(743g) Creating Porous Block Copolymers Using ADMET Depolymerization Mechanisms

Acyclic diene metathesis (ADMET) depolymerization of block copolymers is an emerging field of interest [1]; this method has enormous potential for the production of porous polymers with membrane applications. The depolymerization is achieved by capitalizing on the reversibility of the ADMET reaction. This study demonstrates successful removal of polybutadiene (PB) and polyisoprene (PI) components. From block copolymer results in the formation of porous polystyrene (PS), paving the way for future membrane applications. In this study PS-PB-PS (RCM1, PB = 26 mass%) and PI-PS-PI (V4111, PI = 82 mass%) tri-block copolymers were employed in this ADMET catalysis. Both an Ru based catalyst (Second Generation Grubbs) and an Mo based catalysis (Schrocks) reactions were run under nitrogen, in solution, at various concentrations; each was studied over time to determine the extent of reaction. Degradation of the block copolymers was most rapid when Ru catalyst was used, consistent with previous findings [2]. This difference between the two catalysts was observed using Gel Permeation Chromatography (GPC) and Nuclear Magnetic Resonance (NMR). The two techniques demonstrated not only a drop in molecular weight occurring in the polymer but also a change in the proton spectra. RCM1 demonstrated a very swift drop in molecular weight to approximately one-third of its original value; the final molecular weight is that of one PS block at the outset. The reaction is completed in less than thirty minutes for varying molar ratios of catalyst. The presence of methylene and CH3 end groups in three unique locations on 1H NMR spectra demonstrated chain scissions. Furthermore, increases in peaks attributed to cyclohexene molecules in IR spectra show the product of these scissions. The kinetic rate constant was also measured and the determined to be first ordered [3]. In contrast to some findings, initial results suggest that the catalyst to carbon-carbon double bond ratio influences the kinetic rate constant. Catalyst presence (by an order of magnitude) increases the measured number of broken butadiene carbon-carbon double bonds from an average of 10% to 21% (determined by 1H NMR). The broken carbon-carbon double bonds can be directly correlated with a lower resulting MW. It is likely these additional scissions are releasing small monomeric units. These findings for RCM1 are supported by results of studies on V4111. It has been shown that catalytic ADMET depolymerization can offer removal of carbon-carbon double bonds contained in block copolymers efficiently and effectively. This demonstrates the potential of this method as a viable competitor to current polymer membrane fabrication technology.

1.Lucas, F., Peruch, F., Carlotti, S., Deffieux, A., Leblanc, A. & Boisson, C. Polymer 49, 4935-4941(2008). 2.Craig, S.W., Manzer, J.A. & Coughlin, E.B. Macromolecules 34, (7929-7931). 3.Hummel, K. Pure and Applied Chemistry 54, 351-364(1982).