Ring-Opening Polymerization of Lactones Initiated with Metal Hydroxide-Activated Macrocyclic Ligands: Determination of Mechanism and Structure of Polymers

Abstract

Anhydrous alkali metal hydroxide (KOH, NaOH, and LiOH)-activated macrocyclic ligand complexing metal cations, i.e., coronands 12C4, 15C5, 18C6, DCH24C8, and cryptand C222, were selected for initiation of β-butyrolactone (β-BL) and ε-caprolactone (ε-CL) polymerization. It was found that β-BL polymerizes in the presence of KOH/18C6, KOH/C222, and NaOH/C222 systems. The real initiators in this case are two salts, potassium 3-hydroxybutyrate and potassium trans-crotonate, which are responsible for the formation of two fractions of the obtained polymer. ε-CL underwent polymerization with KOH or NaOH activated by all ligands used or without the ligand but with LiOH/12C4. Using KOH-activated strong ligands, i.e., 15C5, 18C6, or C222, two polymer fractions were generated containing linear and, unexpectedly, also cyclic macromolecules. The mechanism of the studied processes is discussed.. Deprotonation of the monomer and formation of lactone enolate, which decomposes with alkyl-oxygen bond cleavage (with potassium hydride [5] or potassium

Ring-opening polymerization of β-butyrolactone in the presence of alkali metal salts: investigation of initiation course and determination of polymers structure by MALDI-TOF mass spectrometry

“…Chromatographic analysis (GC-MS) of the reaction mixture indicated the formation of trans-crotonic acid isopropyl ester. Formation of macromolecules A and B was reported recently by us in the polymerization of β-BL initiated with anhydrous KOH activated 18C6 [13]. It indicated that KOH really formed in the studied systems and initiated β-BL polymerization.…”

“…In this case, only weak signals of macromolecules A were shown in MALDI-TOF spectrum. No signal of (CH 3 ) 3 CO-end group was found in 13 C NMR spectrum of the polymer. It indicates that t-butoxide anions behave during the initiation step mainly as a strong base.…”

“…Subsequently, 100 MHz, 13 C NMR spectra were recorded in CDCl 3 at 25 °C on a BruckerAvance 400 pulsed spectrometer equipped with 5-mm broadband probe and applying Waltz16 decoupling sequence. Chemical shifts were referenced to tetramethylsilane serving as an internal standard.…”

“…The authors proposed that the primary reaction product is unstable and eliminates KOH, which becomes the real initiator. The course of β-BL polymerization initiated with KOH activated with macrocyclic ligands was recently presented in details by us in [13]. We reported that KOH behaves in such systems as nucleophilic base, which deprotonates the monomer, as well as opens lactone ring in the acyl-oxygen position.…”

Mechanism of ε-caprolactone polymerization in the presence of alkali metal salts: investigation of initiation course and determination of polymers structure by MALDI-TOF mass spectrometry

“…However, the application of NaH for ε-CL polymerization causes acyl-oxygen bond cleavage during initiation [19]. Recently [20], macrocyclic ligands activated by alkali metal hydroxides, i.e., LiOH, NaOH and KOH, have been used successfully for ε-CL polymerization. In all systems, initiation proceeded by acyl-oxygen bond cleavage.…”