Synthesis and Characterization of Model Urethane Compounds

Model urethanes with a narrow distribution are useful in the investigation of structure-property relationships in polyurethanes. Normal random copolymerization leads to statistically distributed segment sizes. A new method has been developed to synthesize model urethanes based on 4,4'-methylenebis [phenyl isocyanate] (MDI) and 1,4-butanediol (BDO). First, one of the hydroxyl groups in BDO is protected by trityl chloride and then the other is chloroformated. After reaction with 4,4'-diaminomethylenebis [benzene] (MDA) and product separation, an asymmetrical intermediate, HO(CH₂)₄OC(=O)NHC₆H₄CH₂C₆H₄NH₂ (BM'NH₂), is isolated. BMB [(HO(CH₂)₄OC(=O)NHC₆H₄)₂CH₂], formed by reaction of MDI with singly protected BDO and followed by deprotection, can be chloroformated and coupled with BMNH₂, making B(MB)₃. The same sequence of reactions using the B(MB)_n as starting materials can be used to synthesize B(MB)_n+2. Increasing insolubility of the reactants B(MB)_n+2 in THF for n > 3 probably necessitates the use of a more powerful solvent such as dimethyl sulfoxide. GPC, elemental analysis, and infrared spectroscopy all indicate that pure monodisperse compounds B(MB)_n, where n = 1, 2, and 3, can be readily produced in multigram quantities suitable for coupling into multiblocks with polyether soft segments. Melting points of the oligomers precipitated from solution increased from 130 to 237 °C as n increased from 1 to 5. However, thermal treatment of the precipitated samples produced several other polymorphs with different melting temperatures.