Abstract
It was shown that selected sodium phenoxide derivatives with different basicity and nucleophilicity, such as sodium p-nitrophenoxide, p-chlorophenoxide, 1-napthoxide, phenoxide and p-methoxyphenoxide, are effective initiators in anionic ring-opening polymerization (AROP) of β-butyrolactone in mild conditions. It was found that phenoxides as initiators in anionic ring-opening polymerization of β-butyrolactone behave as strong nucleophiles, or weak nucleophiles, as well as Brønsted bases. The resulting polyesters possessing hydroxy, phenoxy and crotonate initial groups are formed respectively by the attack of phenoxide anion at (i) C2 followed by an elimination reaction with hydroxide formation, (ii) C4 and (iii) abstraction of acidic proton at C3. The obtained poly(3-hydroxybutyrate) possesses carboxylate growing species. The ratio of the observed initial groups strongly depends on the basicity and nucleophilicity of the sodium phenoxide derivative used as initiator. The proposed mechanism of this polymerization describes the reactions leading to formation of observed end groups. Moreover, the possibility of formation of a crotonate group during the propagation step of this polymerization is also discussed.
Highlights
Poly(β-hydroxyalkanoate)s (PHAs) are biodegradable and biocompatible aliphatic polyesters [1], considered to be future “green” materials for biomedical, food package or drug delivery applications [2,3,4].Several PHAs are produced by many different microorganisms [5], but the most well-known is poly([R]-3-hydroxybutyrate) (PHB)
Results of 1 H NMR and ESI-MSn analyses revealed that the poly(3-hydroxybutyrate)s obtained in anionic ring-opening polymerization (AROP) initiated with the selected phenoxides possess carboxyl end groups and three different types of initial groups: crotonate, phenoxy and hydroxy, occurring in different ratios, depending on the phenoxide used
A typical 1 H NMR spectrum of low molar mass PHB obtained in the polymerization initiated with sodium phenoxide shows the polymer chain characteristic signals: (CH3 at δ = 1.28 ppm, CH2 = 2.5 ppm, CH = 5.25 ppm) and signals corresponding to initial groups: (HO–CH at δ = 4.2 pm, PhO–CH at 4.8 ppm and CH3 CH=CH at 5.8 ppm) (Figure 2)
Summary
Poly(β-hydroxyalkanoate)s (PHAs) are biodegradable and biocompatible aliphatic polyesters [1], considered to be future “green” materials for biomedical, food package or drug delivery applications [2,3,4]. Several PHAs are produced by many different microorganisms [5], but the most well-known is poly([R]-3-hydroxybutyrate) (PHB). Isotactic PHB is a biopolymer produced by microorganisms and used as an energy and carbon source [6]. Synthetic and natural poly(3-hydroxybutyrate)s are very popular for biomedical applications due to their biodegradation and biocompatibility [2,3,4,7]. Coordination polymerization allows for obtaining atactic, syndiotactic and perfectly isotactic PHB depending on the type of coordination catalyst used as well as the way of conducting the reaction Poly([R]-3-hydroxybutyrate) is synthesized by biotechnological processes, applying bacterial fermentation of substrates such as sucrose, corn, cane sugar, etc. [8], or as a result of a chemical reaction, i.e., polymerization of β-butyrolactone (4-methyl-2-oxetanone, BL), using coordination, cationic or anionic ring-opening polymerization [9].
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