Effect of substituents upon melting points of linear polyesters

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AbstractMelting points were determined for several series of linear polyesters containing substituents either in the acid or glycol unit. The number of substituents was progressively increased by preparing copolyesters containing both unsubstituted and substituted glycol or acid units. The following systems were studied: (1) Those with trimethylene and 2‐methyltrimethylene glycols show decreasing melting points as the number of methyl groups is increased, because they introduce tetrahedral asymmetry. If the structrual unit is relatively short, crystallinity is completely destroyed. (2) Those with trimethylene and 2,2‐dimethyltrimethylene glycols show eutectic type melting point curves, since the substituent groups do not introduce asymmetry. The completely substituted polyesters have new crystal structures which give sharp X‐ray patterns. Those from short acids (C2–4) have higher melting points than the unsubstituted polyesters, while the others have lower melting points. The polyesters with an odd number of chain atoms in the unit have higher melting points than the even ones. The ability to pack closely into the crystal lattice appears to be important, a view supported by melting points of certain polyesters of β,β‐dimethylgultaric acid. (3) Those with trimethylene and 2‐methylenetrimethylene glycols and sebacic acid give a eutectic type curve, because of the appearance of a new crystal structure. (4) Those with succinic and α‐methylsuccinic acids and decamethylene glycol give progressively decreased melting points, decreased breaks in heating curves, and an increasingly distorted lattice as the number of methyl groups is increased, because random “head‐to‐head” and “head‐to‐tail” polymerization makes the completely substituted polyester irregular. (5) The polyester of d‐α,β‐dimethoxysuccinic acid and tetramethylene glycol is highly crystalline because the substituent groups are oriented in a regular manner. The meso‐polyester did not crystallize because of more random orientation of substituents. Decamethylene d‐tartrate similarly melts higher than the meso‐tartrate.