57 research outputs found
Mechanistic Insights on Acrylate Insertion Polymerization
Complexes [{(P∧O)PdMe}n] (1n; P∧O ) κ2-P,O-Ar2PC6H4SO2O with Ar ) 2-MeOC6H4) are a
single-component precursor of the (P∧O)PdMe fragment devoid of additional coordinating ligands, which
also promotes the catalytic oligomerization of acrylates. Exposure of 1n to methyl acrylate afforded the two
diastereomeric chelate complexes [(P∧O)Pd{κ2-C,O-CH(C(O)OMe)CH2CH(C(O)OMe)CH2CH3}] (3-meso
and 3-rac) resulting from two consecutive 2,1-insertions of methyl acrylate into the Pd-Me bond with the
same or opposite stereochemistry, respectively, in a 3:2 ratio as demonstrated by comprehensive NMR
spectroscopic studies and single crystal X-ray diffraction. These six-membered chelate complexes are direct
key models for intermediates of acrylate insertion polymerization, and also ethylene-acrylate copolymerization
to high acrylate content copolymers. Studies of the binding of various substrates (pyridine, dmso, ethylene
and methyl acrylate) to 3-meso and 3-rac show that hindered displacement of the chelating carbonyl moiety
by π-coordination of incoming monomer significantly retards, but does not prohibit, polymerization. For
3-meso,3-rac + C2H4 a 3-meso-C2H4, 3-rac-C2H4 an equilibrium constant K(353 K) ≈ 2 × 10-3 L mol-1
was estimated. Reaction of 3-meso, 3-rac with methyl acrylate afforded higher insertion products
[(P∧O)Pd(C4H6O2)nMe] (n ) 3, 4) as observed by electrospray ionization mass spectrometry (ESI-MS).
Theoretical studies by DFT methods of consecutive acrylate insertion provide relative energies of
intermediates and transition states, which are consistent with the aforementioned experimental observations,
and give detailed insights to the pathways of multiple consecutive acrylate insertions. Acrylate insertion
into 3-meso,3-rac is associated with an overall energy barrier of ca. 100 kJ mol-1
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