A Nonlocal Density Functional Study of the Pd(II)-Assisted Copolymerization of Ethylene and CO

We study the homogeneous catalytic copolymerization of olefin and
carbon monoxide.− The catalytic
center is modeled by Pd(II) coordinated to
PH2CHCHPH2.
C2H4 is used as a model for the olefin.
We investigate
the chain propagation mechanism for alternating copolymerization as
well as the side reactions resulting from multiple
insertion of olefin and CO, respectively. We find that strictly
alternating copolymerization is kinetically favored
over homopolymerization of olefin and thermodynamically as well as
kinetically favored over successive multiple
insertions of CO. Insertion of one
C2H4−CO unit into the Pd−ethyl bond yields
−219 kJ/mol, whereas insertion of
a C2H4−C2H4 segment
yields exactly −200 kJ/mol. Insertion of a CO−CO segment
yields only −88 kJ/mol.
Therefore, multiple successive CO insertions are by comparison so
unfavorable as to be ruled out completely. We
explain the experimentally observed preference of strictly alternating
copolymerization over multiple olefin insertions
by the higher adduct formation energy of CO (−78 kJ/mol) as opposed
to only −51 kJ/mol for C2H4.
Furthermore,
the activation barriers for the insertion of a
CO/C2H4 unit into the chain are only 48 and 58
kJ/mol, respectively,
whereas the barrier for C2H4 insertion is 65
kJ/mol. All acyl species encountered are only weakly stabilized
by
agostic interactions, whereas Pd-alkyl species are strongly stabilized
by agostic interactions. The acyl species can
stabilize itself by −31 kJ/mol over the most favorable agostic
conformation by adopting a η2-carbonyl
conformation.
The growing polyketone chain is strongly stabilized by forming
chelate bonds between the carbonyl oxygens and
Pd. The strictly alternating copolymerization pattern originates
from a combination of effects:  On the one hand, the
number of CO units incorporated into the chain is maximized, because CO
(as the better π acceptor) stabilizes the
reactive center more than ethylene during the adduct formation
proceeding insertion and CO also faces a lower
barrier during insertion into the chain. On the other hand,
subsequent multiple insertions of CO are avoided since
they are kinetically as well as thermodynamically highly
unfavorable.