Showing papers by "Jun-Ting Xu published in 2000"

Effect of short chain-branching distribution on crystallinity and modulus of metallocene-based ethylene–butene copolymers

Abstract: In this article, the short chain-branching distribution (SCBD) of some metallocene-based ethylene–butene copolymers was evaluated by DSC, and some conventional ethylene copolymers were also studied for the purpose of comparison. It is found that metallocene-based ethylene copolymers have a relative narrower SCBD. These copolymers were crystallized under different modes, and the crystallinity and initial modulus of them were examined. The metallocene-based ethylene copolymers contain less interfacial regions, and the melting temperatures of them decrease more rapidly with the decrease of density than those of conventional ethylene copolymers. Moreover, the metallocene-based and conventional ethylene copolymers of similar density have close initial modulus when they are quenched or annealed at 100°C, but conventional ethylene copolymers show higher initial modulus when stepwise crystallized from 120°C. These differences in crystallinity and initial modulus were explained based on their differences in short-chain branching distributions. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1709–1715, 2000

Polymer-supported half-titanocene catalysts for the syndiospecific polymerization of styrene

Abstract: Monocyclopendienyltitanium trichloride (CpTiCl 3 ) was supported on polymer carriers with different hydroxyl contents, and the supported catalysts were used for styrene polymerization. The supported catalysts exhibited high activity even at low Al/Ti ratios and increased the molecular weight of the products, indicating that polymer carriers could stabilize the active sites. The polymers prepared with unsupported and supported catalysts were extracted with boiling n-butanone and characterized by carbon nuclear magnetic resonance ( 13 C NMR) and differential scanning calorimetry. The polymers obtained by supported catalysts had a high fraction of boiling n-butanone-insoluble part and high melting temperatures, but 13 C NMR results showed that syndiotacticity decreased compared with that of polymers prepared with an unsupported catalyst. ESR study on the supported catalysts confirmed that the active sites supported on the carrier dropped into the solution and formed active sites the same as those in the unsupported system when they reacted with methylaluminoxane. 13 C NMR analysis showed that the polymerization mechanism of the supported active sites was an active-site controlled mechanism instead of a chain-end controlled mechanism of the unsupported active sites.