Post-polymerisation oxyfunctionalisation of styrene and butadiene-based (co-)polymers using a homogeneous manganese catalyst.

Abstract

Post-polymerisation modification of commodity hydrocarbon-based polymers provides access to functional polymers not readily available through bottom-up synthesis methods. Here, we demonstrate the oxyfunctionalisation of different styrenic and rubbery (co-)polymers using a well-established and robust manganese-based homogeneous catalyst, MnTACN, a 1,4,7-trimethyl-1,4,7-triazacyclononane ligand-bearing di-nuclear tri-μ-oxo bridged Mn(IV) compound, and hydrogen peroxide as a green oxidant. Using various grades of polystyrene (PS) and polybutadiene (PBD), we successfully oxyfunctionalised the polymer backbones with alcohol (PS and PBD), ketone (PS) and epoxide (PBD) functional groups. Under optimised conditions, total functionalisation degrees up to 5% for PS and 18% for PBD can be achieved. Next to the homopolymers, we also show oxyfunctionalisation degrees as high as 11%, of the butadiene-derived part of a styrene-butadiene-styrene block-co-polymer (SBS). These results underscore the versatility of a single catalytic system for the oxyfunctionalisation of various C-H bonds as well as the CC bonds found in these commodity hydrocarbon polymers. Detailed analysis of the oxidised polymers before and after subsequent oxidative cleavage of the installed diol moieties on the PBD backbone suggest that the functional groups are randomly spaced along the polymer backbone. Moreover, this second oxidative cleavage also offers the possibility to selectively break down the polymer backbone after oxyfunctionalisation into a mixture of dialdehyde oligomers consisting of 4 up to 32 monomeric units. For PBD and low/mid M_w PS, oxyfunctionalisation coincided with minimal backbone cleavage or crosslinking, as evidenced by gel permeation chromatography (GPC). For the high molecular weight PS samples and SBS, GPC analysis suggests that backbone cleavage is in contrast more pronounced upon oxyfunctionalisation. The thermal properties of the oxyfunctionalised materials are largely unchanged, with decomposition temperatures decreasing with increasing functionalisation degrees, but overall remaining in the high thermal stability regime.