Respondent
Theme
Mechanisms of N-hydroxyphthalimide effect in radical chain processes of polymerization and oxidation
Defence Date
Annotation
The thesis is devoted to the study of mechanisms of N-hydroxyphthalimide
(NHPI) action in a radical chain processes – polymerization and oxidation. It was
shown that N-hydroxyphthalimide works as initiator in aerobic and anaerobic
polymerizations of vinyl monomers and as catalyst in the processes of oxidation with
molecular oxygen of different classes of organic compounds with the saturated C-H
bonds.
It was studied the EPR and UV / vis-spectra phthalimide-N-oxyl radicals (PINO)
– the key reactivе entity in the radical processes involving NHPI, formed in situ by
oxidation of NHPI or hydrogen abstraction from the O-H bond of NHPI. The kinetics
of the spontaneous decay of PINO radicals in the different solvents has been studied.
It was established that N-hydroxyphthalimide is an efficient initiator of the
radical polymerization of vinyl monomers. The initiation step in the presence of NHPI
proceeds through formation of a complex between NHPI and the vinyl compound due
to hydrogen bonding. The decomposition of this complex leads to the PINO radical
capable of initiating a polymerization chain.
The rate constants of the addition of the PINO radical to π-bonds of vinyl
compounds have been measured. Monomers with donor substituents react considerably
more rapidly with PINO than monomers with acceptor substituents. This fact indicates
the electrophilic character of the PINO radical in the addition reaction to the double
bond of vinyl compounds. In going from styrene to acrylonitrile the value of kadd
decreases very sharply. The reactivity of monomers in the elementary step of addition
of the PINO radical decreases in the order СН2 =С(СН3)С6Н5 > СН2 =СНС6Н5 >
СН2 =С(СН3)СООСН3 > СН2 =СНСОNH2 > СН2 =СНСООСН3 > СН2 =СНСООН >
СН2 =СНCN.
Polymethyl methacrylate obtained in the presence of NHPI has a high content of
syndiotactic sequences. This finding suggests that, under conditions of the vinyl
monomers polymerization, NHPI may have a dual role in the reactions of initiation and
propagation for stereochemical control of the tacticity of polymer chain. The action of
NHPI as stereoregulator is attributed to the capacity of this compound to form a
hydrogen bond with growing macroradicals. The best regulation of the propagation
step is achieved using N-hydroxyphthalimides with substituents in the benzene ring
having hydroxyl groups.
The oxidative polymerization of vinyl monomers (α-methylstyrene, styrene,
methyl methacrylate, acrylonitrile) in the presence of NHPI was studied. It is shown
that oxidation of vinyl compounds under mild conditions (1 atm, ˂ 100 ° C) to form
the corresponding polyperoxides, whose structure was studied by 1H NMR and IR
spectroscopy, thermal stability – by method of thermogravimetry. Аnalysis of
reactivity of the monomers in the oxidative polymerization processes with respect to
radicals of various structures was conducted.
The acenaphthene oxidation with molecular oxygen in the presence of NHPI has
been investigated. It is shown that the main oxidation product is acenaphthene
hydroperoxide. The rate constant of H-abstraction (k H ) from acenaphthene by PINO
has been determined. The kinetic isotope effect and the activation parameters have also
been measured. On the basis of the results of our studies a plausible mechanism for the
oxidation process of acenaphthene with dioxygen catalyzed by NHPI was proposed.
The synergetic effect of the catalytic system cobalt (II) acetate –
N-hydroxyphthalimide (NHPI) was studied during the oxidation of alkylbenzenes
(cumene, ethylbenzene, toluene, p-xylene). It was established that
N-hydroxyphthalimide is included in the internal coordination sphere of the metal; the
reaction of NHPI with cobalt ions in the internal sphere leads to the formation of
phthalimide-N-oxyl radicals that than participate in the catalytic cycle of the oxidation
of organic substrates. The stability constants of complexes of Со(ІІ)-NHPI and Со(ІІ)-
RH were determined.
The kinetics of the reactions of hydrogen atom abstraction from the C–H bonds
of substrates of different structures by phthalimide-N-oxyl radicals is studied, the rate
constants of this reaction are measured. It is shown that in addition to the
thermodynamic factor Coulomb forces and donor–acceptor interactions affect the
reaction between phthalimide-N-oxyl radicals and substrate molecules, altering the
shape of the transition state. This favors the tunneling of hydrogen atoms and leads to
a substantial reduction in the activation energy of the process. It is shown that the lack
of internal rotation reactionary alkyl group in molecules of acenaphthene, fluorene and
tetralin increases of reagents entropy. It causes their high reactivity in the reactions of
hydrogen abstraction.
The influence of salts of manganese (II) on the efficiency of the catalyst system
NHPI / Co(OAc)2 in the oxidation of p-xylene was studied. Ions Mn (II) show high
reactivity with respect to peroxy radicals and determine rate and selectivity of the
oxidation of p-xylene to terephthalic acid.
The effect of N-hydroxyphthalimide structure on the efficiency of catalysis
system NHPI / Cu(I) in the oxidation of cumene was studied. It is shown that the
introduction of four phenyl substituents in the benzene ring of NHPI leads to a sharp
increase of the rate of oxidation. It was also revealed a positive effect of bromide anion
and a non-polar solvent chlorobenzene in the cumene oxidation process.
It is shown that the cause of the high selectivity of oxidation processes involving
NHPI is a sharp increase in the chain length. Phthalimide-N-radical in oxidative
processes working as an mediator and can efficiently transfer a hydrogen atom from
the substrate to the radical chain carrying.
Keywords: N-hydroxyphthalimide, initiation, catalysis, radical chain processes,
polymerization, vinyl monomers, oxidation, molecular oxygen, alkylarenes, kinetics
mechanism.