Characterization of linear and cyclic polylactic acids and their solvolysis products by electrospray ionization mass spectrometry

Abstract

Linear and cyclic polylactic acids (PLAs) were characterized using electrospray ionization mass spectrometry (ESI-MS) as part of our ongoing investigation of the hydrolysis mechanism of biodegradable polymers. The condensation oligomers of linear polylactic acid (LPLA) were synthesized by thermal dehydration of L-lactic acid. The trimer and tetramer base polymers of cyclic polylactic acid (CPLA) were obtained by cyclization reactions of lactic acid trimers and tetramers, respectively. In the ESI-MS/MS measurement, LPLA yielded three types of product ion series, while CPLA yielded only one type, from which the repeated units of CPLA were removed. The MS/MS spectrum of the NH4+ adduct ion for both cyclic and linear PLA showed loss of one ammonia molecule. The postsource decay (PSD) spectrum of CPLA by matrix-assisted laser desorption ionization (MALDI) mass spectrometry was similar to the ESI-MS/MS spectrum, while that of LPLA was different. In addition, the degradation of cyclic and linear PLAs by solvolysis was investigated. Solvolysis with anhydrous MeOH was quite feasible, but did not readily occur in the presence of even a small amount of water in the MeOH solvent.

Mechanistic study of the collision-induced dissociation of sodium-cationized polylactide oligomers: A joint experimental and theoretical investigation

“…First of all, as exemplified in Scheme 1, the losses of 295 and 90 u, respectively, routes (a) and (b), can be considered as end-group specific and must arise through Mc-Lafferty-like rearrangements that involve the migration of the hydrogen atom from a pendant methyl group to the oxygen atom of the carbonyl group of the ester moiety. Those results are fully supported by previous papers on the CID dissociations of sodiumcationized PLA oligomers [13,20]. The involvement of six-membered ring transition states is consistent with the efficiency of such decompositions [14].…”

“…Our findings are totally in line with the results from Osaka et al for CID experiments on Na ϩ -cationized linear PLA [20]. In particular, they argued that three dissocia- tions pathways occur to account for the production of all CID fragment ions.…”

“…In direct connection with the scope of our work, two recent papers have reported the CID characterization of sodium-cationized PLA oligomers [13,20]. Both papers describe that the dissociations of collisionally-excited cations are always initiated by a hydrogen atom migration through a favorable six-membered ring transitionstate ultimately leading to backbone cleavages at the ester binding sites.…”

“…They obtained some evidences for the occurrence of this third dissociation route when investigating the CID behavior of the solvolysis products obtained by degradation in methanol of linear poly(lactic acid). Indeed, in the CID spectrum of their [M n ϩ MeOH ϩ Na ϩ ] cations (M ϭ lactic acid monomer unit), they observed three types of product ions, i.e., the A and B series together with the distinct C ion series [20]. Their main conclusion was that the production of C fragment ions probably involves a rearrangement of the hydrogen atom at the end-group [20].…”

“…Indeed, in the CID spectrum of their [M n ϩ MeOH ϩ Na ϩ ] cations (M ϭ lactic acid monomer unit), they observed three types of product ions, i.e., the A and B series together with the distinct C ion series [20]. Their main conclusion was that the production of C fragment ions probably involves a rearrangement of the hydrogen atom at the end-group [20]. To shed further light on this intriguing dissociation pathway, we decided to study in more details the CID behavior of end-group modified polylactides.…”