Air-Stable (Phenylbuta-1,3-diynyl)palladium(II) Complexes: Highly Active Initiators for Living Polymerization of Isocyanides

A family of air-stable (phenylbuta-1,3-diynyl)­palladium­(II)
complexes
were designed and prepared in a facile synthetic procedure. Their
structures were characterized by 1H and 13C
NMR, MS, and X-ray analysis. These Pd complexes were revealed to efficiently
initiate the polymerization of phenyl isocyanides in a living/controlled
chain growth manner, which led to the formation of poly­(phenyl isocyanide)­s
with controlled molecular weights and narrow molecular weight distributions. 13C NMR analysis indicated the isolated poly­(phenyl isocyanide)
was of high stereoregularity. The Pd unit at the end of the polymer
chain could undergo further copolymerization with phenyl isocyanide
monomers to give block copolymers. It was also found that incorporation
of an electron-donating group on the phenyl group of the Pd complex
could improve the catalytic activities. Furthermore, these Pd complexes
were tolerant to most organic solvents and applicable to a wide range
of isocyanide monomers including alkyl and phenyl isocyanides and
even phenyl isocyanide with bulky substituents at the ortho position
and diisocyanide monomers. Therefore, this polymerization system is
versatile in the preparation of well-defined polyisocyanides with
controlled sequence. Bi- and trifunctional Pd complexes with two and
three Pd units incorporated onto the same phenyl ring were designed
and synthesized. They were also able to initiate the living polymerization
of phenyl isocyanide to afford telechelic linear and star-shaped polyisocyanides
with controlled molecular weights and narrow molecular weight distributions.