Ziegler-Natta catalysts alternating copolymers

Alternating copolymers have been obtained by copolymerisation of ethylene and cycloolefins (using a large excess of cycloolefin) in the presence of vanadium-based Ziegler-Natta catalysts such as V(Acac)3-AlEt2Cl and VC14 AlEt2Cl ... 

Explain why butadiene forms alternating copolymers with ethylene in copolymerisation with some Ziegler-Natta catalysts. 

The 1,2-disubstituted olefmic monomers will usually not homopolymerize with the Ziegler-Natta catalysts. They can, however, be copolymerized with ethylene and some a-olefins. Due to poorer reactivity, the monomer feed must consist of higher ratios of the 1,2-disubstituted olefins than of the other comonomers. Copolymers of cw-2-butene with ethylene, where portions of the macromolecules are crystalline, form with vanadium-based catalysts. The products have alternating structures, with the pendant methyl groups in erythrodiisotactic arrangements. Similarly, vanadium-based catalysts yield alternating copolymers of ethylene and butadiene, where the butadiene placement is predominantly rm/w-1,4. ... 

Modifications to the Ziegler-Natta catalyst system have led to the preparation of alternating copolymers from olefins, such as ethylene and propylene, and diolefins, such as butadiene and isoprene. In typical systems (Furukawa 1972, 1974a, b) the catalyst is prepared at very low temperatures (-70°C or below) from three components a... 

Pasch and co workers [45] hyphenated TREF fractionation with SEC-FTIR to measure the compositional heterogeneity within a commercial impact PP copolymer and demonstrated successfully that the combination of TREF and SEC-FTIR provides a simple alternative to more time-consuming conventional ways of characterizing impact PP copolymers of complex heterogeneity. Hasan et al. [46] correlated the TREF, C NMR and SEC results of PP produced by different Ziegler-Natta catalysts in terms of catalyst preparation method, polymerization conditions, type of co-catalyst, and evaluated the performance of the catalyst with respect to several aspects including the grinding effect on the surface catalyst performance, etc. Their results show that the polymerization activity, isospecificity and the isospecificity distribution of active sites of TiCls catalysts are mainly... 

Acrylonitrile-butadiene copolymers may also be prepared by the use of Ziegler-Natta catalysts. Catalysts such as a triethylaluminium, aluminium chloride and vanadium chloride combination give copolymers with a high degree of alternation. Such copolymers appear to have improved mechanical properties but have not attracted much commercial interest. 

Low molecular weight liquid nitrile rubbers with vinyl, carboxyl or mercaptan reactive end groups have been used with acrylic adhesives, epoxide resins and polyesters. Japanese workers have produced interesting butadiene-acrylonitrile alternating copolymers using Ziegler-Natta-type catalysts that are capable of some degree of ciystallisation. 

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. 

The copolymerizations between monoolefins and dienes have been considered to be of practical and theoretical importance. As reported in the literatures ethylene-butadiene and propylene-butadiene copolymers can be prepared with conventional Ziegler-Natta titanium-based or vanadium-based catalysts. The copolymer composition and monomer sequence distribution strongly depend on the catalyst system and polymerization conditions. Alternating copolymers were synthesized when the catalyst components were mixed at the... 

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