Quantitative description of the metathesis polymerization/ depolymerization equilibrium in the 1,4-polybutadiene system, 2 - Unusual behaviour at lower temperature

The product spectrum at the metathesis polymerization/depolymerization equi librium is very complex because of several subequilibria (ring/chain- equil ibrium, equilibrium between rings of different size, equilibrium between ch ains of different length, configuration equilibrium for the chains, configu ration equilibria for the rings). Continuing our work on the 1,4-polybutadi ene (resp. cyclobutene) system, investigations were carried out in toluene at low temperatures. The results reveal (for temperatures below 3 degrees C ) the existence of a solution/precipitation equilibrium in addition to the subequilibria known until now. The reason for this is that with decreasing temperature the trans amount increases and, when a critical value and thus a critical block length of trans units is reached, crystallization occurs w ith remarkable negative enthalpy, forcing the polymeric chains to precipita te. The degree of polymerization must be larger than 30 for the precipitati on. At a feed concentration lower than the integral turning point (it is eq ual to the saturation concentration of all rings), no precipitation is obse rved even at lower temperatures than 3 degrees C. The results obtained show that the whole equilibrium system must be divided into two parts, a soluti on region which is dominated by the exothermic build-up of the all-trans cy clic trimer and a heterogeneous area determined by the polymeric chains pre cipitated. These are more stable than any ring in the system. Even the form ation of the all-trans cyclic trimer occurs endothermically in the back bit ing reaction in this region.