Synthesis and Characterization of Amine-Bridged Bis(phenolate) Yttrium Guanidinates and Their Application in the Ring-Opening Polymerization of 1,4-Dioxan-2-one

A series of neutral yttrium guanidinates
supported by an amine-bridged
bis­(phenolate) ligand were synthesized, and their catalytic behaviors
for the ring-opening polymerization of 1,4-dioxan-2-one (p-dioxanone, PDO) were explored. Metathesis reactions of amine-bridged
bis­(phenolate) yttrium chlorides LLnCl­(THF) [L = Me2NCH2CH2N­{CH2-(2-OC6H2-tBu2-3,5)}2] with corresponding lithium guanidinates
generated in situ in a 1:1 molar ratio in THF gave the neutral yttrium
guanidinates LY­[R2NC­(NR1)2] [R1 = −Cy, R2N = −N­(TMS)2 (1), −NiPr2 (2), −N­(CH2)5 (3);
R1 = −iPr, R2N = −NiPr2 (4) −NPh2 (5))]. These complexes were well characterized by elemental
analyses, IR, and NMR spectroscopy. The definitive molecular structures
of these complexes were determined by single-crystal X-ray analysis.
It was found that these complexes can efficiently initiate the ring-opening
polymerization (ROP) of PDO, and the catalytic activity is affected
by the nature of the guanidinate groups with the active sequence of 1 > 2 ≈ 3 ≈ 4 > 5. The influences of reaction conditions
such as polymerization time, polymerization temperature, and molar
ratio of monomer to initiator on the polymerization were also investigated.
The polymerization kinetics of PDO catalyzed by complex 1 is first-order with respect to monomer concentration, and the apparent
activation energy amounts to 30.8 kJ mol–1. The
mechanistic investigations showed that the ROP of PDO proceeded through
a coordination–insertion mechanism with a rupture of the acyl–oxygen
bond of the monomer. MALDI-TOF mass spectrum analysis of the oligomer
revealed that there are two kinds of polymer chains in this catalytic
system, e.g., the linear chains H–[OCH2CH2OCH2CO]n–OH and the
PPDO macrocycles.