Syntheses and polymerizations of alpha-amino acid N-carboxyanhydrides (NCAs) were reported for the first time by Hermann Leuchs in 1906. Since that time, these cyclic and highly reactive amino acid derivatives were used for stepwise peptide syntheses but mainly for the formation of polypeptides by ring-opening polymerizations. This review summarizes the literature after 1985 and reports on new aspects of the polymerization processes, such as the formation of cyclic polypeptides or novel organometal catalysts. Polypeptides with various architectures, such as diblock, triblock, and multiblock sequences, and star-shaped or dendritic structures are also mentioned. Furthermore, lyotropic and thermotropic liquid-crystalline polypeptides will be discussed and the role of polypeptides as drugs or drug carriers are reviewed. Finally, the hypothetical role of NCAs in molecular evolution on the prebiotic Earth is discussed.
Syntheses of cyclic polymers including cyclic homopolymers, cyclic block copolymers, sun-shaped polymers, and tadpole polymers are discussed on the basis of a differentiation between synthetic methods and synthetic strategies (e.g., polycondensation, ring-ring equilibration, or ring-expansion polymerization). Furthermore, all synthetic methods are classified as kinetically or thermodynamically controlled reactions. Characteristic properties of cyclic polymers such as smaller hydrodynamic volume, lower melt viscosities, and higher thermostabilities are compared to the properties of their linear counterparts. Furthermore, the nanophase separation of cyclic diblock copolymers is discussed.
Sn(II)2-ethylhexanoate (SnOct2) was reacted with 2 equiv of benzyl alcohol at 20 °C, and a liberation of octanoic acid in a rapid equilibration was found. When the temperature was raised to 180 °C in steps of 40 °C, esterification of benzyl alcohol and octanoic acid was observed up to a conversion of 90%. This esterification was catalyzed by Sn(II) and not by the protons of the free octanoic acid. The esterification liberated Sn(OH)2, which finally precipitated in the form of SnO. This precipitate proved to be a good initiator for the polymerization of lactide above 120 °C. Analogous results were obtained with 1-decanol, triethylene glycol monomethyl ether, and neopentane diol. When SnOct2 was reacted with methyl lactate at 20 °C, a chelate complex of one Sn with two lactate ligands was formed, liberating almost all octanoic acid. At higher temperatures, esterification of octanoic acid with methyl lactate and transesterification of the methyl group (yielding methyl octoate) were observed. The latter esterification was predominant at higher temperatures, and a Sn lactate (1:1) complex precipitated under all circumstances. This complex proved to be an initiator for polymerizations of L-lactide. Polymerization of L-lactide initiated with neat SnOct2 at 180 °C yielded polylactides having octanoate end groups, and the molecular weights paralleled the monomer/initiator ratio.
The theory of step‐growth polymerizations including the cascade theory is discussed in the light of new results focussing on the role of cyclization reactions. The identification of cyclic oligomers and polymers in reaction products of step‐growth polymerizations has been eased considerably by means of MALDI‐TOF mass spectrometry. Experimental examples concern syntheses of polyesters, polycarbonates, polyamides, polyimides, poly(ether sulfone)s, poly(ether ketone)s and polyurethanes. It was found in all cases that the percentage and molecular weight of the cycles increases when the reaction conditions favor high molecular weights. In the absence of side reactions all reaction products will be cycles when conversion approaches 100%. Cyclization may even take place in the nematic phase but even‐numbered cycles are favored over odd‐numbered ones due to electronic interactions between mesogens aligned in parallel. In contrast to Flory's cascade theory, cyclization also plays a decisive role in polycondensations of abn‐type monomers, and at 100% conversion all hyperbranched polymers have a cyclic core. Furthermore, it is demonstrated that in a2+b3 polycondensations intensive cyclization in the early stages of the process has the consequence that either no gelation occurs or the resulting networks consist of cyclic and bicyclic oligomers as building blocks. Finally, a comparison between cyclization of synthetic polymers and biopolymers is discussed.
Schematic representation of a network structure mainly consisting of cyclic oligomers and multicyclic building blocks as derived from “a2” + “b3” polycondensation.magnified imageSchematic representation of a network structure mainly consisting of cyclic oligomers and multicyclic building blocks as derived from “a2” + “b3” polycondensation.
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