Metallocene-catalysed

The second aspect, predicting reaction dynamics, including the quantum behaviour of protons, still has some way to go There are really two separate problems the simulation of a slow activated event, and the quantum-dynamical aspects of a reactive transition. Only fast reactions, occurring on the pico- to nanosecond time scale, can be probed by direct simulation an interesting example is the simulation by ab initio MD of metallocene-catalysed ethylene polymerisation by Meier et al. [93]. 

Meier, R.J., Van Doremaele, H.J., larlori, S., Buda, F. Ab-initio molecular dynamics study of metallocene-catalysed ethylene polymerization. J. Amer. Chem. Soc. 116 (1994) 7274-7281. 

Mention has already been made in this chapter of metallocene-catalysed polyethylene (see also Chapter 2). Such metallocene catalysts are transition metal compounds, usually zirconium or titanium. Incorporated into a cyclopentadiene-based structure. During the late 1990s several systems were developed where the new catalysts could be employed in existing polymerisation processes for producing LLDPE-type polymers. These include high pressure autoclave and... 

Metallocene-catalysed polyethylenes exhibit the general characteristics of polyethylene as noted in the introductory paragraph of Section 10.5. Furthermore... 

Metallocene-catalysed very low density polyethylene (m-VLDPE) has become available with densities of as low as 0.903. This is of use for sealing layers of multi-layer films since sealing can commence at lower temperatures than with conventional materials such as LLDPE and EVA (see Section 11.6) with the polymer seal exhibiting both cold strength and hot tack strength. 

Since the last edition several new materials have been aimounced. Many of these are based on metallocene catalyst technology. Besides the more obvious materials such as metallocene-catalysed polyethylene and polypropylene these also include syndiotactic polystyrenes, ethylene-styrene copolymers and cycloolefin polymers. Developments also continue with condensation polymers with several new polyester-type materials of interest for bottle-blowing and/or degradable plastics. New phenolic-type resins have also been announced. As with previous editions I have tried to explain the properties of these new materials in terms of their structure and morphology involving the principles laid down in the earlier chapters. 

Report 88 Plasticisers - Selection, Applications and Implications, A.S. Wilson. Report 115 Metallocene-Catalysed Polymerisation, W. Kaminsky, University of Hamburg. 

Obviously, there exists severe interplastics competition, e.g. PP vs. ABS, clarified PP vs. PS, PA, PVC, HDPE and PS (Table 10.7). A wide range of cross-linked and thermoplastic elastomer applications, from footware to automotive parts and toothbrushes, are adopting new metallocene-catalysed polyolefin elastomers (POEs). These low-density copolymers of ethylene and octene were first accepted as impact modifiers for TPOs, but now displace EPDM, (foamed) EVA, flexible PVC, and olefinic thermoplastic vulcanisates (TPVs). Interpolymer competition may also result from... 

Report 115 Metallocene-Catalysed Polymerisation, W. Kaminsky, University of Hamburg. 

A metallocene-catalysed polymerization was used to produce syndiotactic polystyrene but its marketing was recently suspended. 

Beagan, C.M., McNally, G.M., and Murphy, W. R., Metallocene Catalysed Polyethylene in Blown Film Applications-A Comparison Between Monoextruded Blended Films and Coextruded Films, SPEANTEC Tech. Papers, 44, 128 (1998)... 

The effect of blending LDPE with EVA or a styrene-isoprene block copolymer was investigated (178). The properties (thermal expansion coefficient. Young s modulus, thermal conductivity) of the foamed blends usually lie between the limits of the foamed constituents, although the relationship between property and blend content is not always linear. The reasons must he in the microstructure most polymer pairs are immiscible, but some such as PS/polyphenylene oxide (PPO) are miscible. Eor the immiscible blends, the majority phase tends to be continuous, but the form of the minor phase can vary. Blends of EVA and metallocene catalysed ethylene-octene copolymer have different morphologies depending on the EVA content (5). With 25% EVA, the EVA phase appears as fine spherical inclusions in the LDPE matrix. The results of these experiments on polymer films will apply to foams made from the same polymers. 

Blends of ethylene-vinyl acetate (EVA) copolymer with metallocene-catalysed elastomeric ethylene-alpha-olefin copolymer were investigated and were found to be immiscible in the melt and solid state but mechanically compatible. The morphology (SEM), thermal (DSC), rheological (viscosity), mechanical (including tensile, shear thinning and elastic behaviour) and optical properties of EVA-rich and ethylene-alpha-olefin copolymer-rich blends were studied and the results are discussed in terms of processibility in film applications. 24 refs. 

