Kinetic Investigation on the Cationic Polymerization of o-Phthalaldehyde: Understanding Ring-Expansion Polymerization

Depolymerizable polymers are of interest for engineering applications, especially when their decomposition products can be recycled back into high-value monomers. Polyphthalaldehyde and its derivatives form cyclic polymer products which can depolymerize rapidly from the solid state. To facilitate the development of these materials into technologies, the polymerization kinetics of this system was investigated. A microflow reactor was used to examine the cationic polymerization of o-phthalaldehyde with boron trifluoride diethyl etherate as the polymerization catalyst. By utilizing a microflow reactor, mass and heat resistances could be minimized to probe the kinetics of the reaction. High molecular weight polymer, >270 kDa, was formed in 10 s. Kinetic and molecular weight data indicate that two polymerization regimes exist. The first regime is characterized by controlled growth rates to moderate molecular weight polymers. The second regime occurs above a threshold concentration, where intermolecular chain transfer dominates the polymerization kinetics, resulting in an exponential growth of the polymer molecular weight via fusion of adjacent polymer chains. A ring-expansion polymerization model is proposed to explain these polymerization results. These findings enable greater synthetic control to achieve targeted polymer properties for poly(aldehydes) and identifies kinetic methods that can be employed to assess the activity of other polymerization catalysts.