Mechanistic insights into polar monomer insertion polymerization from acrylamides

Area of Science:

• Organometallic chemistry
• Polymer chemistry
• Catalysis

Background:

• Palladium complexes are effective catalysts for olefin polymerization and copolymerization.
• Acrylamide monomers present unique challenges in polymerization due to their polar amide groups.
• Understanding monomer insertion mechanisms and catalyst inhibition is crucial for controlling polymer properties.

Purpose of the Study:

• To investigate the copolymerization of N-Isopropyl acrylamide (NIPAM), N,N-dimethyl acrylamide (DMAA), and 2-acetamidoethyl acrylate (AcAMEA) with ethylene using a palladium catalyst.
• To elucidate the role of polar additives in inhibiting the polymerization process.
• To determine the insertion regioselectivity and kinetics of different acrylamide monomers into the palladium-methyl bond.

Main Methods:

• Copolymerization reactions employing a specific palladium catalyst precursor, [(P^O)PdMe(DMSO)].
• Inhibition studies using various nonpolymerizable polar additives to quantify retardation effects.
• Kinetic analysis by determining pseudo-first-order rate constants for monomer insertion.
• Structural characterization of reaction products using electrospray ionization mass spectrometry (ESI-MS) and single crystal X-ray diffraction.

Main Results:

• Reversible kappa-O-coordination of free amide groups to the palladium center significantly retards polymerization.
• Polymerization inhibition increases with the polarity of the additive, with DMSO causing the most significant retardation.
• Monomer insertion rates follow the order DMAA < AcAMEA < NIPAM < methyl acrylate.
• Consecutive insertion products of NIPAM and both 2,1- and 1,2-insertion products of DMAA were identified and structurally characterized.

Conclusions:

• The palladium-catalyzed copolymerization of acrylamides with ethylene is sensitive to polar functional groups, leading to catalyst inhibition.
• The coordination of amide groups to the palladium center plays a critical role in modulating polymerization kinetics.
• Understanding these interactions allows for better control over polymerization processes and the design of novel polymer architectures.