Well-Defined Polyamide Synthesis from Diisocyanates and Diacids Involving Hindered Carbodiimide Intermediates
journal contributionposted on 11.01.2011, 00:00 by An-Liu Chen, Kuan-Liang Wei, Ru-Jong Jeng, Jiang-Jen Lin, Shenghong A. Dai
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We have uncovered a novel polycondensation strategy for the synthesis of well-defined polyamides of narrow molecular weight distributions based on modifications of our sequential self-repetitive reaction (“SSRR”) previously developed for diisocyanate−dicarboxylic acid polymerization. In our newly discovered SSRR polyamide formation mechanism, a small amount of hindered carbodiimide, N,N′-bis(2,6-diisopropylphenyl)carbodiimide (iPr-CDI) or a hindered isocyanate such as 2,6-diisopropylphenyl isocyanate (iPr-NCO), was introduced to the polymerization as an initiator, followed by simultaneous addition of diisocyanates and diacids monomers. By using this new reaction mode, the SSRR mechanism produces polyamide products of narrow molecular weight distributions with their dispersities reduced to 1.2−1.4, which is far lower than a range of >2.5 found in regular SSRR reactions. Significantly different from a conventional step-growth or standard SSRR reaction, the formation of a polymer backbone is preferential when the diacid is added to the requisite iPr-CDI in the first step, followed by a rearrangement to form amide and fragmented components for SSRR. The control of molecular weight is mainly attributed to the acid addition favoring the unhindered poly-CDI intermediates in the middle of the growing chains over the hindered-CDI at the chain terminals. It appears that the formation of a “hindered isocyanate” and the subsequent formation of a “new hindered-CDI” at the terminal end of growing amide-chains in each SSRR cycle force the acid again toward the preferred unhindered CDI sites dictating the observed outcome. This simple polyamide synthesis methodology is unique and unconventional, and it could significantly facilitate the development of tailored-made polyamides from a variety of diisocyanates and diacids.