SEC Analysis of Poly(Acrylic Acid) and Poly(Methacrylic Acid)

The accurate characterization of molar-mass distributions of poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMAA) by size-exclusion chromatography (SEC) is addressed. Two methods are employed: direct aqueous-phase SEC on P(M)AA and THF-based SEC after esterification of P(M) AA to the associated methyl esters, P(M) MA. P(M) AA calibration standards, P(M) AA samples prepared by pulsed-laser polymerization (PLP), and PAA samples prepared by reversible addition-fragmentation chain transfer (RAFT) are characterized in a joint initiative of seven laboratories, with satisfactory agreement achieved between the institutions. Both SEC methods provide reliable results for PMAA. In the case of PAA, close agreement between the two SEC methods is only observed for samples prepared by RAFT polymerization with weight-average molar mass between 80 000 and 145 000 g mol(-1) and for standards with peak molar masses below 20 000 g mol(-1). For standards with higher molar masses and for PLP-prepared PAA, the values from THF-based SEC are as much as 40% below the molar masses determined by aqueous-phase SEC. This discrepancy may be due to branching or degradation of branched PAA during methylation. While both SEC methods can be recommended for PMAA, aqueous-phase SEC should be used for molar-mass analysis of PAA unless the sample is not branched.