No abstract

Benzoxazine (BOX) functionalized polyurethanes (PU) are introduced to provide a conceptually new thermal curing mechanism for polyurethanes. 3,4-Dihydro-3-methyl-2H-1,3-benzoxazine (P-m) was carefully oligomerized through thermal treatment. In a straightforward synthesis the newly formed hydroxyl groups are used for end-capping reactions with isocyanate-terminated polyurethane prepolymers. The isocyanate reactive hydroxyl content (IRH) of the benzoxazine oligomer was investigated in detail via 1 H NMR spectroscopy, HPLC-MS, indirect potentiometric titration in various solvents, and comparison with model substances and found to be strongly influenced by hydrogen bonding. The corresponding polyurethane/benzoxazine hybrid materials (PU/BOX) can cross-link at elevated temperatures and do not suffer from shelf-life issues or outgassing of blocked isocyanates. The thermally activated curing reaction was investigated via rheology and DSC. Significant improvements over state-of-the-art systems based on phenol-capped PU prepolymers are shorter curing times, increased moduli, and drastically increased glass transition temperatures.

The total rates of reaction between four cyclic esters (β-butyro-, γ-butyro-, γ-valero- and δ-valero-lactones) and the OH radical have been measured relative to the rate of reaction of a reference compound, ethene, at room temperatures. The measurements show that the rates increase with increasing ring size. Theoretical calculations on the four lactones with the inclusion of a fifth, α-methyl-γ-butyrolactone, are broadly in agreement with this picture but provide a more insightful view of the sites at which hydrogen atom abstraction occurs in each molecule.

Photopolymers with unique curing capabilities were produced by combining living cationic polymerization with network formation and restricted polymer motion. A vinyl ether diol was synthesized as a functional building block and reacted with isophorone diisocyanate to form a highly functionalized vinyl ether polyurethane as a model system with high crosslinking ability. When using a cationic photoinitiator, fast polymerization is observed upon short UV irradiation. Curing proceeds in the absence of light and under ambient conditions without oxygen inhibition. Cationic active sites become trapped dormant species upon network-induced vitrification and surprisingly remain living for several days. The polymerization can be reactivated by additional UV irradiation and/or raised temperature. The curing behavior was studied in detail by using UV and FT-NIR coupled rheology and photo-DSC to simultaneously study spectroscopic and mechanical information, as well as thermal effects.