Characterization of organometallic copolymers and copolymerization by size-exclusion chromatography coupled with trace metal- and mass-sensitive detectors

Abstract

An important class of controlled-release, biocidal triorganotin-containing copolymers (OMPs) is produced by the free radical initiated reaction of a triorganotin methacrylate (typically tributyltin methacrylate, TBTM) with methyl methacrylate. In earlier studies, we have demonstrated that size-exclusion chromatography (SEC) on μStyragel, coupled with an in-line graphite furnace atomic absorption detector (GFAA) quantifies at least two well-resolved tin-bearing molecular weight fractions observed in many different commercial OMP formulations: a polymer fraction, and, probably, unreacted TBTM. When tetrahydrofuran is used as the eluent, a third component, believed to be an ionic tin-containing compound, is adsorbed and slowly eluted. We now verify that a dilute solution of acetic acid in the tetrahydrofuran efficiently desorbs this species. If a one percent solution of acetic acid in tetrahydrofuran is introduced into the mobile phase as a discrete, injected volume several minutes after the injection of the polymer formulation, there appears in the SEC—GFAA chromatogram: the polymer peak, the monomer peak, several minutes of tailing, and, finally a spike, with little additional tailing. However, when a one percent solution of acetic acid in tetrahydrofuran is used as the eluent, the high polymer peak is substantially reduced in size owing to acid hydrolysis of polymeric esters to which the tributyltin is chemically bonded; the low-molecular-weight peak is substantially increased. Thus, delayed injection of acetic acid serves the purpose of avoiding acid hydrolysis of the polymer. In the work described, only trace amounts of acetic acid (0.5 μl to 5.0 μl) were introduced by injection into the tetrahydrofuran mobile phase, whereas prolonged exposure to dilute solutions of acetic acid in tetrahydrofuran — e.g., introduced by a gradient method— could be expected to shorten column life. Partial decomposition of esters was found in partially reacted OMPs after one year of storage at −780°C.