The synthesis and structure–property relationships of one-component urethane adhesives were studied. NCO-terminated urethane prepolymers were prepared from monomeric diphenylmethane-4,4′-diisocyanate, difunctional polypropyleneglycol polyols and their mixtures. The syntheses were monitored by in-line FTIR-ATR spectroscopy. The rate of the urethane group formation increased with increasing free isocyanate (NCO) concentration in the bulk. The average molecular weight and Brookfield viscosity of the urethane prepolymers increased with decreasing NCO/OH ratio, at fixed polyol composition. When prepolymers contained longer polyol segments, the number average molecular weight increased, while Brookfield viscosity decreased due to higher content of soft segments, which made urethane chains more flexible. The prepolymers’ average molecular weights affected their glass transition temperature values, particularly when prepolymers had shorter soft segments. With increasing content of polyol with higher molecular weight and decreasing content of polyol with lower molecular weight, the glass transition temperature decreased. At lower NCO/OH ratios the rigidity and stiffness of the adhesive bond in wood-to-wood specimens increased with increasing free NCO content, whereas beyond a critical free NCO content, high stiffness was responsible for decrease in adhesion strength. The cure time of adhesives increased with increasing polyol molecular weight and decreasing free NCO content.