Raman spectroscopic characterization of high-vinyl polybutadienes produced from anionic polymerization Saran Poshyachinda, Howell G. M. Edwards* and Anthony F. Johnson Chemistry and Chemical Technology, Department of Chemical Engineering, University of Bradford, Bradford, West Yorkshire BD7 1DP (Received 6 January 1990; accepted 2 March 1990) High-vinyl polybutadienes have been prepared by anionic polymerization using n-butyllithium as initiator in cyclohexane solvent and complexing agents in order to change the mode of addition of monomer to (1,2)- or vinyl addition. The microstructure of these polymers was characterized by Raman spectroscopy using the Raman active carbon-carbon double bond stretching bands of vinyl, cis and trans structures at 1640, 1650 and 1664cm -1, respectively. Band areas were analysed with the aid of a curve analysis program and the data thus acquired used for subsequent calculations. The living-end concentration was determined from the number average molecular weight of the polymer obtained by gel permeation chromatography, which made it possible to calculate a more accurate value for the molar ratio, r, of the complexing agent to living polyanions than is possible by more conventional means. It has been observed that 1,2-dipiperidinoethane (DIPIP) is a very effective complexing agent for the control of polymer microstructure as is diglyme at 0°C but their effectiveness decreases with increasing temperature. (Keywords: anionic polymerization; high-vinyl polybutadiene; Raman spectroscopy; complexing agent; mierostructure) INTRODUCTION The polymerization of (1,3)-butadiene gives rise to pro- ducts which have a (1,4)-polybutadiene or (1,2)-poly- butadiene structure or a mixture of these. The (1,4)- addition products may comprise the cis or trans isomer while the (1,2)-addition may be isotactic, syndiotactic or atactic, or some mixture of these structural types (Figure 1). Anionic polymerization of butadiene in non-polar media with alkyllithium initiators results in polybuta- dienes with low-vinyl content. Langer I found that the introduction of a relatively small amount of N,N,N',N'- tetramethylethylenediamine (TMEDA) into the polymer- izing solution gave rise to polybutadienes with a predominantly vinyl microstructure with atactic pendant vinyl groups. It is thought that TMEDA forms a complex with living polybutadienyllithium chain ends and changes the mode of addition of monomer from primarily (1,4)- to (1,2)-addition. Antkowiak et al. 2 reported the effects of temperature and concentration of complexing agent on the microstructure of the resulting polybutadienes as characterized by infrared spectroscopy. Polybutadienes with vinyl microstructure as high as 80% can be ob- tained using diglyme as the complexing agent at 30°C. Later, 1,2-dipiperidinoethane (DIPIP) was found to be the most effective complexing agent for butadiene polymerization 3,4. Determination of the microstructure of polybutadienes by infrared spectroscopy is not simple because the method requires calibration and, more importantly, the carbon-carbon double bond stretching vibration, v(C=C), of a trans unit is infrared inactive. Therefore, * To whom correspondence should be addressed 0032-3861/91/020033845 © 1991 Butterworth-Heinemann Ltd. 338 POLYMER, 1991, Volume 32, Number 2 the infrared absorption of the skeletal backbone of the polybutadienes cannot be used and carbon-hydrogen deformations of the three microstructures were selected for the analyses 5. Cornell et al. 6 reported the first Raman spectroscopic studies of polybutadienes. The micro- structure was quantitatively determined from the v(C=C) Raman bands of vinyl, cis and trans units at 1640, 1650 and 1664 cm- ~, respectively, from peak height determination. This method suffered from the inherent disadvantage that derives from the incomplete resolution of the v(C=C) Raman bands arising from the three microstructures, especially when one or more of the components is in a low species concentration. In this work we report the use of Raman spectroscopy for the microstructure determination of high-vinyl poly- butadienes produced anionically in cyclohexane using an alkyllithium initiator with different complexing agents. A curve analysis program applied to the computerized data acquisition enables the band areas of each compo- nent band to be evaluated and makes it possible to obtain accurate microstructure compositions from samples which contain relatively small amounts of one or more components. In addition, laser excitation of the Raman spectra gives a means of testing sample homogeneity and surface microstructure composition. EXPERIMENTAL The experiments were carried out under high vacuum using well established techniques T which have been described fully in an earlier paper 8. The solvent, cyclo- hexane (May and Baker, h.p.l.c, grade), was distilled from n-butyllithium prior to use. (1,3)-Butadiene (Aldrich) was also freshly distilled from n-butyllithium into the reaction vessel. The complexing agents, diglyme (Aldrich) and