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

Abstract

The product spectrum at the metathesis polymerization/depolymerization equilibrium is very complex because of several subequilibria (ring/chain-equilibrium, equilibrium between rings of different size, equilibrium between chains of different length, configuration equilibrium for the chains, configuration equilibria for the rings). Continuing our work on the 1,4-polybutadiene (resp. cyclobutene) system, investigations were carried out in toluene at low temperatures. The results reveal (for temperatures below 3°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 with remarkable negative enthalpy, forcing the polymeric chains to precipitate. The degree of polymerization must be larger than 30 for the precipitation. At a feed concentration lower than the integral turning point (it is equal to the saturation concentration of all rings), no precipitation is observed even at lower temperatures than 3°C. The results obtained show that the whole equilibrium system must be divided into two parts, a solution region which is dominated by the exothermic build-up of the all-trans cyclic trimer and a heterogeneous area determined by the polymeric chains precipitated. These are more stable than any ring in the system. Even the formation of the all-trans cyclic trimer occurs endothermically in the back biting reaction in this region.