Ionic liquid-functionalized LDH as catalytic-initiating nanoparticles for microwave-activated ring opening polymerization of epsilon-caprolactone

The paper is focused on investigation of layered double hydroxides (LDH) functionalized with ionic liquid (IL) as novel catalytic/initiating system for ring opening polymerization (ROP) of epsilon-caprolactone. Firstly, the Ca2+/Al3+ LDH nanoparticles were synthesized via a co-precipitation method followed by the anion exchange using phosphonium IL with decanoate anions producing the modified LDH with interlayered IL-anions. Then, the progress of microwave-assisted polymerization in the presence of the modified LDH was studied showing the anionic mechanism of ROP initiated by the water molecules, which were adsorbed on the LDH surface and intercalated in the LDH galleries. The whole polymerization was significantly accelerated by microwave energy, which induced molecular rotations of the intercalated IL-anions leading to LDH delamination and exfoliation into individual nanosheets, which made the catalytic sites (IL-anions) easily accessible for the epsilon CL molecules. The kinetics of microwave-assisted ROP thus showed a 4.6-fold increase of the reaction rate compared to the polymerization performed under conventional heating. Activation enthalpy and entropy were calculated for the microwave-assisted polymerizations initiated by the modified LDH. The use of microwave-active catalytic-initiating LDH nanoparticles enables to synthesize the organometallic catalyst-free PCL or, at higher LDH loadings, the highly-exfoliated PCL/LDH nanocomposites, which are highly desirable for e.g. packaging materials.