Well-Defined Polyamide Synthesis from Diisocyanates and Diacids Involving Hindered Carbodiimide Intermediates

Abstract

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 diisocyanatedicarboxylic 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 1.21.4, 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

High elongation thermoplastic Polyester‐urethanes based on widely available diacid intermediates

“…Under the ordinary conditions used for polyurethane synthesis, in particular those used in this work (110°C, see below) and in the absence of any suitable catalyst, carboxylic acids are very scarcely reactive towards isocyanate groups . However the maximal “degree of polymerization,” P , that can be calculated for every final segmented polyurethane from the proportion between OH and COOH groups is always high enough to ensure appropriate molar masses for the proper comparison of thermal and mechanical properties.…”

Recent development of synthetic preparation methods for guanidines via transition metal catalysis

“…This reaction can be accelerated by many catalysts including metal carbonyls or compounds containing the PQO moiety. [112][113][114][115] However, only symmetric carbodiimide can be prepared by condensation of isocyanate. The aza-Wittig reaction between iminophosphoranes (Ph 3 PQNR) and isocyanate is then developed, in which iminophosphoranes are used as imino transfer reagents in the reaction with isocyanate.…”

Mechanistic aspects on molecular structure formation of polymeric networks from diisocyanates with amidine compounds

“…Isocyanates are also of great importance for new conceptions in polymer chemistry. Recently, isocyanates in conjunction with imine compounds have been proved as an innovative source for new polymer synthesis strategies . Imines are organic molecules that contain a carbon–nitrogen double bond (CRN) such as, amidines, guanidines, azines, and azomethines.…”

“…However, it could be shown that diisocyanates are suitable to react with aprotic imine compounds as reactants to mediate step‐growth polymerization . Furthermore, imine compounds and isocyanates can interact with each other while generating radicals for the polymerization of acrylates .…”