BACKGROUND

Low number average molar mass (Mn) (meth)acrylic acid polymers, such as poly(acrylic acid) (PAA), poly(methacrylic acid) (PMAA), and salts thereof, are commercially useful as dispersants. However, the controlled polymerization of acrylic acid (AA) and methacrylic acid (MAA) with targeted Mnis difficult, especially for polymers with Mnbelow 20 kilograms per mole (kg/mol). Although there are methods for preparing low MnPAA and PMAA with narrow molecular weight distribution, or molar mass dispersity (Ð), expensive chain transfer agents (CTA) and complex processes or equipment are often required, which results in a high cost for the PAA or PMAA produced by these methods.

Reverse iodine transfer polymerization (RITP) has found utility in the controlled polymerization of methyl acrylate and n-butyl acrylate. In RITP, an ethylenically unsaturated monomer is polymerized in the presence of molecular iodine (I2) and a radical polymerization initiator. A simplified chemical mechanism of RITP is depicted inFIG. 1. InFIG. 1, 101represents an inhibition period in which the CTA, A-Mn-I (n≥20), is formed in situ, A. is a carbon-centered radical derived from the radical polymerization initiator and M is an ethylenically unsaturated monomer.102represents the polymerization period, wherein A-Mn. A-Mm. represent propagating polymer chains, kexis the degenerative chain transfer rate constant, and kpis the propagation rate constant. In the degenerative chain transfer equilibrium, both a new propagating polymer chain, A-Mn., and a new CTA, A-Mm-I, are simultaneously produced.

A major problem with RITP is the use of I2. I2is only slightly water-soluble, so RITP is conducted in organic solvents. If it is to be used with water as the solvent, it has to be dissolved in the monomer before use. Moreover, I2is reactive toward a large number of chemical compounds, and is subject to disproportionation into iodate and iodide in water. I2is also potentially dangerous upon storage and handling, in view of the risk of formation of I2vapor. It causes skin irritation, serious eye irritation, is harmful when inhaled, may cause drowsiness or dizziness, and causes damage to organs through prolonged or repeated exposure. Thus there are many health and safety issues associated with the use of I2in a manufacturing process.

Due to their commercial importance, it is desirable to have a polymerization method to produce PAA and PMAA of controlled Mnand narrow molecular weight distribution (low Ð), with high monomer conversion. It is also desirable that the method does not use I2due to the problems described above, and that transition metal catalysts, and expensive and problematic CTAs, such as cobalt complexes, nitroxides, dithioesters, thiocarbamates, xanthates, and p-toluenesulfonyl iodide, not be used. It is also desirable that, unlike the above methods, the direct polymerization of (meth)acrylic acid does not require organic solvents, and can be conducted in water instead. Moreover, the polymerization method should provide linear polymers having low Mnand Ð under mild conditions and provide high monomer conversion in short time.

BRIEF DESCRIPTION

A method of making a (meth)acrylic acid polymer comprises reverse iodine transfer polymerization of an ethylenically unsaturated monomer comprising (meth)acrylic acid, salt thereof, or combination thereof, in the presence of: a radical polymerization initiator; an oxidant; an iodide salt; and a solvent. The method of making the (meth)acrylic acid polymer can comprise reverse iodine transfer polymerization of 5 to 500 equivalents of the ethylenically unsaturated monomer comprising (meth)acrylic acid, salt thereof, or combination thereof, in the presence of 1 to 3 equivalents of the radical polymerization initiator, 0.2 to 1 equivalent of the oxidant, and 1 equivalent of the iodide salt. (Meth)acrylic acid polymer solutions are made by these methods.

