Quantitative description of the metathesis polymerization depolymerizationequilibrium in the 1,4-polybutadiene system, 1 - Influence of feed concentration and temperature

As reported recently, our results about the ring distribution in the polybu tadiene (resp, cyclobutene) system at the metathesis polymerization/depolym erization equilibrium differ strongly from literature data published earlie r. To support our findings with thermodynamic data, the product spectrum in toluene was determined at equilibrium in dependence of the feed concentrat ion (0.1 to 1.8 mol [=C4H6=]/L) and the temperature (4 degrees C to 50 degr ees C). For better understanding of the ring-chain distribution we introduc ed the turning point. This is the feed concentration at which the different ial change of the ring equilibrium concentration equals that of the chains. In contrast to the terminology used until now, as for example "cut off poi nt" or "critical concentration", the turning point describes what exactly h appens in the system and has a physical meaning not only for the integral r ing concentration but also for the concentration of every individual ring. The integral turning point was determined, it is temperature dependent. A s trong preference for the cyclic trimers (especially for the all-trans one) was observed, which is the only ring size to show a temperature influence a s a whole. The all-trans cyclic trimer is built the most exothermically in comparison with any other cyclic geometric isomer. The ctt-isomer is built endothermically. Tetramers are disfavoured regarding the pentamers. The amo unt of trans double bonds in the trimers at 25 degrees C is 97%. The trans amount decreases with increasing ring size and approaches a trans/cis ratio of about 83/17 at 25 degrees C. In the whole temperature region the polyme ric chains show a most probable distribution. The trans/cis ratio in the po lymeric fraction as a function of the temperature follows a clear tendency in accordance with the thermodynamic prediction. The trans respectively cis double bonds are distributed randomly across the chains. For all cases men tioned, quantitative thermodynamic data are presented.