Gamma-poly(glutamic acid) ester derivatives: Solution and liquid crystalline properties

Several water-insoluble {dollar}alpha{dollar}-esters of natural origin {dollar}gamma{dollar}-Poly(Glutamic Acid), {dollar}gamma{dollar}-PGA, were synthesized for the first time by reaction of {dollar}gamma{dollar}-PGA with corresponding halides of desired side chains. The derivatives included ethyl, n-propyl, i-propyl, n-pentyl, n-dodecyl, benzyl, 4-cyano-4{dollar}spprime{dollar}-heptyloxybiphenyl and 4-cyano-4{dollar}spprime{dollar}-nonyloxybiphenyl {dollar}alpha{dollar}-esters with high degrees of substitution. Due to reduced polarity and disruption of strong inter- and intra-chain hydrogen-bonding by the side-groups, most of these derivatives showed solubility in a wider range of organic solvents than underivatized, natural origin. {dollar}gamma{dollar}-PGA. This disruption of hydrogen-bonding also resulted in the progressive reduction of Tg's of these polymers as the side chains were lengthened. For the n-dodecyl derivative, side chain crystallization was induced and a melting transition was hence observed for the first time in a derivative of natural origin {dollar}gamma{dollar}-PGA. In contrast, underivatized {dollar}gamma{dollar}-PGA decomposes before melting. When the more bulky i-propyl side-group was introduced, the mobility of the polymer backbone appeared to be restricted, causing the Tg to be higher than that of the parent polymer. Solubility and thermal properties were found to be sensitive also to the degree of substitution.; Optical studies into the conformational properties of {dollar}gamma{dollar}-glutamate polymers, having thus far been restricted to aqueous solutions of underivatized {dollar}gamma{dollar}-PGA in the free acid and ionized forms, have not been able to resolve the question of whether it can achieve a regular conformation in solution. The n-pentyl {dollar}alpha{dollar}-ester showed a wide range of solubility in organic solvents, and was hence the subject of the first ever investigation into the conformation of a {dollar}gamma{dollar}-glutamate polymer in non-aqueous solvents. Evidence obtained from {dollar}sp1{dollar}H and {dollar}sp{lcub}13{rcub}{dollar}C NMR and circular dichroism(CD) indicated that this polymer possesses a helical conformation in 2,2,2-trifluoroethanol(TFE), and a flexible {dollar}beta{dollar}-sheet type conformation in 1,1,1,3,3,3-hexafluoro-2-propanol(HFIP) and trifluoroacetic acid(TFA). A conformational transition is seen in TFE, upon gradually increasing the temperature, or upon the addition of gradually increasing amounts of TFA. The transition due to TFA appears to be that of a helix to a {dollar}beta{dollar}-type conformation. A different transition appears to take place with increasing temperatures.; Along with the study of the conformational order of these derivatives in solution, ordered systems were specifically developed by the introduction of mesogenic cyanoalkoxybiphenyl groups on the side chain. We have documented the ability of {dollar}gamma{dollar}-PGA to form thermotropic side chain liquid crystalline polymers upon derivatization by these mesogens with two spacer lengths of 7 and 9 methylene units(CB7PGA & CB9PGA). CD of CB7PGA showed that while the side chain was decoupled from the main chain in dilute solution, main chain chirality was seen to be successfully transferred to the side chain mesogen upon formation of a mesophase. This transfer of chirality also takes place in CB9PGA. This is the first example of a chiral thermotropic polymer having been synthesized by taking advantage of the chirality in the main chain of a natural origin polymer.