Stereospecificity in Anionic Polymerization

Bywater, S.

doi:10.1007/978-94-009-3175-6_13

Cited by 4 publications

References 22 publications

(13 reference statements)

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Active centre structure and the stereoregulation process in anionic polymerization

Bywater

1992

Makromolekulare Chemie. Macromolecular Symposia

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The mechanism of stereoregulation in the anionic polymerization of butadiene and isoprene is discussed in terms of what is known about the structure of the active centres. This includes the effect of the presence of cis and trans isomers and the dynamics of their interconversion and in the isoprene case, the effect of two distinct types having 1.4 and 4.1 structures.The difflcult task of elucidation of the stereoregulation process is made much easier in anionic polymerization because of the good stability of the active centres under most conditions. Particularly important is the ability to characterize them by NMR spectroscopy in the form of oligomeric models but also by optical spectroscopy during actual polymerization processes. The first indication of interest mechanistically from N M R spectroscopy was the appearance of cis and trans forms of the model compounds t.bu. CH&H-CHz CH, Li+ (1) and t.bu. C--CHZC(CH,)~CH-~ L i ' (2) which mimic the active centres in butadiene and isoprene polymerizatioqrespectively. A cross-check with optical spectroscopy revealed that generally the two forms absorb at different wavelengths (trans at longer h) so that isomerization can be monitored during actual polymerization. The relative amounts of each depend on counter-ion and solvent but allylic lithium compounds generally, largely independent of the hydrocarbon chain attached, prefer the trans form in hydrocarbon solvents and cis form in polar solvents.This observation is of obvious interest in terms of the classic recipe for the preparation of highly cis-1.4 polyisoprene in non-polar solvents by lithiumbased initiation (3). One might expect that the predominant trans active centres would lead to a polymer rich in trans units. Since the exact opposite is observed then it is clear that the equilibrium state is not attained under most conditions. A study of the early stages of isoprene addition (4). together with measurements of the cis + trans isomerization rates, showed that the 0 1992 Huthig & Wepf Verlag, Basel

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Active centre structure and the stereoregulation process in anionic polymerization

Bywater

1992

Makromolekulare Chemie. Macromolecular Symposia

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Ion‐pair structure in anionic polymerization. Effect of temperature and association on an allylic lithium compound in two polar solvents

Bywater

Black

1994

Macromolecular Symposia

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NMR studies are reported on enriched samples of a one-unit model of the active centres present in butadiene polymerization. Concentrations between 2 x M and 1M, and temperatures between -120" and 50" were investigated. The effect of ion-pair association on charge distributions in diethyl ether solutions is described. The effect of temperature on the overall structure in tetrahydrofuran solutions is discussed.The use of spectroscopic methods to study charge delocalized ion-pairs has contributed much to our knowledge of their structure in solution. Much of this refers to ion-pairs derived from aromatic compounds where optical spectroscopy has proved to be extremely valuable."' This technique is of less value in the case of allylic ion-pairs of the type present in anionic diene polymerization, although interesting information can still be ~btained.".~' I3C NMR studies on single (or two unit) monomer models, on the other hand, can give evidence on charge distribution and bond rotations in the active ~entre.'~"' The low natural abundance of the I3C nucleus however, means that concentrations in the range of 0.5 to 1.0 M have generally been used. In many cases (particularly with lithium compounds) the species observed is then in an associated state. The sensitivity of the NMR method can obviously be improved if I3C enriched samples are available, so that concentrations -lo3 M can be used. A comparison made with results obtained at higher concentrations may also be of value. This communication reports on I3C NMR spectra obtained on neopentylallyllithium (t.C,H,-CH,-CH=CH'CH,Li') prepared a) with 2-13C enriched butadiene or b) with a mixture of 55% 1-I3C enriched butadiene and 45% of 2-I3C enriched butadiene. The former samples were used generally in the high concentration experiments, where they are particularly valuable in the 0

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Polymerization of conjugated dienes initiated by soluble organosodium compounds in hydrocarbon solvents

Arest-Yakubovich¹,

Pakuro²,

Zolotareva³

et al. 1995

Polymer International

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Polymerization of butadiene and isoprene in hydrocarbon solvents initiated by unsolvated organosodium compounds was studied. It was found that the polymer molecular weight and the MWD are determined mainly by chain transfer to solvent and polymer, and no chain transfer to monomer was observed even in the case of isoprene. The overall polymerization rate is proportional to the concentrations of the monomer and the initiator. Apparent chain propagation rate constants were found to be 0.11 litre mo1-ls-l for butadiene and 0.065 litre mol-ls-' for isoprene polymerization in heptane at 30°C. It is suggested that associated (dimeric) forms of polydienylsodium active centres play an important role in chain propagation, being responsible for a stronger chain transfer and a greater l,Zbutadiene, or 3,4-isoprene, unit content than in polymerization with other alkali metals.

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Stereospecificity in Anionic Polymerization

Cited by 4 publications

References 22 publications

Active centre structure and the stereoregulation process in anionic polymerization

Active centre structure and the stereoregulation process in anionic polymerization

Ion‐pair structure in anionic polymerization. Effect of temperature and association on an allylic lithium compound in two polar solvents

Polymerization of conjugated dienes initiated by soluble organosodium compounds in hydrocarbon solvents

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