We show that when poly(ethylene glycol) (PEG) is dissolved in isobutyric acid at temperatures below about 55 °C, the polymer molecule can form helices. Small-angle neutron scattering indicates that in pure isobutyric acid and in isobutyric acid-rich aqueous solutions the polymer chains form stiff rods that coexist with polymer coils when the polymer molecular weight is 2.38 × 10 4 , 2.13 × 10 5 , and 2.87 × 10 5 g/mol, but that at the lower molecular weight of 1.73 × 10 2 , only the polymer rods form. The addition of chiral dopants causes a net optical rotation in the solution, indicating that the rods are actually helices. Above about 60 °C in deuterated isobutyric acid (and above about 70 °C in hydrogenated isobutyric acid), the helices convert to coils. In water, the PEG molecules form coils which persist over the entire temperature range studied (25-60 °C).

Reversible polymerizations occur in inorganic, organic, and biological molecules. The thermodynamic states at which monomer-to-polymer reactions become favorable can be viewed as second-order phase transitions and can be profitably treated by the statistical mechanics of phase transitions. The nature and development of the microscopic structure of a solution of active polymers in equilibrium with the monomers pose interesting issues for the physical chemist.

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We have previously shown that poly(ethylene glycol) (PEG) assumes a helical conformation in isobutyric acid. We now show that the formation of helices by PEG in isobutyric acid requires the presence of a trace amount of water:  We can make the helices coil and uncoil by adding or removing trace water to/from the solvent. We also show that the similar polymer poly(ethylene imine) (PEI) forms helices in isobutyric acid and that PEG forms helices in isopentanoic and n-propanoic acids but not in isobutanol or n-butanol. PEI (M n = 21.8 kg/mol) forms only helices in isobutyric acid, whereas PEG (M n = 21.0 kg/mol) forms a mixture of helices and coils. Helical PEI (M n = 21.8 kg/mol) shows a helix-to-coil transition when the temperature is increased to about 50 °C, while PEG (M n = 21.0 kg/mol) shows a helix-to-coil transition at 40−45 °C. When the trace water in the solution is D2O, the PEG (M n = 21.0 kg/mol) helix-to-coil transition moves to a higher temperature of about 56 °C, perhaps due to stronger hydrogen bonding.

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The aggregation of monomers into polymers, whether by covalent or noncovalent interactions, is often reversible and frequently occurs with the entropy and enthalpy of the aggregation sharing the same sign. In such a case, the aggregation goes forward or reverses, depending on such variables as temperature and composition, rather like a phase transition. We explore the physical chemistry of three such systems: an organic monomer (alpha-methylstyrene), an inorganic monomer (sulfur), and a biopolymer (actin). We compare the available theories and experiments and list issues still open.

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