Abstract
Various alkali metal salts were applied as initiators for ε-caprolactone anionic ring-opening polymerization in tetrahydrofuran at room temperature. It was observed that potassium methoxide (MeOK), potassium isopropoxide (i-PrOK) and potassium tert-butoxide (t-BuOK) nonactivated or activated by 18-crown-6 (18C6) initiated polymerization mainly by deprotonation of the monomer. In the case of potassium hydride (KH), its basicity increased with the ability of the ligand for cation complexation. For example, KH without ligand or with weak ligands for K+ as 12C4 reacted exclusively by ring opening. However, in the presence of strong ligands, as 15C5, 18C6 or cryptand C222, basicity of H− increased with the ability of the ligand for cation complexation. In the last case, ~ 32% of the monomer was deprotonated. In these systems, gaseous H2 evolved during the initiation. Deprotonation of the monomer by some initiators resulted in macromolecules with reactive aldehyde group or lactone ring as starting groups. They took part in the reaction with potassium alkoxide active centers of growing chains leading to the formation of branched poly(ε-caprolactone)s. Sodium hydride (NaH) was inactive, but in the presence of 15-crown-5 or 18-crown-6 initiated polymerization exclusively by ring opening. MALDI-TOF mass spectrometry supported with 13C NMR and SEC was used for analysis of the polymers obtained. Mechanism of the studied processes was proposed and discussed.
Highlights
Poly(ε-caprolactone) (PCL) is an important polymer due to its mechanical properties, miscibility with other polymers and biodegradability [1]
Various alkali metal salts were used for initiation of ε-caprolactone anionic ringopening polymerization in tetrahydrofuran at ambient temperature
Potassium hydride nonactivated or activated by weak ligands as 12C4 or DCH24C8 reacted by ring opening, but in the presence of strong ligands as 15C5, 18C6 or C222 initiation occurred by ring opening and monomer deprotonation
Summary
Poly(ε-caprolactone) (PCL) is an important polymer due to its mechanical properties, miscibility with other polymers and biodegradability [1]. It is produced by polycondensation of 6-hydroxyhexanoic acid or ring-opening polymerization (ROP) of ε-caprolactone (ε-CL). Copolymers including PEG and PCL unit are good transporter for hydrophobic drugs delivery [14]. The synthesis of poly(CL-b-EG-b-CL) ABAtype block copolymers has been performed by “click” chemistry method [15]. Another type of copolymers’ architecture is graft one where copolymers have a branching chain structure attaching polymer units to another polymer backbone [16]. By synthesis of a novel macroinitiator (RAFT-ROP agent), poly[CL-bEPCH-CL-(g-n-BMA)] block–graft copolymers were prepared [17]
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