Metallocenes alternating copolymers

Leclerc and Waymouth (119) and, independently, Arndt et al. (120) synthesized alternating copolymers of ethylene and propylene with zirconocene catalysts. The ethylene/propylene (EP) copolymerizations were carried out at 30 and 60°C for each of four metallocene catalysts (Me2C(3-RCp)(Flu)) ZrCl2 (R = H, Me, lsoPr, tertBu) (Fig. 10). As the size of the substituent increased, there were distinct changes in the copolymerization behavior and in the polymer microstructure. 

The alternating copolymers are characterized both by a glass transition temperature (130°C for ethylene/norbornene copolymers) and a melting point (295°C for the totally alternating copolymer). The melting point and the crystallinity of these copolymers may be influenced by choice of the metallocene and the conditions of polymerization. Compared with the statistical copolymers, the alternating structures show better resistance to nonpolar... 

Metallocene catalysts can produce both random and alternating copolymers of ethylene and propylene [85]. At present there does not appear to be any commercial utilization of alternating copolymers. They were reported to form in polymerizations catalyzed by bridged fluorenyl catalysts [85]. 

While most conventional catalysts failed to copolymerize st)n ene, halfsandwich-metallocene catalysts proved very effective in styrene copolymerization. In addition to random copolymers, specific catalysts were found to produce alternating copolymers. Semetz et al. [15] determined copolymerization parameters of random ethylene/styrene copolymerization using single site (CpMe4)SiMe2N(tert.Bu)TiCl2/MAO to be... 

Most metallocenes produce copolymers with a statistical stmcture, and a few produce polymers with an alternating stmcture. Statistical copolymers are amorphous if more than 10-15 mol% of cycloolefins are incorporated into the polymer chain. The glass transition temperature can be varied over a wide range by selection of norbomene as cycloolefin and variation of the amount of norbomene incorporated into the polymer chain [104]. 

Randomly incorporated ethylene introduces defects along the backbone. These defects dismpt crystallization, reducing the modulus, melting point, and heat of fusion. The incorporation of random ethylene also reduces haze. Butene has also been used as a comonomer in PP. With the development of metallocene catalysts, even higher a-olefins such as hexene could be incorporated. While these alternative copolymers are now technically feasible, they have not seen commercial... 

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. 

Block copolymers can be produced from terminally borane-containing polyolefins. These borane-containing POs can be synthesized by the metallocene-catalyzed (co)polymerization of olefin(s) monomer with 9-BBN as a chain transfer agent or by the metallocene catalyzed copolymerization of olefins with allyl-9-BBN [55,56], as referred to above. Alternatively, borane-containing POs were prepared by hydroboration of terminally unsaturated PO, for instance, terminally vinyl PE and terminally vinylidene PP [33-35,57]. Such method could produce diblock copolymers, such as polyethylene-block-poly(methyl methacrylate) (PE-fo-PMMA), polypropylene-foZock-poly(methyl methacrylate) (PP-fc-PMMA), polypropylene-foZock-poly(butyl methacrylate) (PP-fc-PBMA), and PP-fc-PS. 

Metallocene catalysts show low r values, which allows easy incorporation of bulky cycloolefins into the growing copolymer chain. Surprisingly, the ethylene reactivity ratio in copolymerisation with cyclopentene in the presence of a (ThindCH2)2ZrCl2-based catalyst (r = 2.2) and in copolymerisation with norbornene in the presence of catalysts characterised by Cs and Ci symmetry (ri 3.4 and 3.1 respectively) is considerably lower than that for the copolymerisation of ethylene with propylene (r = 6.6 at 37 °C). Various catalysts produce copolymers of structures that are between statistical and alternating [468]. 

Metallocene catalysis is an alternative to the traditional Ziegler-Natta vanadium-based catalysis for commercial polyolefin production, e.g. the use of metallocene-catalyzed ethylene alpha-olefin copolymers as viscosity index modifiers for lubricating oil compositions [23]. The catalyst is an activated metallocene transition metal, usually Ti, Zr or Hf, attached to one or two cyclopentadienyl rings and typically activated by methylaluminoxane. Metallocene catalysis achieves more stereo-regularity and also enables incorporation of higher alpha-olefins and/or other monomers into the polymer backbone. In addition, the low catalyst concentration does not require a cleanup step to remove ash. 

ACRONYMS, ALTERNATIVE NAMES mLLDPE, metallocene PE, single site catalyzed LLDPE (SSC), polyolefin plastomers (POP), homogeneous ethylene copolymers... 

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