Copolymerisation alternating copolymers

Though important results have already been obtained in the carbonylation of olefins, the field still remains open. Development of more active, efficient and stable catalysts based also on less expensive metals will make the carbonylation processes more attractive. Carbonylation of less common olefins, including functionalised ones, has to be explored in more depth. Other important targets are the efficient living copolymerisation, the multiple olefin insertion producing non-alternating copolymers and the selective synthesis of unsaturated products like acrylates and methacrylates. 

The factors that control the strictly alternating copolymer chain with no detectable errors (e. g., microstructures involving double insertion of ethene) have been the object of detailed studies since the discovery of the first Pd" catalysts for the alternating alkene/CO copolymerisation [11]. Sen was the first to demonstrate that double carbonylation is thermodynamically unfavorable and to suggest that the higher binding affinity of Pd" for CO relative to ethene inhibits multiple ethene insertions, even in the presence of very low concentrations of CO [12]. Therefore, once a palladium alkyl is formed, CO coordination ensures that the next monomer will be a CO molecule to generate the acyl complex. 

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 ... 

Copolymerisation of conjugated dienes and alkenes can lead to random or alternating copolymers, depending on the applied catalyst and copolymerisation conditions [7],... 

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

As in the case of homopolymerisation, no problem of isomerisation of propagating species exists in the copolymerisation of norbornene with carbon monoxide and hence 2,3-bicyclo[2.2.1]hept-2-ene units, as the only norbornene monomeric units appear in alternating copolymers with carbon monoxide [27] ... 

Alternating copolymers of cycloolefins and carbon monoxide have also been obtained in the case of cyclopentene the copolymerisations were run using cationic Pd(II) complexes modified by an achiral l,3-bis(diphenylphosphino)-propane ligand or a chiral 2,4-bis(diphenylphosphino)pentane ligand. 

Oxirane/cyclic acid anhydride alternating copolymers of controlled molecular weight with a narrow molecular weight distribution were found by Aida et al. [188,189] to be formed under mild conditions when copolymerising ethylene oxide and phthalic anhydride in the presence of the (tpp)AlCl-quater-nary phosphonium salt catalyst system. The copolymerisation carried out with (tpp)AlCl alone proceeded very slowly, and the product was not polyethylene terephthalate) but contained ether linkages in considerable amount. The development of the living character and the tendency towards alternation of the copolymerisation when using the aluminium porphyrin catalyst, coupled with a quaternary salt, have been postulated [188,189] to be due to the formation of... 

Copolymerisations of heterocyclic and heterounsaturated monomers generally lead to random copolymers with a prevailing content of the heterocyclic monomer and to alternating or nearly alternating copolymers, depending on the kind... 

Catalysts derived from reaction systems such as triethylaluminium-water and triethylaluminium-water-acetylacetone [225], triethylaluminium-triphe-nylphosphine [226], triethylaluminium-pyrogallol [209] and rare-earth metal phosphonate- triisobutylaluminium-glycerol [227] appeared to be effective in the copolymerisation of propylene oxide and carbon dioxide, yielding high molecular weight polypropylene ether-carbonate)s (Table 9.4) but not the respective alternating copolymer which is polypropylene carbonate). 

The copolymerisation of ketene and acetaldehyde was carried out with ethyl-zinc diphenylamide as the catalyst. It was found to yield an alternating copolymer [scheme (43)], the respective polyester of highly isotactic structure (Table 9.3) [279] ... 

Alternating copolymers may be considered as homopolymers with a structural unit composed of the two different monomers. Random copolymers are obtained from two or more monomers, which are present simultaneously in one polymerisation reactor. In graft polymerisation a homopolymer is prepared first and in a second step one or two monomers are grafted onto this polymer the final product consists of a polymeric backbone with side branches. In block copolymerisation one monomer is polymerised, after which another monomer is polymerised on to the living ends of the polymeric chains the final block copolymer is a linear chain with a sequence of different segments. 

Scheme 5.19 Synthesis of a diene and a diacrylate to be copolymerised further to give AB-alternating copolymers by ALTMET polymerisation [68]...

Alternating copolymerisation. This occurs if r = t2 = 0, that is, each radical reacts exclusively with the other monomer, not with its own (Figure 2.5). In this case d[Mi]/d[M2] = 1 and for each monomer feed composition = in = 1- An example is the copolymerisation of vinyl acetate (Mi) with maleic anhydride (M2), for which n = 0.055, f2 = 0.003 and r r2 = 0.00017. Alternating copolymers can crystallise only if the copolymers are purely alternating and if the product is stereoregular or if the monomers are symmetrical. 

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]. 

In the case of ethylene/carbon monoxide copolymerisation with nickel- and palladium-based catalysts, a strictly alternating high molecular weight copolymer is formed (average molecular weight in the range 10 x 103 100 x 103).When more developed catalysts are used, the copolymerisation conditions can be mild a temperature of 25 °C combined with a pressure of ca 20 atm. The obtained copolymer, poly(ethylene-c// -carbon monoxide), poly(l-oxytrimethylene)... 

Palladium-based catalysts bearing chiral ligands have also been found to be capable of the stereospecific copolymerisation of allylbenzene [492] and its derivatives [493] with carbon monoxide the formed copolymers appeared to be of an alternating, isotactic structure. 

In general, copolymerisation of conjugated dienes with coordination catalysts is difficult because of the strong selectivity of the catalysts towards monomers of the particular type. The use of suitable catalysts, however, makes it possible to obtain copolymers of conjugated dienes with various monomers. Obtained copolymers can be random or alternating, depending on the kind of comonomers and catalysts. 

Since oxiranes are representative heterocyclic monomers containing an endo-cyclic heteroatom, and the most commonly polymerised of such monomers, they have been subjected to copolymerisations with heterocyclic monomers containing both an endocyclic and an exocyclic heteroatom. Coordination copolymerisations of heterocyclic monomers with different functions are focused on oxirane copolymerisation with cyclic dicarboxylic acid anhydride and cyclic carbonate. However, the statistical copolymerisation of heterocyclic monomers with an endocyclic heteroatom and monomers with both endocyclic and exocyclic heteroatoms have only a limited importance. Also, the block copolymerisation of oxirane with lactone or cyclic dicarboxylic acid anhydride is of interest both from the synthetic and from the mechanistic point of view. Block copolymerisation deserves special interest in terms of the exceptionally wide potential utility of block copolymers obtained from comonomers with various functions. It should be noted, however, that the variety of comonomers that might be subjected to a random, alternating and block polymerisation involving a nucleophilic attack on the coordinating monomer is rather small. 

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