A review is presented of the nitrogen autoclave process for the manufacture of crosslinked polyolefin foams. Process and product developments over the last few years are summarised and future possibilities are described. Process developments include use of higher temperatures and pressures to produce foams having densities as low as 10 kg/cub.m. Product developments include foams based on HDPE/LDPE blends, propylene copolymers and metallocene-catalysed ethylene copolymers. The structure and properties of these foams are compared with those of foams produced by alternative processes. 5 refs. 

It is generally adopted that the catalytically active species in the metallocene-catalysed polymerization is a 14-electron cation. As an example, the mechanism of activation of an unbridged zirconocene catalyst is presented in Fig. 9.5-4, top. In the first two steps the activation by MAO, resulting in the 14-electron cation, is shown. The same cation can be generated by N,N -dimethylanilinium-tetrakis(pentafluorophenyl)borate and methylated metallocenes. As side-products methane and an amine are formed. TiBA can also be involved in the activation, which is not shown in Fig. 9.5-4, bottom. On the other hand, TiBA acts as a scavenger in the polymerization. The above-mentioned reactions take place in the absence of the monomer and are performed before the catalyst is used in the polymerization process. 

The LCB in metallocene-catalysed ethene polymerisation is considered to occur via a copolymerisation reaction where a vinyl-terminated polyethene chain is reinserted into a growing chain. Thus, the choice of the catalyst used will be extremely crucial. When the prerequisites of LCB are fulfilled, the process conditions will then be even more important [44, 60]. 

PP-b-PMMA (Mn = 22220, Mw/Mn = 1.14) was produced by CRP via another route. Terminally vinyl PP (Mn = 3100, Mw/Mn = 1.45, isotactic-ity = 32%) prepared using a zirconocene catalyst was converted to terminally brominated PP via PP-SiH prepared by hydrosilylation [70]. The resulting PP-b-PMMA was purified by extraction of unreacted PP with diethyl ether. Poly(ethylene-co-butene)-bZocfc-poly(methyl methacrylate) (EBR-b-PMMA) was synthesized through the bromination of terminally hydroxy-lated EBR (Mw = 3600 g/mol, Mw/Mn = 1.05), which was commercially available [71]. An atactic PP/PMMA had been synthesized by a combination of metallocene catalyses, Cp2ZrCl2 and Me2Si(CpMe4)(.W-f-Bu)TiCl2, and ATRP [72]. 

Variation of Cs symmetric metallocenes leads to Cj symmetric ones (Fig. 15). If a methyl group is introduced at position 3 of the cyclopentadienyl ring, stereospecificity is disturbed at one of the reaction sites so every second insertion is random. A hemiisotactic polymer is produced [119-121]. If sterie hindrance is bigger (for example a tert-butyl group is introduced instead of the methyl group) stereo selectivity is inverted and the metallocene catalyses the production of isotactic polymers (Table 10) [117,118,122-125],... 

Gomez-Elvira and co-workers [12] studied thermal stability at 90 "C to 140 °C of predominantly isotactic metallocene catalysed PP samples ranging from 3,000 to 41,000 molecular weight by chemiluminescent analysis and compared results with those obtained for predominantly isotactic Ziegler-Natta PP (ZNiPP) of increasing molecular weight. The structural differences of these two types of polymer is sufficiently different to expect very different thermal stability behaviour between them. 

Figure 1.11 An example of the chemiluminescence experiment showing the determination of the induction time. The curve corresponds to a sample of metallocene catalysed PP = 19,000, oxidised in at 140 °C. Reproduced with permission from J.M. Gomez-Elvira, P. Tiemblo, M. Elvira, L. Matisova-Rychla and J. Rychly, Polymer Degradation and Stability, 2004, 85, 873.

Zotefoams has added new F grades to its ED-15 polyethylene foam range to meet the needs of the automotive and aerospace markets. The range of foams is the lightest available for crosslinked foam with a density of 15 kg/m, some 30% lighter than any other polyolefin foam, and is made in LDPE and metallocene catalysed polyolefins. 

J. Pe6n, C. Dominguez, J.F. Vega, M. Aroca, J. Martinez-Salazar, Viscoelastic behaviour of metallocene-catalysed polyethylene and low density polyethylene blends use of the double reptation and Palieme viscoelastic models. J. Matra-. Sci. 38,4757-4764 (2003)... 

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