An efficient iterative synthesis was utilized to prepare hard-segment extenders of uniform chain lengths with multiple hydrogen-bonding sites for uses in preparation of supramolecular thermoreversible polyurethanes (TRPUs). The unique feature of our iterative synthesis is based on two alternative addition reactions to a dual-functional intermediate, 4-isocyanato-4′(3,3-diethyl-2,4-dioxoazetidino)diphenylmethane (assigned as [MIA]), where (1) the more reactive isocyanate group of [MIA] was reacted first with anilines or secondary amine group of 1-(2-aminoethyl)piperazine followed by (2) addition of a more reactive primary aliphatic amine group of 1-(2-aminoethyl)piperazine to react with the more selective azetidine-2,4-dione group of the [MIA]. With this iterative synthetic approach, three generations of supramolecular extenders were prepared, and each were added to isocyanate-prepolymers of varied chain lengths to form supramolecular "pseudo-triblock" TRPUs with a similar hard segment content of about 41%. It was found that both supramolecular extenders and their respective TRPUs showed distinctive glass transition temperatures (T g ). Thermal degradation temperatures (T d ) of both TRPUs and extenders increase with increasing hard-segment chain lengths. Different degrees of phasesegregation were present in the synthesized TRPUs and have shown to be enhanced in the same general trend. Investigations by variable-temperature FT-IR and circular scanning treatment of DSC on the TRPU [G2-41] with second generation extenders, between 25 and 180°C revealed thermal reversibility due to the increasing (at room temperature) and diminishing of hydrogen bonding (at 180°C) among the multiple hydrogen-bonding hard-segment chains. The results of SAXS analyses of TRPUs with first and second generation extenders further showed that these TRPUs formed self-assembling phase-segregated domains. The TRPU [G2-41] was found to exhibit the most prominent phase-segregation observed by SAXS with formation of uniform hard-segment domains of 40 nm. In the meantime, it also behaves like a polyurethane elastomer with a high elongation of 651% possessing the best mechanical properties found among the series. This study further demonstrated that a structural balance between uniform chain-length hard-segment extenders and their connecting soft segment play a dominant role on performances of "pseudo-triblock" polyurethane systems through molecular self-assembling.
Two new synthetic methodologies for making aryl- and alkyl-bis(azetidine-2,4-dione)s have been accomplished in this study. A new family of N,N'-benzophenonyl bis(azetidine-2,4-dione)s (2) has been successfully synthesized through autoxidation of readily available diphenylmethane bis(azetidine2,4-dione)s (1) in 70-80% yields. In addition, the synthesis of N,N'-alkylene bis(azetidine-2,4-dione)s (3) was accomplished through sensitized photocyclization of N,N'-alkylene bis(N-formyl-2-methylacrylamide)s in anew three-step process from alkylene diamines in overall yields of 63-80%. The prepared bis(azetidine-2,4-dione)s were converted to polymalonamide elastomers in two steps by reacting first with a long-chained polyether diamine of 2000 g center dot mol(-1) molecular weight to form the prepolymers. Subsequently, the prepolymers were melt-polymerized with hexamethylene diamine to produce the final elastomeric polymalonamides. The present study further investigated the reactivities of these aryl- and alkyl-bis(azetidine-2,4-dione)s as well as their elastomeric property differences due to structural variations. The study found that the order of the intermediate's reactivity toward amines is (2) > (1) > (3). This implies that aryl azetidine-2,4-diones, (1) and (2), are more reactive than their aliphatic counterpart (3) toward amines in their ring-opening reactions. Furthermore, the ketone-conjugated aromatic intermediates (2) were found to produce the highest molecular weight polymalonamides in short reaction time and result in polymalonamides with overall superior mechanical properties. For the final elastomeric polymalonamides produced, we found that aromatic polymalonamides were mostly amorphous, but aliphatic elastomeric polymalonamides were fibrous semicrystalline. All of the polymalonamides based on aryl bis(azetidine-2,4-dione)s possess temperature-resistant properties of >350 degrees C. The study indicated that the carbonyl activated bis(azetidine-2,4-dione)s, such as (2), can serve as the best candidate intermediates for making elastomeric polyamides via the rapid reaction injection molding (RIM) process similar to those practiced in polyurethanes
In this study, we synthesized a dual-functional building intermediate, 4-(3,3-diethyl-2,4-dioxoazetidin-1-yl)benzoyl chloride (DEDA-BC), from readily available starting materials, including 4-isocyanatobenzoyl chloride and p-tolyl isocyanate. In its iterative syntheses of hard segments, we first treated the highly reactive acid chloride of DEDA-BC with the monoamine (aniline) or the diamine (4,4′-methylenedianiline, 4,4′-MDA) to form first-generation azetidien-2,4-dione intermediates. We then reacted these derivatives with 4-aminobenzylamine at the more-selective azetidine-2,4-dione group of DEDA-BC to form the first-generation of benzyl amine extenders. Using this alternating method, we obtained high yields of supramolecular extenders of various chain lengths (n = 1–3) in a systematic manner, without the need for tedious purification steps, under catalyst-free conditions. The mono- and diamine extenders with numbers of repeating units ranging from one to three were synthesized precisely through this new iterative synthetic approach. The molar mass increases between each generation were 365 g mol–1 for the monoamine series and 730 g mol–1 for the diamine series. The three generations of supramolecular extenders possessed the distinctive characteristics of multiple hydrogen bonding moieties and narrow molecular weight distributions. Their gelation phenomena in THF revealed that these amine extenders underwent supramolecular assembly, through intermolecular hydrogen bonding, to form organogels. We used these well-defined extenders with various chain lengths in the preparation of polyurethane (PU) elastomers. Small-angle X-ray scattering revealed well-defined microdomains in the morphologies of the PU elastomers presenting multiply hydrogen-bonded terminal groups. The tensile and thermal properties of the prepared PUs were dependent on the effects of the content of hard segments, the chain length, and the strength of hydrogen bonding.
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