A method is described which allows molecular modeling to be combined with a group additive property model to estimate glass transition temperatures of linear polymers. Tg is assumed to be a function of conformational entropy and mass moments of the polymer. These two molecular properties are estimated in terms of the torsion angle units composing the polymer using conformational energy calculations. A “universal” Tg equation is derived using 30 structurally diverse polymers and multidimensional linear regression analysis. “Designer” Tg equations are also derived specifically for acrylate and methacrylate polymers. The work described here demonstrates how molecular modeling can be combined with group additivity theory to yield open‐ended models that are not restricted by lack of requisite group additive parameters and take advantage of three‐dimensional molecular information.
The logarithm of the partition coefficient (log P) of low-molecular-weight organic compounds is a physicochemical parameter used extensively in structure-biological activity studies to model interactions of the compounds with nonpolar phases in vitro and in vivo. The partition coefficient can be determined between water and a number of nonpolar solvents. The most common nonpolar solvent is 1-octanol, but solvents such as benzene, carbon tetrachloride, and chloroform are frequently used as models for the nonpolar phases. The functional relationship between chemical structure and partitioning is not well-understood. In this paper, partition coefficient data for 50 solutes in six nonpolar solvent systems are analyzed by using principal components analysis. The objective of the work is to explore the relationship between solute structure and partitioning behavior for simple organic compounds. Two structural factors are found to be important, with the isotropic surface area being the most important. The isotropic surface area can be used to estimate log P in some solvents and as an independent variable in quantitative structure-activity relationships (QSAR). This is illustrated by estimating the rate of epidermal diffusion of steroids.
The relationship between chemical structure of a solute and the logarithm of its partition coefficient (log P) between the aqueous and a nonpolar phase is poorly understood. We have recently shown that the variation in log P data for 50 low molecular‐weight organic solutes in 6 aqueous‐non‐polar solvents is a function of two structural features. The main feature accounts for ≈︁60% of the variation in the log P data, and is uniformly weighted in all 6 nonpolar solvent systems. This suggests that it is related to the aqueous solution properties of the solute. The first feature is termed the isotropic surface area, or the surface area associated with the nonpolar portion of the solute when the solute is considered to be a hydrated complex. The hydrated solute complex is termed a supermolecule with waters of hydration occupying hydrogen bonding sites on the functional groups of the solute. Empirical rules for formation of the super molecule are discussed.
In this report the analysis is extended to log P data for 72 solutes in the 6 nonpolar solvent systems. The results of the analysis are essentially unchanged for this more extended data set and the second factor is tentatively identified. The second structural feature accounted for ≈︁35% of the variation in the log P data was not equally weighted in all solvents and is difficult to interpret structurally.
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