Alternating copolymers transition metals

Fluoride activation of Si-C bonds toward electrophiles has recently been exploited to synthesise alternating thiophene-perfluoroarene copolymers without using transition metal catalysis. This has the advantage of leading to products that are devoid of even traces of metal residues <2006JA2536>. Here the electrophiles are perfluoroarenes (ttF) the potential nucleophilic sites are the 2- and 5-positions of 3,4-dibutoxy-2,5-bis(trimethylsilyl)thiophene. The reaction is initiated with catalytic fluoride ion, which is regenerated with each C-C bond formed (Equation 106). 

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 high oxophilicity of early transition metal catalysts (titanium, zirconium, or chromium) causes them to be poisoned by most functionalized olefins, particularly the commercially available polar comonomers. However, there are examples of copolymerizations with special substrates or with very high levels of a Lewis acid incorporated into the polymerization system to protect the polar functionality through complexation. " Alternative routes to polar copolymers involving metathesis of cyclic olefins and functionalization of the resulting unsaturated polymer or metathesis of polar cycloolefins followed by hydrogenation to remove the resulting unsaturation have been published.The cost of these multistep... 

Although presently lacking industrial importance, alternating copolymers can be made from propylene and butadiene, also from propylene and isoprene. Copolymers of propylene and butadiene form with vanadium- or titanium-based catalysts combined with aluminum. alkyls. The catalysts have to be prepared at very low temperature (-70 C). Also, it was found that a presence of halogen atoms in the catalyst is essential.Carbonyl compounds, such as ketones, esters, and others, are very effective additives. A reaction mechanism based on alternating coordination of propylene and butadiene with the transition metal was proposed by Furukawa. ... 

Scheme 31.3 Inset Reactions of NHCs with themselves, transition metals, and electrophiles. General strategy for forming homopolymers (11), organometallic polymers (12), or alternating copolymers (13) from a facially opposed bis(NHC) (10).

Nearly 40 years have evolved between the first discovery by Reppe of transition-metal-catalyzed CO/ethene copolymerization and the discovery in 1983, at Shell, of a class of highly active, high yield palladium catalysts for the synthesis of high molecular weight, perfectly alternating CO/ethene copolymers [PK-E, Fig. 1], This class of catalysts is also active for the co- and terpol5unerization of CO with alkenes other than ethene, both simple aliphatic and heteroatom functionalized, thus providing access to a family of completely new polyketone polymers. 

The incorporation level of norbornene, as well as its distribution and enchainment orientation, have a great influence on the properties of E/NB copolymers. Norbornene copolymerization with early and late transition metal complexes happens exclusively by double bond addition (2,3-insertion). The norbornene unit has a 2,3-cis-exo-orientation (Figure 16.14a), and two stereocenters are formed in the polymer chain. Copolymers with isolated norbornene units therefore have different possible tacticities than alternating copolymers (Figure 16.14b). 

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