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A kinetic model was developed to describe the propylene polymerization behavior of fluxional, two-state metallocene catalysts. In particular, the pentad and molecular weight distributions can be described as well as other parameters of interest, such as the weight fraction of crystallizable sequences and the isotactic sequence length distribution, in terms of fundamental kinetic constants and polymerization conditions that pertain to these two-state catalyst systems. The model was used in an attempt to describe the polymerization behavior of two, prototypical, fluxional catalyst systems, (2-PhInd)2ZrCl2/MAO (1) and (2-p-CF3PhInd)2ZrCl2/MAO (2). The model can accurately reproduce the pentad distributions observed in PP prepared using these catalysts and the response of the distribution to changes in polymerization conditions, specifically changes in [C3H6] at constant T. These studies illustrate that the rate of state-to-state interconversion is slower but of comparable magnitude to the rate of monomer insertion and that the states have similar stability and reactivity. The broad molecular weight distributions previously observed with this family of catalysts can be described by the model. However, the model predicts that the state-to-state interconversion rate has to be significantly slower than the rate of formation of dead polymer chains, and this is inconsistent with the rate estimated from the response of the pentad distribution to changes in the rate of propagation (i.e., [C3H6]). Recent work where propylene polymerizations using 1 were carried out to low conversion indicate that the broad MWD seen in earlier studies is partly related to variations in [C3H6] during polymerization.

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A kinetic model to describe the propylene polymerization behavior of ansa-metallocene catalysts was derived. The model can predict n-ad stereosequence distributions, polymer crystallinity, and related properties as well as distinguish between extremes in kinetic behavior expected for such catalysts. In particular, where polymer microstructure is sensitive to changes in [C3H6], the model can provide reliable estimates of kinetic parameters of interest, including ratios between rates of some of the significant reaction steps involved in polymer microstructure formation. The model is applied to a description of the polymerization behavior of some simple symmetrical [Me2C(Cp)(Flu)MCl2; M = Zr, Hf] and unsymmetrical [Me2Y(Cp)(Ind)MCl2; M = Zr, Hf; Y = C, Si] ansa-metallocene catalysts, activated with methyl aluminoxane. With the former two catalysts, the Zr catalyst operates very close to the kinetic quenching limit where chain inversion (or chain back-skip) is slow compared to monomer insertion, while for the Hf analogue, these two processes have more comparable rates. In the more complicated, unsymmetrical systems, both Zr- and Hf-based systems (Y = Si) operate under conditions where inversion is much faster that propagation, whereas for the Hf catalyst (Y = C), intermediate behavior is observed, and the corresponding Zr complex (Y = C) produces poly(propylene) where propagation is faster than inversion.

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ABSTRACT:The preparation of nanocomposites of poly (ethylene terephthalate) (PET) and lamellar zirconium phosphorous compounds by melt extrusion was investigated. Two types of zirconium phosphorous compounds were synthesized by the direct precipitation reaction method: a-zirconium bis(monohydrogen orthophosphate) monohydrate (ZrP) and organic-inorganic hybrid layered zirconium phenylphosphonate (ZrPP). Composites containing 2 and 5 wt % ZrP and ZrPP were prepared in a twin-screw extruder and specimens were obtained by injection molding. The extent of dispersion of the layered filler in the composite matrix was investigated by X-ray diffraction and transmission electron microscopy (TEM). The crystallization and thermal properties were analyzed by differential scanning calorimetry and thermogravimetry, and the mechanical properties were evaluated by tensile tests. Whereas ZrP-containing composites showe characteristic diffraction peaks at 2y 11.78 (d ¼ 7.54 Å ), indicative of no delamination, ZrPP showed practically no low-angle diffraction peak at 2y 5.58 (d ¼ 15.24 Å ), indicating loss of the layered order. TEM images of ZrPP particles indicated the formation of an intercalated/partially delaminated nanocomposite. The behavior was attributed to the higher affinity of the polyester with phenyl groups on the platelet surface of ZrPP. In both cases, the addition of the fillers increased the crystallization rate and the modulus.

in Wiley InterScience (www.interscience.wiley.com).ABSTRACT: Conventional and chain extended-modified solid-state polymerization (SSP) of postconsumer poly(ethylene terephthalate) (PET) from beverage bottles was investigated. SSP was carried out at several temperatures, reaction times, and 2,2 0 -bis-2-oxazoline (OXZ) or pyromellitic anhydride (ANP) concentrations. The OXZ was added by impregnation with chloroform or acetone solution. Higher molecular weights were reached when the reaction was carried out with OXZ, resulting in bimodal distribution. The molecular weights of the flakes reacted at 230 C for 4 h were 85,000, 95,000, and 100,000 for samples impregnated with 0, 0.5, and 1.25 wt % OXZ solution, respectively. In the case of reactions with ANP, branched chains were obtained. The thermal and thermal-mechanical-dynamic properties of these high-molecular-weight recycled PET were determined. For OXZ-reacted samples, the reduction of crystallinity was observed as the reaction time was increased, becoming evident the destruction of the crystalline phase. The chain extended samples did not show changes in thermal relaxations or thermal degradation behavior.

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