Abstract
In this study, low molecular weight poly(δ-valerolactone) (PVL) was synthesized through bulk-ring openings polymerization of δ-valerolactone with boric acid (B(OH)3) as a catalyst and benzyl alcohol (BnOH) as an initiator. The resulting homopolymer was characterized with the aid of nuclear magnetic resonance (NMR) and mass spectrometry (MS) techniques to gain further understanding of its molecular structure. The electrospray ionization mass spectrometry (ESI-MS) spectra of poly(δ-valerolactone) showed the presence of two types of homopolyester chains—one terminated by benzyl ester and hydroxyl end groups and one with carboxyl and hydroxyl end groups. Additionally, a small amount of cyclic PVL oligomers was identified. To confirm the structure of PVL oligomers obtained, fragmentation of sodium adducts of individual polyester molecules terminated by various end groups was explored in ESI-MSn by using collision induced dissociation (CID) techniques. The ESI-MSn analyses were conducted both in positive- and negative ion mode. The comparison of the fragmentation spectra obtained with proposed respective theoretical fragmentation pathways allowed the structure of the obtained oligomers to be established at the molecular level. Additionally, using matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS), it was proven that regardless of the degree of oligomerization, the resulting PVL samples were a mixture of two types of linear PVL oligomers differing in end groups and containing just a small amount of cyclic oligomers that tended to be not visible at higher molar masses.
Highlights
Lactones are cyclic esters and a very important polymer building material
We report the synthesis of biodegradable poly(δ-valerolactone) via a bulk-ring opening polymerization of δ-valerolactone with the use of boric acid as a nontoxic metal-free catalyst
A comparison of the fragmentation spectra presented in Figures 4 and 5 with the respective theoretical fragmentation pathways shown in Schemes 2 and 3 clearly indicated that the main A series corresponded to sodium adducts of PVL chains terminated by benzyl ester and hydroxyl end groups (Figure 4, Scheme 2), while series B was ascribed to PVL oligomers with hydroxyl and carboxyl end groups (Figure 5, Scheme 3)
Summary
Lactones are cyclic esters and a very important polymer building material. The synthetic aliphatic polyesters derived from lactones have a unique position among biodegradable polymers. Many practical applications of these materials in medicine arise from their degradability in vivo, which can be achieved through simple hydrolysis of the ester backbone in aqueous environments, such as body fluids, to form products that are broken down to nontoxic metabolites [3,4,5] This group of polymers has been widely investigated both in terms of synthesis pathway as well as in structural characterizations [6,7,8,9,10,11]. MALDITOFMS and ESIMS are suitable for polymer analysis because they are sensitive and nonaveraging techniques that provide detailed information about the structure of individual molecules (including end group analysis) in a polymer sample [27,28,30,31] These techniques are widely used with increasing success for characterization of synthetic copolymers. The chemical structure of the individual poly(δ-valerolactone) chains was unequivocally established by investigating the fragmentation product patterns of the individual molecular ions
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