Initiator-Monomer Complex Formation

There are many reports on polymerization of different monomers showing different degrees of kinetic complexity where the observed effects are well explained and understood on the basis of equilibrium complex formation between the initiator used and the monomer (Walling and Heaton, 1965 Ghosh and Billmeyer, 1969 Ghosh andBanerjee, 1974). 

Problem 6,26 Formation of complex (C) between initiator (I) and monomer (M) leads to nonideal kinetic behavior in many cases. A sin iified scheme of chain initiation based on the concept of equilibrium complex formation between initiator and monomer may be given as 

The rate of initiation is given by Ri = 2 /[C]. Inserting Eq. (P6.26.2) in this equation and then combining with Eq. (6.23) gives 

Equation (P6.26.3) describes a change in reaction order with respect to monomer from 1.5 to 1 with increasing [M]- Equation (P6.26.3) may be further transformed to  

According to Eq. (P6.26.4), a plot of [M] /R vs. [M] should give a straight line such that the quotient of the slope and the intercept is equal to K. 

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The results of the static and dynamic DPT calculations for the methyl acrylate copolymerization suggest that there are two factors inhibiting the polar co-polymerization in the Ni-catalyst case (1) the initial 0-complex formation (2) a difficult chelate opening prior to insertion of the next monomer. Both of those factors may be overcome by the use of the complexes with reduced oxophihcity of the metal on the catalyst. 

The IR data suggest an anomalous decrease of the D157S/D 610 value with changing composition of the monomer mixture at constant total concentration of initial monomers. This concirms that the formation of a coordination complex between the carbonyl group and the tin atom at an equimolar TBSM-to-MA ratio is highly probable. 

Having commented briefly on the first two parts of my new theory (the self-ionisation and the initiation by AlX+2), it is appropriate to consider the complex formation between monomer and metal halide, expressed by Equation (ii), which we have mentioned in the previous section. This complex formation actually provides an easy and plausible explanation for some of the hitherto rather obscure phenomenological differences which are observable when monomer and aluminium halide solutions are brought together in different ways we can distinguish three such techniques which give very different results. 

By this complex formation with monomer the A1X3 is effectively deactivated. It is important to realise that these experiments not only provide convincing evidence for the initiation by ions derived from the initiator, but also an almost insuperable obstacle to Kennedy s allylic hydride theory [9] and any other theory which involves a reaction of monomer with A1X3 molecules as the initiation step. 

The complex formation with monomer also provides a simple and logical explanation for the decrease of the rate-constant for the attack of a cation, R+, such as a trityl ion from an initiator, on a monomer, with increasing m this can be shown thus ... 

Complex formation takes place in an organic solvent or in a water/monomer mixture by reaction of the macroligand with a metal compound (e.g. a Cu(I)-ha-lide). It is supposed that the conditions in the reaction mixture are comparable to those in conventional emulsion polymerization, where monomer droplets stabilized by surfactant molecules coexist with monomer swollen micelles [64]. Reaction sites are presumably the hydrophobic core of the micelles and the monomer droplets as well. Initial results of the micellar-catalyzed ATRP of methyl methacry-... 

Photopolymerization of acrylamide by the uranyl ion is said to be induced by electron transfer or energy transfer of the excited uranyl ion with the monomer (37, 38). Uranyl nitrate can photosensitize the polymerization of /S-propiolactone (39) which is polymerized by cationic or anionic mechanism but not by radical. The initiation mechanism is probably electron transfer from /S-propiolactone to the uranyl ion, producing a cation radical which propagates as a cation. Complex formation of uranyl nitrate with the monomer was confirmed by electronic spectroscopy. Polymerization of /J-propiolactone is also photosensitized by sodium chloroaurate (30). Similar to photosensitization by uranyl nitrate, an election transfer process leading to cationic propagation has been suggested. 

