Isotactic and syndiotactic polymethacrylic acids (PMA * s) have been prepared and studied by pH titration, enthalpy titration and interaction with copper(II).The syndiotactic polyacid (85% syndiotactic triads by NMR of the derived polymethyl methacrylate) is similar to ordinary PMA (57% syndiotactic). Isotactic PMA (90% iso tactic triads) is insoluble at low degrees of neutralization (alpha); it is a weaker acid by about 0.4 pK independent of alpha and level of neutral salt, and binds copper(II) about three times as strongly (equilibrium dialysis). The alpha dependence of the copper binding ratio indicates that there are two carboxylate ions in the syndiotactic complex, one in the isotactic.The visible and ultraviolet absorbancies of the copper- PMA complex are quite sensitive to tacticity, especially at high pH where the possibility of a tacticity index is indicated.Microcalorimetric determination of the heat of neutralization as a function of alpha shows that tacticity in fluences acidity primarily through the dissociation enthalpy. The dissociation enthalpy decreases with increasing alpha, more rapidly in water than in salt solution, the concomitant increase in the free energy of dissociation being due to the rapid decrease in entropy of dissociation.In the presence of copper(II) there are large effects on the heat of reaction with base, corresponding, in general, with the alpha regions of large pH effect. The reaction of syndiotactic PMA with copper(II) is endothermic to the ex tent of about 8 kcal per mole of copper.The experimental results have been interpreted in terms of a helical structure analogous to that of crystalline, stereoregular polymethyl methacrylate; in the syndiotactic helix the carboxyl groups are associated in physically distinct pairs separated by similar pairs of alpha methyl groups; in the isotactic chain the carboxyl and methyl groups alternate. Solubility differences are attributed to differences in hydrogen bonding capacities in accord with this conformation. The titration curves of "atactic" and isotactic poly acrylic acids were only marginally different; there are indications that in this system, the "atactic" polyacid is predominantly isotactic. Crosslinking of polyacrylic acid (microsol copolymer with divinylbenzene) causes a "parallel" shift in the titration curve without change of shape.The tacticity of PMA’s prepared from the monomer by gamma ray initiation in water at various alphas indicates that charge favors syndiotactic placement to about the same extent that would be predicted from the effect of tacticity on the free energy of dissociation.From the effect of polymerization temperature on tacticity, the enthalpy and entropy of activation for iso tactic placement are estimated to be greater than for syndiotactic placement by 1.2 kcal/mole and 1.5 e.u. respectively in the absence of charge.An attempt to study the proton NMR spectra of PMA’s in D2O failed for lack of signal strength and resolution under conditions which gave good spectra of structurally similar monomeric acids.