSA)], and 13C nuclear magnetic resonance (NMR) polymer characterization results were used, as appropriate, to model the catalyst active-center distribution, ethylene sequence (equilibrium crystal) distribution, and lamellar thickness distribution (both continuous and discrete). Five different types of active centers were predicted in each catalyst, as corroborated by the SSA experiments and complemented by an extended X-ray absorption fine structure (EXAFS) report published in the literature. 13C NMR spectroscopy also supported this active-center multiplicity. Models combined with experiments effectively illustrated how and why the active-center distribution and the variance in the design of the supported MAO anion, having different electronic and steric effects and coordination environments, influence the concerned copolymerization mechanism and polymer properties, including inter- and intrachain compositional heterogeneity and thermal behaviors. Copolymerization occurred according to the first-order Markovian terminal model, producing fairly random copolymers with minor skewedness toward blocky character. For each copolymer, the theoretical most probable ethylene sequences, n E MPDSC‑GT and n E MPNMR‑Flory, as well as the weight-average lamellar thicknesses, L wav DSC–GT and L wav SSA DSC, were found to be comparable. To the best of our knowledge, such a match has not previously been reported. The percentage crystallinities of the homo- and copolymers increased linearly as a function of L MPDSC‑GT. This indicates that the homo- and copolymer chains folded excluding the butyl branch. The results of the present study will contribute to developing future supported metallocene catalysts that will be useful in the synthesis of new grades of ethylene−α-olefin linear low-density polyethylenes (LLDPEs).

(nBuCp)2ZrCl2‐catalyzed ethylene‐4M1P copolymerization: Copolymer backbone structure, melt behavior, and crystallization

“…The experimental procedure reported in the literature was followed. [38][39][40] The data were acquired for each cycle and handled using the TA explorer software. The measured X c was subsequently used to calculate the polymer material density q polym , following the rule of additivity of volumes of polyethylene amorphous and crystalline phases:…”

Metallocene-catalyzed ethylene−α-olefin isomeric copolymerization: A perspective from hydrodynamic boundary layer mass transfer and design of MAO anion

“…The instrument was calibrated using indium. The experimental procedure reported in the literature was followed [23,24]. The data were acquired for each cycle and handled using the TA explorer software.…”

“…For this purpose, a Bruker 600 MHz AVANAC III spectrometer (Bruker BioSpin, Rheinstetten, Germany) was used. The details of this instrument and the NMR operating conditions and procedure are available in one of our earlier publications [24]. The spectra were recorded using DEPT (distortionless enhancement by polarization transfer) 135 pulse sequence, and they were analyzed applying Bruker Topspin 2.1 software (Bruker BioSpin, Rheinstetten, Germany).…”

“…Accordingly, different active site ion-pairs [Zr] + [MAO] n − are likely to prevail in the experimental zirconocene catalysts. This subject-metallocene active center distribution-has been particularly detailed in some of our recent publications [20,24].…”

“…12 mutually compares the Gibbs-Thompson (GT) lamellar thickness distributions of the as-synthesized polyethylenes. We calculated them applying Gibbs-Thompson (GT) equation, the related literature expressions [24,41,[74][75][76][77], and the DSC data. We valued the most probable lamellar thickness L MPDSC GT from the peak of each (equilibrium melting temperature of the copolymer) was found as reported in the literature [41,[74][75][76][77].…”