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Anionic Polymerization of -Butyrolactone Initiated with Sodium

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Anionic Polymerization of -Butyrolactone Initiated with Sodium polymers Article Anionic Polymerization of β-Butyrolactone Initiated with Sodium Phenoxides. The Effect of the Initiator Basicity/Nucleophilicity on the ROP Mechanism Adrian Domi ´nski 1, Tomasz Konieczny 1, Magdalena Zi˛eba 1, Magdalena Klim 1 and Piotr Kurcok 1,2,* 1 Centre of Polymer and Carbon Materials, Polish Academy of Sciences, 34, M. Curie-Sklodowskiej St., 41-819 Zabrze, Poland 2 Faculty of Mathematics and Natural Science, Jan Dlugosz University in Czestochowa, 13/15, Armii Krajowej Ave., 42-200 Czestochowa, Poland * Correspondence: [email protected] Received: 14 June 2019; Accepted: 20 July 2019; Published: 22 July 2019 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. Keywords: β-butyrolactone; anionic ring-opening polymerization; phenoxides; polymerization mechanism; β-lactones; polyhydroxybutyrate 1. Introduction 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 [2–4]. 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 [2–4,7]. 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]. 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. The most common coordination catalysts are based on metals such as aluminum, tin, yttrium, zinc and lanthane [10–13]. The polymerization of β-butyrolactone can also be carried out using cationic initiators Polymers 2019, 11, 1221; doi:10.3390/polym11071221 www.mdpi.com/journal/polymers Polymers 2019, 11, 1221 2 of 12 Polymers 2019, 11, x FOR PEER REVIEW 2 of 12 such as trifluoromethanesulfonic acid or hexafluorophosphate. However, the cationic polymerization cationicis characterized polymerization by a low is molar characterized mass of theby a resulting low molar polymer, mass of which the resulting seems to polymer, be unattractive which [seems14,15]. to be Anionicunattractive ring-opening [14,15]. polymerization (AROP) of BL is considered one of the most interesting methods,Anionic due ring-opening to the simplicity poly ofmerization the reaction, (AROP) lack of toxicityBL is considered of the resulting one of polymer, the most inexpensive interesting methods,catalysts anddue to the the possibility simplicity of of preparation the reaction, of lack tailor-made of toxicity materials. of the resulting Additionally, polymer, AROP inexpensive allows catalystsfor obtaining and the poly(3-hydroxybutyrate) possibility of preparation with of thetailor-made desired microstructure materials. Additionally, depending AROP on the allows absolute for obtainingconfiguration poly(3-hydroxybutyrate) of the monomer used with (R, Sthe or desir a R,Sed mixture) microstructure and the depending method of carryingon the absolute out the configurationreaction [16–18 of]. the Since monomer the 1990s, used many (R, scientistsS or a R,S havemixt describedure) and the the method mechanism of carrying of anionic out ring-opening the reaction [16–18].polymerization Since the of 1990s,β-butyrolactone many scientists with various have described initiators, the i.e., mechanism alkali metal of alkoxides, anionic ring-opening carboxylates, polymerizationnaphthalenides, of hydrides β-butyrolactone or basic organocatalysts. with various initiators, These studies i.e., alkali have shownmetal alkoxides, that the initiation carboxylates, step of naphthalenides,β-butyrolactone polymerizationhydrides or basic strongly organocatalysts. depends on These the typestudies of anionichave shown initiator that used the initiation [16,18–20 ].step of β-butyrolactoneWhen alkali metal polymerization naphthalenide strongly is used depends in BL anionic on the polymerization,type of anionic initiator the initiation used of[16,18–20].β-lactone ring-openingWhen alkali polymerization metal naphthalenide occurs asis used a result in BL of anionic exclusive polymerization, deprotonation the at initiation the C3 carbon of β-lactone of the ring-openingBL ring by the polymerization naphthalenide. occurs The as unstable a result of enolate exclusive formed deprotonation in this reaction at the isC3 rearranged carbon of the to theBL ringcrotonic by the acid naphthalenide. salt. The unsaturated The unstable acid enolate salt formed formed is ain real this initiator reaction of is therearranged polymerization, to the crotonic where acidcenters salt. of The propagation unsaturated are acid carboxylate salt formed ions is [19 a, 21real]. Theinitiator same of situation the polymerization, occurs in the where case of centers applying of propagationan alkali metal are hydridecarboxylate as anions initiator [19,21]. inThe the same BL polymerizationsituation occurs processin the case [22 of,23 applying]. Recently, an alkali basic metalorganocatalysts hydride as likean initiator guanidine in the (TBD), BL polymerization amidine (DBU) process and phosphazene [22,23]. Recently, (BEMP) basic were organocatalysts investigated likeas β -lactoneguanidine initiators (TBD), dueamidine to effi (DBU)cient polymerization and phosphazene of BL (BEMP) in mild were conditions investigated [24,25]. as In β addition,-lactone initiatorsCoulembier due et to al. efficient [26] proved polymerization that polymerization of BL in ofmild BL initiatedconditions with [24,25]. TBD mainlyIn addition, proceeds Coulembier with initial et al.deprotonation [26] proved of βthat-lactone polymerization and crotonate of with BL a protonatedinitiated with base TBD counterion mainly (e.g., proceeds TBDH+ )with formed initial as a deprotonationreal initiator of of this β-lactone polymerization and crotonate [20,26 with]. In a the protonated case of the baseβ-lactone counterion ring (e.g., opening TBDH with+) formed potential as acovalent real initiator linkage of initiator-3-hydroxyalkanoatethis polymerization [20,26]. In unit the formation,case of the thereβ-lactone are two ring main opening classes with of potential initiators covalentdescribed: linkage weak initiator-3-hydroxyalkanoate nucleophiles such as carboxylic unit acid formation, salts and there strong are nucleophiles,two main classes i.e., of alkali initiators metal described:alkoxides (seeweak Figure nucleophiles1). such as carboxylic acid salts and strong nucleophiles, i.e., alkali metal alkoxides (see Figure 1). FigureFigure 1. 1. ββ-butyrolactone-butyrolactone molecule molecule with with carbon carbon descripti descriptionon and and possible possible nucleophile nucleophile attack attack positions positions inin anionic anionic BL BL polymerization. polymerization. In β-lactones AROP, the initiation reaction by a weak nucleophile, e.g., a salt of carboxylic acids, In β-lactones AROP, the initiation reaction by a weak nucleophile, e.g., a salt of carboxylic acids, proceeds according to the SN2 mechanism. The initiator attacks the C4 carbon of the β-lactone ring proceeds according to the SN2 mechanism. The initiator attacks the C4 carbon of the β-lactone ring with alkyl-oxygen bond cleavage (b in Figure1). The carboxylate ion formed in this reaction is a with alkyl-oxygen bond cleavage (b in Figure 1). The carboxylate ion formed in this reaction is a center of the chain growth. The activity of the initiator and the chain propagation center strongly center of the chain growth. The activity of the initiator and the chain propagation center strongly depend on the cation–anion interactions, the size of the counterion and the solvent polarity. The depend on the cation–anion interactions, the size of the counterion and the solvent polarity. The activity of the initiating system can be easily manipulated [27–31]. When the polymerization initiator activity of the initiating system
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