Terpene Coordinative Chain Transfer Polymerization: Understanding the Process through Kinetic Modeling

The interest in the Coordinative Chain Transfer Polymerization (CCTP) of a family of naturally occurring hydrocarbon monomers, namely terpenes, for the production of high-performance rubbers is increasing year by year. In this work, the synthesis of poly(β-myrcene) via CCTP is introduced, using neodymium versatate (NdV₃), diisobutylaluminum hydrade (DIBAH) as the catalytic system and dimethyldichlorosilane (Me₂SiCl₂) as the activator. A bimodal distribution in the GPC signal reveals the presence of two populations at low conversions, attributable to dormants (arising from reversible chain transfer reactions) and dead chains (arising from termination and irreversible chain transfer reactions); a unimodal distribution is generated at medium and high conversions, corresponding to the dominant species, the dormant chains. Additionally, a mathematical kinetic model was developed based on the Method of Moments to study a set of selected experiments: ([β-myrcene]₀:[NdV₃]₀:[DIBAH]₀:[Me₂SiCl₂]₀ = 660:1:2:1, 885:1:2:1, and 533:1:2:1). In order to estimate the kinetic rate constant of the systems, a minimization of the sum of squared errors (SSE) between the model predicted values and the experimental measurements was carried out, resulting in an excellent fit. A set of the Arrhenius parameters were estimated for the ratio [β-myrcene]₀:[NdV₃]₀:[DIBAH]₀:[Me₂SiCl₂]₀ = 660:1:2:1 in a temperature range between 50 to 70 °C. While the end-group functionality (EGF) was predominantly preserved as the ratio [β-myrcene]₀:[NdV₃]₀ was decreased, higher catalytic activity was obtained with a high ratio.