Mono and bi-alkyne functionalised polyethylene glycols : polymeric substrates in palladium catalysed oscillatory carbonylation reactions

Abstract

In this work, the family of oscillatory carbonylation reactions was expanded to include mono and bi-functionalised polymeric substrates. As research around intelligent / stimuli responsive materials progress, it is increasingly important to expand capabilities of chemical oscillators, with a view of merging these two areas to achieve fully pulsatile polymeric materials, capable of autonomous volume changes over longer (i.e. weeks, months) periods of time. This study focused on mono alkyne functionalised polyethylene glycols (A-PEG2000) and bi-alkyne functionalised polyethylene glycols (A-PEG2000-A) as polymeric substrates in oscillatory carbonylation reactions. The work presented here systematically evaluates oscillatory and non-oscillatory reaction profiles recorded in experimental studies. The studies were designed to grasp the reaction processes and elucidate reaction mechanisms responsible for observed trends. Extensive studies were undertaken at different polymeric substrate, PdI2 and KI (originally added to aid PdI2 dissolution) concentrations. Across all studies, the concentration of the catalytic mixture consisting of PdI2 and KI in methanol ranged from 3 - 9 mM for KI and 15.1 - 60.4 μM for PdI2. Mono alkyne substrate concentrations ranged from 0.508 - 3.55 mM while the bi-alkyne substrate concentration ranged from 0.254 - 3.04 mM. The influence of methanol perturbation during the reaction and varying KI addition times was also investigated. Comparison of pH profiles for both substrates at the same molecular concentrations or same alkyne group concentrations was likewise assessed. At constant catalyst concentration (KI/PdI2), as the substrate concentration increased, the amplitudes and period of the pH oscillations increased in reactions with mono alkyne substrates. On increasing the bi-alkyne polymeric substrate concentrations, the size and amplitudes of pH oscillations varied, and was significantly dependent on the catalyst / substrate concentration. Increasing PdI2 concentration at constant KI and substrate concentrations increased [H+] generated via autocatalytic conversions of both alkyne functionalised polymer substrates. More [H+] was formed in reactions employing bi-alkyne substrates due to increased concentrations of alkyne groups (two alkyne ends per chain). Delaying KI addition times at constant PdI2 and mono alkyne substrate concentration shifted the reactions from oscillatory to non-oscillatory modes. Furthermore, increasing KI concentrations induced occurrence of oscillations in both substrates. Simple, complex, Canard and mixed mode oscillations, as well as other complex non-oscillatory features and pH transitions / offsets, were observed in some reactions as the mono alkyne and bi-alkyne polymer substrates and catalytic concentrations were altered. This work confirms the feasibility of pH oscillations with mono and bi-alkyne substrates in Pd-catalysed oxidative carboxylation reaction and opens new avenues in nonlinear chemical dynamics and intelligent polymeric materials. A merger of intelligent soft materials and this pH oscillator which exhibits extended oscillation duration in batch mode could potentially create self-oscillating regulatory devices for a range of applications including drug delivery and soft robotics.