Photooxidation of Eosin with periodate ion has been used to initiate the polymerization of acrylonitrile in aqueous solution [187]. Addition of acrylonitrile to a periodate solution shifts the absorption maximum from 220 to 280 nm. This spectral change is interpreted as being due to complex formation between the monomer and oxidizing agent. The rate of photopolymerization increases linearly with the absorbed light intensity and monomer concentration. The observed intensity dependence indicates the main chain terminator is not produced photochemically. Polymer is not formed when the concentration of periodate ion is lower than 0.5 mM and the rate of polymerization is independent of its concentration for higher values. 

A number of other polar monomers have been polymerized with butyllithium, nominally in hydrocarbon or aromatic solvents. In almost all cases the monomer concentration was so high that the effective dielectric constant was much greater than in a pure hydrocarbon. All show rather complex behaviour. The degree of polymerization of the polymer formed is always much higher than the initial monomer-catalyst ratio so that a simple scheme involving only initiation and propagation reactions is not applicable. Only precipitable polymer was isolated, so it is not sure if the low initiator efficiencies are due to low polymer formation or to side reactions of butyllithium with the monomer. In addition most systems studied stop before complete conversion of the monomer. Evidently the small fraction of active polymer chains formed... 

Heterocyclic monomers containing both endocyclic and exocyclic heteroatoms such as cyclic esters (lactones, lactide, carbonates) and cyclic anhydrides undergo coordination polymerisation or copolymerisation involving complex formation between the metal atom and the exocyclic heteroatom [100,124]. Polymerisation of /1-lactones is representative of such coordination polymerisations with catalysts containing an Mt-X active bond the initiation and propagation steps are as follows ... 

Mechanism. The ability of S02 to participate in complex formation with unsaturated hydrocarbons and a host of other compounds such as amines, ethers, phenols, and aromatic hydrocarbons is known (2, 6, 11, 22). SO has been found to initiate polymerization of some monomers... 

While considering the rate-enhancing effect of bromobenzene in MMA polymerization initiated by AIBN, Henrici-Olive and Olive (19) noted that the effect can be explained as the consequence of electron donor—acceptor complex formation between polymer radicals and monomer or solvent molecules. Based on this view, these authors have shown that in polymerization in active solvents (which enhance the rate), the degree of polymerization Pn appears as a linear function of M2/Rp with... 

Acrylamide, 2-methyl-5-vinylpyridine and /V-vinylpyrrolidone can be polymerized under similar conditions, and also after decomposition of a monomer-peroxide complex. On the other hand, styrene, methyl methacrylate, isoprene, methyl acrylate, vinyl acetate and ascorbic acid do not polymerize under these conditions. Complex formation between persulphate and these monomer donors is more favourable energetically [165]. The complex is more stable, it is not decomposed into initiating radicals and polymerization does not occur. 

Monomer complexes play an important role even in non-radical processes. In coordination polymerizations, the interactions of monomers with catalysts are evidently of greatest importance without them this type of addition would not be possible. The formation of unstable complexes between the electrophilic initiator and nucleophilic monomer is also necessary in cationic polymerizations. The idea that under certain conditions the formation of stable complexes between initiator and monomer may prevent polymerization [171] is now frequently accepted [172-174]. 

An electron acceptor such as maleic anhydride forms complexes with many donors, amongst which is vinyl acetate [80, 189, 190], Its existence at 363 K was proved by UV spectroscopy, ll NMR and by the formation of an alternating copolymer [80]. The complex is not formed above 363 K. From the two monomers, a statistical copolymer is formed, its composition depending on the ratio of initial monomer concentrations. 

Not many initiators belong to this class even though the halogenoacetic, fluorosulphonic, and other acids are included. A detailed analysis of their polymerization mechanism is obscured by complex formation with monomer and with solvent, by the occurrence of aggregates, and by anion reactions in acids with an unstable anion. In spite of its apparent simplicity, initiation by Bronsted acids has not yet been investigated in detail. The pseudo-cationic polymerization of styrene is an instructive example. 

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