Core-First Synthesis of Three-Armed Star-Shaped Polymers by Rare Earth Metal-Mediated Group Transfer Polymerization

Addressing polymer topologies is
one of the key methods for tailoring
polymer properties. Herein, we report for the first time on the core-first
synthesis of three-armed star-shaped polymers with adjustable molecular
weights via rare earth metal-mediated group transfer polymerization
(REM-GTP). Based on the versatility of REM-GTP, enabling polymerization
of a broad variety of functional monomers not accessible via conventional
techniques, a novel and fast method toward directed polymeric structures
was established. Therefore, the trinuclear catalyst was synthesized
by 3-fold C–H bond activation of 1,3,5-tris­(3,5-dimethyl-4-pyridinyl)­benzene
using Cp2YCH2TMS­(THF) as precursor complex.
Kinetic investigations in comparison to monometallic Cp2Y­(sym-collidinyl) on the polymerization of diethyl
vinylphosphonate (DEVP) and 2-isopropenyl-2-oxazoline (IPOx) evidenced
activity of all three metal centers. However, in REM-GTP generally
occurring incomplete initiation provoked by the interaction of initiators
and monomers, potential impurities, and applied reaction conditions
led to a distribution of stars, long linear, and short linear polymers
originating from chain growth in three, two, and one direction, respectively.
For further visualization PIPOx produced by the trinuclear complex
was converted into P­(IPOx-graft-2-ethyl-2-oxazoline)
using living cationic ring-opening polymerization. AFM scans confirmed
the occurrence of the three types of polymer. Additionally, comparable
solely linear PDEVP and PIPOx were synthesized by dinuclear complexes
generated by C–H bond activation of 1,3-bis­(3,5-dimethyl-4-pyridinyl)­benzene
and 1,4-bis­(3,5-dimethyl-4-pyridinyl)­benzene using Cp2YCH2TMS­(THF) as precursor. In the case of PDEVP, the mass
fraction of the low molecular weight polymer, being formed by chain
growth in one direction, was accessible via GPC analysis. Further
stochastic examinations on the incomplete initiation for multinuclear
complexes corroborated our findings accurately.