Glycidyl ethers from acyclic terpenes : a versatile toolbox for multifunctional poly(ethylene glycol)s with modification opportunities
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Abstract
Multifunctional poly(ethylene glycol) copolymers (mfPEGs) are accessible via the anionic copolymerization of functional epoxides with ethylene oxide (EO). Glycidyl ethers are conveniently synthesized from bio-renewable alcohols and epichlorohydrin (ECH). Herein, we present the synthesis of a series of acyclic terpenyl glycidyl ethers (TGEs) and their subsequent copolymerization with ethylene oxide (EO) via anionic ring-opening polymerization (AROP). The resulting library of copolymers with varied side chain length and comonomer composition comprises molar masses in the range of 4800 to 8300 g mol−1 and narrow molar mass distributions (Đ = 1.06–1.13). For the copolymerization of the TGEs with EO, detailed 1H NMR in situ kinetic studies revealed a change from ideally random to slight gradient copolyether microstructures with increasing chain length and hydrophobicity of the respective TGE. The living nature of AROP provides control of molar masses, and optimized reaction conditions, such as low reaction temperatures and a weakly bound cesium counterion, suppress the well-known proton abstraction of monosubstituted epoxides. Since the incorporation of the terpenyl side chains impedes crystallization, thermal properties of the copolyethers can be tailored by the monomer feed ratio. Subsequently, hydrogenation and thiol–ene click reactions at the side chain double bonds were carried out as post-polymerization modifications. The application of potassium azodicarboxylate (PADA) in the diimide reduction of the polymers was demonstrated to possess vast potential for the full hydrogenation of the novel copolymers, offering facile purification options. Overall, the copolymerization of EO and TGEs gives access to biobased, tailormade polyethers with various options for post-functionalization.