Insights into post-polymerisation modification of bio-based unsaturated itaconate and fumarate polyesters via aza-michael addition: Understanding the effects of C=C isomerisation.

T J Farmer,J H Clark,A Pellis,D J Macquarrie,J W Comerford

doi:10.1002/pola.29079

Abstract

ABSTRACTDevelopment of renewable bio‐based unsaturated polyesters is undergoing a renaissance, typified by the use of itaconate and fumarate monomers. The electron‐deficient C=C bond found on the corresponding polyesters allows convenient post‐polymerisation modification to give a wide range of polymer properties; this is notably effective for the addition of nucleophilic pendants. However, preservation of unsaturated functionality is blighted by two undesirable side‐reactions, branching/crosslinking and C=C isomerisation. Herein, a tentative kinetic study of diethylamine addition to model itaconate and fumarate diesters highlights the significance of undesirable C=C isomerisation. In particular, it shows that reversible isomerisation from itaconate to mesaconate (a poor Michael acceptor) is in direct competition with aza‐Michael addition, where the amine Michael donor acts as an isomerisation catalyst. We postulate that undesired formation of mesaconate is responsible for the long reaction times previously reported for itaconate polyester post‐polymerisation modification. This study illustrates the pressing need to overcome this issue of C=C isomerisation to enhance post‐polymerisation modification of bio‐based unsaturated polyesters. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018, 56, 1935–1945

Highlights

An area of the particular interest has been in the field of polymer synthesis using these sustainably sourced building blocks as monomers or monomer precursors
The aza-Michael addition was performed without catalyst at 21 8C with a 3:1 ratio of DEA:DMI under solvent free conditions, as would be typical for additions onto unsaturated polyesters (UPEs) or unsaturated monoesters (Fig. 1).4(e,f),13,15 Reaction times of up to 72 h were required for yields above 80% of adduct 3
This observation could highlight positive benefits for polymers modified in this manner as high regio-selectivity leading to a single constitutional repeat unit will increase the likelihood of crystalline regions forming in modified polymer, possibly improving strength and stability of the resultant polyester. This comprehensive study of the aza-Michael addition onto itaconates has shown how a competing pathway exists between the desired addition and the undesired regioisomerisation to the mesaconate unit

Summary

Introduction

Over the last decade there has been a growing interest in the utilisation of bio-derived platform molecules for the synthesis of higher value products, triggered for the most part by the US DOE report on Top Value Added Chemicals from Biomass. An area of the particular interest has been in the field of polymer synthesis using these sustainably sourced building blocks as monomers or monomer precursors. More recently there has been increasing focus toward functionalisable polymers and in particular, the polymerisation of common platform molecules, itaconic acid, and fumaric acid with a range of diols such as 1,2ethanediol, 1,2-propanediol, 1,3-propanediol, 1,4-butanediol, and glycerol and to produce novel, 100% bio-derived unsaturated polyester resins (UPEs).. More recently there has been increasing focus toward functionalisable polymers and in particular, the polymerisation of common platform molecules, itaconic acid, and fumaric acid with a range of diols such as 1,2ethanediol, 1,2-propanediol, 1,3-propanediol, 1,4-butanediol, and glycerol and to produce novel, 100% bio-derived unsaturated polyester resins (UPEs).3 Synthesis of these polymers typically employs well-established melt polymerisation methods along with well-known metal-centred catalysts (Ti, Al, Sn, Zn).. Plastics such as poly(lactic acid) (PLA), poly(butylene succinate) (PBS), and poly(ethylene furanoate) (PEF) demonstrate how polymers with favourable properties can be partly or wholly derived from platform molecules. More recently there has been increasing focus toward functionalisable polymers and in particular, the polymerisation of common platform molecules, itaconic acid, and fumaric acid with a range of diols such as 1,2ethanediol, 1,2-propanediol, 1,3-propanediol, 1,4-butanediol, and glycerol and to produce novel, 100% bio-derived unsaturated polyester resins (UPEs). Synthesis of these polymers typically employs well-established melt polymerisation methods along with well-known metal-centred catalysts (Ti, Al, Sn, Zn). due to the unsaturated nature of the dicarboxylates, they often undergo undesired side reactions such as isomerisation, radical crosslinking, and Ordelt saturation (an oxo-Michael addition, where an RAOH end-group attack the conjugated C@C through a b-addition, [Scheme 1(C)].4(a,b)

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Conclusion