Synthesis of linear polyglycerols with tailored degree of methylation by copolymerization and the effect on thermorheological behavior

Abstract

We introduce a two-step strategy for the synthesis of linear polyglycerols (linPG-OHx/OMey) with an adjustable degree of methylation (y=DM100). Ethoxy ethyl glycidyl ether (EEGE) and glycidyl methyl ether (GME) were copolymerized via the “activated monomer” polymerization technique, using tetraoctylammonium bromide (NOct4Br) as an initiator and triisobutylaluminum (i-Bu3Al) as a catalyst under mild conditions. Subsequent acidic cleavage of the acetal protective groups generates linear polyglycerols with different degree of methylation (DM) by varying the GME comonomer content between 10 and 91%. Size exclusion chromatography (SEC) evidenced good control over molecular weight and narrow to moderate polydispersity (PDI = 1.2–1.8). 1H NMR spectroscopy confirmed ideally random copolymerization of EEGE and GME (in situ1H-NMR kinetics) and provided perfect agreement of the comonomer content with the targeted values. Thermoresponsive behavior in solution and lowering of cloud points with increasing degree of methylation was observed. Furthermore, the differently methylated polyglycerols were investigated with respect to their rheological properties in the melt. Comparison with the fully hydroxylated and permethylated polyglycerol provides new insights into the dynamic behavior of functional polyethers. A tremendous influence of DM on zero-shear viscosity and differences of up to 5 decades were observable at the same reference temperature (273 K). The trend of glass transition temperature and zero-shear viscosity in dependency of degree of methylation was described by mixing rules. To understand the changes in zero-shear viscosity, the “sticky” Rouse model was applied and led to an estimated association lifetime of stickers, i.e. hydroxyl groups of τs=4.9±1.8μs.