Diene polymerization mechanism

Despite the intensive investigation of the stereospecific polymerization of 1,3-dienes by transition metal catalysts in the past 25 years, the detailed mechanism governing stereselectivity is still not well understood. Various proposals on the diene polymerization mechanism have been 14-17... 

Mechanisms have been proposed to explain stereoselectivity in 1,3-diene polymerizations, but these processes are less understood than the polymerization of ethylene and 1-alkenes [Peluso et al., 1997]. The ability to obtain cis 1,4-, trans 1,4-, and st 1,2-polymers from 1,3-butadiene, each in very high stereoregularity, hy using different initiators has great practical utility for polymer synthesis even if it is not well understood why a particular initiator gives a particular stereoregular polymer. 

The diene polymerizations catalyzed by organometallic compounds are probably further examples of diene insertion reactions of metal alkyls (25). But there is little evidence on the detailed mechanisms of these polymerizations. 

B17. — —.and W. L. Miller The formation of linear polymers from diene monomers by a cyclic polymerization mechanism. I. The structure of poly-(diallylammonium halides). J. Am. Chem. Soc. 80, 3615 (1958). 

III. Mechanism of Diene Polymerization with Polar Ligands. 65... 

Most reaction models which describe the mechanism of diene polymerization by Nd catalysts have been adopted from models developed for the polymerization of ethylene and propylene by the use of Ti- and Ni-based catalysts systems. A monometallic insertion mechanism which accounts for many features of the polymerization of a-olefins has been put forward by Cossee and Arlman in 1964 [624-626]. Respective bimetallic mechanisms date back to Patat, Sinn, Natta and Mazzanti [627,628]. The most important and generally accepted mechanisms for the polymerization of dienes by Nd-based catalysts are discussed in the following. 

A detailed review on the multiple aspects of the reaction mechanisms of diene polymerization with transition-metal catalysts was published by Porri, Giarrusso and Ricci in 1991 [486]. 

Although heterobimetallic complexes with alkylated rare-earth metal centers were proposed to promote 1,3-diene polymerization via an allyl insertion mechanism, details of the polymerization mechanism and of the structure of the catalytically active center(s) are rare [58,83,118-125]. Moreover, until now, the interaction of the cationizing chloride-donating reagent with alkylated rare-earth metal centers is not well-understood. Lanthanide carboxylate complexes, which are used in the industrial-scale polymerization of butadiene and isoprene, are generally derived from octanoic, versatic, and... 

Scheme76 Modified allyl insertion mechanism of the neodymium-based 1,3-diene polymerization taking into account organoaluminum-mediated chain transfer [148]...

In our studies an attempt was made to obtain detailed information on the mechanism of stereospecific diene polymerization initiated by several catalyst systems. To this end the structure and some chemical reactions of compounds used as models of the system "catalyst-growing polymer chain were studied in detail. 

According to present ideas, the initiation mechanism of diene polymerization on ZN centres does not substantially differ from that of alkene initiation. Experiments support the idea according to which the chain is propagated on the transition metal atom it may be bound to the metal either by a a, or by a n allylic bond... 

It is apparent that the bridged ring monomers in which the double bond is made more reactive by steric strain are comparable in reactivity with ethylene, whereas, as would be expected, the aliphatic dienes are more like a-olefins in reactivity. Baldwin and van Strate have critically discussed this work [244], drawing attention to the possibilities of E/P reactivity ratios being changed by the presence of the diene and of diene polymerization by cationic mechanisms. They identify as... 

With [Ni(C3H5)I]2 as a trans catalyst and [Ni(C3H5)02CF3]2 as a cis catalyst the allyl insertion mechanism has been proven directly by H- and C-NMR spectroscopic measurements as the principle catalytic reaction for chain growth in diene polymerization [24, 25] (cf. Scheme 2). 

Scheme 2. Schematic representation of the allyl insertion mechanism as the catalytic principle of chain growth in the complex-catalyzed diene polymerization.

To derive the reaction mechanisms of the complex-catalyzed stereospecific diene polymerization the following stereochemical and mechanistic aspects, which are considered as well established today, must be taken into account. 

While several explanations may be given for these findings, we focus here on our contention that this result supports the importance of allyl isomerization in diene polymerization. We postulate initially that the loss of bond forming stereospecificity in the diene polymerization is the result of allyl isomerization. This is particularly well supported by the completely stereorandom bond formation found for trans polymer. This is satisfactorily accounted for by rapid isomerization of the label, via primary allyls, to a random(l l) mixture of syn and anti deuterium. Any other mechanism for randomization, for example... 

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