The Effect of NMR Setting Parameters on Molecular Weight Determination of Polyether Diols

Abstract

Nuclear magnetic resonance (NMR) is an advanced technique for the molecular weight (MW) determination of polymers at quantitative conditions. In this study, we investigate the effect of liquid ¹H-NMR instrumental setting parameters on the MW determination of polyether diols, namely poly(ethylene glycol) (PEG) and poly(tetramethylene oxide) (PTMO) diols, using hydroxymethylene groups as chain-ends. Our results show that the protons in chain-ends have larger spin-lattice relaxation time (T₁) than those in main chains. To let most of the excited protons relax to the equilibrium state, the delay time (d₁) should be much larger than T₁ of end-groups. When ¹³C decoupling is inactive, the relative errors can be greater than 60%, due to the ¹³C-coupled proton satellite peaks, which can overlap with chain-end groups or be misassigned as chain-ends. The optimal quantitative NMR conditions for the MW estimation of polyethers are revealed below: standard pulse with inverted gated ¹³C decoupling pulse sequence, 32 scans, 2.0 s acquisition time in 90 degree of flip angle and 30 s d₁. The MWs determined from ¹H quantitative NMR are all smaller than those from SEC which are relative to polystyrene (PS) standards, since the size of polyether chains is larger than that of PS with the same MW. In addition, the MW obtained from SEC for PTMOs shows larger overestimation than PEGs, suggesting PEG chains are more flexible than PTMO’s.