Equibinary polymers

The kinetics of butadiene polymerized by bis(7T-allyl nickel trifluoro-acetate) has been studied by Teyssie et al. [173]. Equilibrium constants for the formation of the complex (with aromatic compounds which give the equibinary polymer), calculated from cryoscopic measurements or polymer micro-structure, are 6—7 and 26—35 respectively for benzene and nitrobenzene. The higher concentration of active centres compared with... 

These results show clearly that there cure two different and specific catalytic sites in equilibrium (separated, or on the same binuclear complex), vhich yield respectively the high cis and the equibinary polymer. 

This "dual-site mechanism (v ch might cLLso arise from bi-molecular interactions between 2 allyl-oonplexes), has received preliminary support from severail types of experiments suffice it to ronind the polymerization (36) of isqprene by od alt ocnplexes which yields a (cis-1,4-3,4) equibinary polymer, but v erein the vinyl-type 3,4 units (and not the cis-1,4 ones) are selectively replaced by methylmethacrylate units this vinyl ocmononer is present. 

Several procedures are available for preparing polybutadienes that contain equal amounts of 1,4-cis and 1,2- units [104,105]. The structures of these equibinary" polymers have been investigated extensively by cmr spectroscopy [21,22,106,107]. The olefinic carbons of the 1,4-cis units are particularly useful for determining cis unit centered triad distributions, and some of the aliphatic carbon resonances have provided measures of dyad distributions. The... 

Catalyst complexation with a Lewis base or other electron donor may affect the polymer microstructure in different ways. If the added component occupies one coordination site, a monomer coordinates to another site of the active species with one double bond, i.e. as an s-trans-rf ligand, which gives rise to the formation of trans-1,4 monomeric units via the pathway (a)-(b) [scheme (10)]. Depending on the lifetimes of metal species complexed with the monomer and with the Lewis base or the other donor [scheme (11)], mixed cis-1,4/trans- 1,4-polybutadienes or an eb-czs-1, 1 A trans-1,4-polymer can be formed. One should mention in this connection that equibinary cis-l,A/trans- 1,4-butadiene polymers can also be formed in systems without the addition of a Lewis base or other electron donor in such cases, the equilibrium of the anti-syn isomerisation is not shifted and there are equal probabilities for the reaction pathways involving coordination of a transoid monomer and a cisoid monomer [7]. 

It is worth mentioning that/ under certain conditions/Specific catalytic systems can lead to the formation of butadiene or isoprene polymers containing equal amounts of cis and trans units (73). For these polymers the term "equibinary polydienes" has been proposed. It now appears that depending on the nature of the solvent and on the temperature of polymerization/ the distribution of the two isomeric units can range from purely random/ to highly alternate or even to sequential (16). These results hint at an interesting new type of catalytic control. 

Fig.4 demonstrates the possibility to control, by the add-tion of specific ligand, the formation of polymers containing exactly equal amounts of cis and trans isomers. It was shewn later that this type of behaviour is quite general in coordination catalysis, and the name "equibinary polydienes" has been coined for these polymers containing equal amounts of 2 different isomeric units. Other typical exaitples include the equibinary (1,4 cis or trans -1,2) polybutadienes (33), (cis 1,4-3,4) polyisoprene (34), (1,2-3,4) polyisoprene, dbtained in the presence of various conplexes of nickel, molybdenum and cobalt. 

A priori an equibinary oonposition could result either from a mixture of equal amounts of horoo-cis and hono-trans polybutadienes, or alternatively from a randan, or an alternate, or even a stereobLock placement of cis and trans units in the same chadns. The first possibility could be easily discairded, since the polymers obtained cannot be separated into different oonpositions by classical fractionnation methods, and the product has none of the characteristic physical properties of the pure cis- and transpolybutadienes. ... 

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