Low molecular weight acrylic acid polymers provide a range of useful properties including dispersing, scale inhibition, antiprecipitation, soil dispersing and antisoil redeposition. Utility can often be improved by a) shortening the acrylic acid chain particularly to a degree of polymerization less than 100 which can be described as a telomer and b) by introducing a terminal group that contributes functionality to the telomer. Functionality could be derived from a group that confers hydrophobicity to enhance adsorption of polymer on low polarity surfaces or from a group that imparts reactivity to allow post polymerization reactions.
Use of chain transfer agents in the polymerization of acrylic acid is the usual means to control molecular weight and to introduce terminal groups. The terminal groups produce an increasingly important effect as chain length of the acrylic acid is shortened. It is with telomers that the terminal group most dramatically affects properties and performance.
Mercaptans, including alkyl, hydroxyalkyl and carboxyl containing, have been used for molecular weight control of polyacrylic acids. By acting as chain transfer agents, they also contribute an end group to the polymer chain although the selection available is limited. The mercaptan chain transfer agents, however, suffer from the serious disadvantage of imparting objectionable odor to the polymers or telomers. Because of this, the products can not be used in many applications particularly as detergent additives. Attempts to remove the odor by chasing after polymerization to remove residual mercaptan or by oxidation of residual mercaptan to a nonvolatile sulfonate have added additional processing cost and have generally not been successful. The low odor threshold of parts per billion for most mercaptans make the task of achieving odor free material and maintaining it during storage almost impossible. Poor storage stability is a result of the reversibility of the mercaptan addition reaction. Exposure of the product to elevated temperatures can lead to loss of end group and performance properties as well as increased odor.
The lower alkyl secondary alcohols, particularly isopropanol and secondary butanol, have also been employed as chain transfer agents to produce low molecular weight polyacrylic acid. These low boiling volatile alcohols, however, present processing difficulties and additional expense as their use requires pressure reactors to allow the high temperatures necessary for them to effectively control molecular weight. This is especially difficult when preparing the very low molecular weight telomers. Furthermore isopropanol or secondary butanol do not provide terminal groups that impart significant functionality; contain too few carbons for hydrophobicity and produce a tertiary hydroxyl on the terminal group with little reactivity. U.S. Pat. No. 4,774,303 does reveal the use of C.sub.2 to C.sub.6 secondary alcohols where the upper limit C.sub.6 alcohol would contribute hydrophobicity. However, this reference teaches use of a phosphorous or phosphonic acid or salt as a necessary coreactant.