Thermoresponsive Properties of Poly[oligo(ethylene glycol) sorbate]s Prepared by Organocatalyzed Group Transfer Polymerization

Abstract

The current contribution presents the synthesis of poly[oligo(ethylene glycol) sorbate]s (PREGmSs) by the t-Bu-P4-catalyzed group transfer polymerization (GTP) method, their postpolymerization modification at the backbone double bond, and thermoresponsive properties in aqueous solution. Four oligo(ethylene glycol) sorbate monomers (REGmSs), including 2-(2-methoxyethoxy)ethyl sorbate (MeEG2S), 2-(2-(2-methoxyethoxy)ethoxy)ethyl sorbate (MeEG3S), 2-(2-(2-ethoxyethoxy)ethoxy)ethyl sorbate (EtEG3S), and 2-(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)ethyl sorbate (MeEG4S), are newly designed to study the thermoresponsive properties of their related polymers. The t-Bu-P4-catalyzed GTP of REGmS only proceeds in a 1,4-regioselective addition manner to quantitatively produce the backbone trans-rich (ca. 80%) PREGmS. The complete hydrogenation and epoxidation of the backbone double bonds result in hydrogenated and epoxidized polymers (PREGmS-H2 and PREGmS-epoxy, respectively), which have different main-chain structures and rigidity from their parent PREGmS. Given the 1,4-substituted structure of a sorbate monomer, the chemical structure determination of PREGmS-H2 by 13C NMR spectra demonstrates that the above polymers have an erythro-rich (ca. 60%) structure in diastereochemistry and a diisotactic/disyndiotactic ratio around 1:1 in stereoregularity. The thermoresponsive properties of PREGmS, PREGmS-H2, and PREGmS-epoxy in aqueous solutions are studied in detail from various aspects in terms of polymer mass concentration, molecular weight, length and end group of the oligo(ethylene glycol) side chain, and main-chain structure by turbidity, 1H NMR spectroscopy, and dynamic light scattering (DLS) methods, which are discussed below.