| dc.description.abstract | A general kinetic scheme for the polymerization of vinyl monomers, initiated by dipivaloyl methane chelates of ferric and cupric, was postulated in Chapter III. The results of the spectral investigation complementary to the polymerization results now allow us to probe deeper into the actual mechanism involved in the different stages.
The effects of hydroquinone and diphenyl picryl hydrazyl in the present polymerization systems clearly indicate a free radical mechanism. The value of ? at different temperatures for styrene and methyl methacrylate agree fairly well with those obtained with conventional free radical initiators. Further, these values and the square root dependency of the rate of polymerization on the chelate concentration strongly favour a normal propagation reaction and bimolecular termination reaction.
Even though direct thermal decomposition was found to be possible for ferric acetylacetonate, the dipivaloyl methane chelates were found to be rather inert. Thermal decomposition studies carried out in this laboratory for ferric and cupric dipivaloyl methides also lend support to this—the thermal stability of ferric and copper chelates are very stable towards decomposition and suggested that resonance effect involving copper plays a very important role in determining the stability of the chelates.
The selectivity exhibited by the chelates was the main factor which demanded the involvement of the monomer in the initiation step. The spectral results too are consistent with the formation of a complex between the chelate and the monomer, the monomer acting as the donor and the chelate as the acceptor. The nature of the complex and its possible structure have been discussed in Chapter IV. In this Chapter we would make an attempt to find out the most probable route through which the complex can initiate free radical polymerization.
One route for the radical production is as suggested by Lingamurthy and Palit for ferric laurate. Ferric laurate was found to initiate the polymerization of methyl methacrylate. The observation that ferric laurate initiated polymerization is inhibited by hydroquinone or benzoquinone indicated a free radical mechanism. These investigators rule out the possibility of redox polymerization, for no reduction of ferric to ferrous could be detected either during the polymerization or at the end.
A mechanism, very similar to that of the conventional free radical type is proposed. Iron in the ferric laurate has an electronic configuration 3d?. That means, it can accept electrons since all the 5d orbitals are singly occupied and according to Hund’s Rule pairing up is possible. So a vinyl monomer with labile ? electrons can act as the donor and they have suggested the following mechanism:
Initiation:
Fe + C=C ? Fe + C•
Propagation:
Fe–C• + C=C ? Fe–C–C•
(Termination by combination is suggested in view of the abnormally high molecular weight obtained. And the authors state further: “In spite of the fact that we are in favour of a free radical mechanism, our experimental results do not exclude the possibility of a Zeigler type mechanism, in which the laurate radical moves to the end of the chain during the propagation step...”).
But a mechanism of the above type seems to be improbable in either ferric dipivaloyl methide initiated or cupric dipivaloyl methide initiated systems. The main counter argument in the case of ferric dipivaloyl methide will be the steric hindrance offered by the bulky tertiary butyl groups for such a coordination to remain intact throughout the propagation reaction. The second and more important fact is the detection of ferrous and cuprous in the respective systems which favour redox initiation.
The above arguments quite clearly refute the possibility of a Zeigler type catalysis which Lamomote and colleagues have proposed for ferric alkyl dipyridyls. Because a mechanism of the following nature invariably puts down the condition that the resultant polymer is most likely to have some kind of stereo specificity.
X-ray diffraction pattern of polymethyl methacrylate polymerized in presence of ferric dipivaloyl was found to be identical with the X-ray diffraction pattern obtained for polymethyl methacrylate polymerized with azo-bis-isobutyronitrile or even unsensitized thermally polymerized samples of polymethyl methacrylate. These evidences are strong enough to decide in favour of discarding a Zeigler type mechanism.
A mechanism proposed by Bamford for manganese(III) trifluoroacetylacetonate initiated polymerization is also possible.
The observations by a group of Japanese workers that copper chelates, especially bis 3-substituted acetylacetonates, are stable in benzene when heated in the absence of the monomer, but decomposition of the chelate and simultaneous polymerization when heated in the presence of styrene, are quite relevant in this respect. They also observed a monomer order greater than unity. An alternate mechanism considering the coordination of the monomer to the chelate, or more specifically, the reaction of the monomer with a thermally excited state of the chelate has been postulated.
They point out that M need not necessarily be a vinyl monomer; organic halogen compounds or pyridine which would alter the total electronic distribution in the chelate will facilitate radical production.
There are ample examples when such a mechanism is assumed to be operating, for instance in the ferric nitrate initiated polymerization of methyl methacrylate, the following step is accepted:
Fe(NO?)? + M ? Fe(NO?)?–M ? Fe(II) species
If we suppose the production of radicals in ferric and cupric initiated polymerization systems in a similar way, then the most feasible route will be:
Me(DPM)n + M ? (DPM)n.Me* ? (DPM)n-1.Me + R•
From spectral data, we assume the structure:
R = –C–CH?
Considering the decomposition of this species is said to be that in which one Ag? ion is coordinated at the vinylic side chain and the other at the aromatic nucleus. Accordingly, the vinylic bond in styrene is quite basic. The presence of the neighbouring methyl group actually will be increasing the electronic density of the double bond in methyl methacrylate, similar to the acidity of aliphatic acids because of their electron-withdrawing capacity. Hence, in methyl acrylate too, the double bond will have enough electron density to function as a donor towards the ferric chelate.
From spectral data, we find that in the case of ferric dipivaloyl methide, only those monomers which are capable of bringing about spectral shifts are polymerized by ferric dipivaloyl methide. That is, spectral shifts are observed only for styrene, methyl methacrylate, and methyl acrylate, and these monomers are polymerizable with the chelate as initiator.
The powerful electrophilic group –C?N will cause the downflow of ? electrons into itself, thus decreasing the density of the electron cloud about the double bond in acrylonitrile. And, quite possibly, that is the reason why acrylonitrile does not induce a shift in the spectrum of ferric dipivaloyl methide and why it is not polymerizable.
Coming to cupric dipivaloyl methide, the arguments put forward for ferric dipivaloyl methide with respect to styrene, methyl methacrylate, and methyl acrylate hold good here also. The higher polymerization rate with cupric dipivaloyl methide and its capacity to polymerize acrylonitrile can be explained under one heading, and that is the peculiarity of ligand-metal bonding in cupric ?-diketonates.
The chelate resonates between two possible structures and, as discussed in Chapter IV, the bonding is ionic. So a slight push from the donor molecule is sufficient to knock the ligand out of plane and establish a complexation. Another reason will be the coulombic attraction of the electrophilic carbon of the vinyl group towards the nucleophilic oxygen. This electrostatic force brings the chelate and the monomer molecule to close proximity, and hence complexation and subsequent cleavage can occur in two ways:
1. Homolytic cleavage of the bond between the vinylic carbon and copper.
2. Rearrangement of the ligand to enhance complexation through the methylene carbon.
Thus, the free radical nature of initiation, the selectivity of the metal chelate, and the reduction of the metal ion are accounted for by this mechanism, which successfully explains the salient features of the polymerization reaction initiated by ferric dipivaloyl methide and cupric dipivaloyl methide. | |
