Cationic chain polymerization inhibition

It Is well known that cationic polymerization may be Inhibited by nucleophilic species In the reaction mixture however, vinyl ethers appear to be less sensitive than epoxies to presence of nucleophiles. The high reactivity of the vinyl ether group with the growing cationic chain Is often favored over termination by a nucleophilic species. Water may act as a nucleophile and It has been reported that high atmospheric humidity conditions will Inhibit the photocuring of epoxles(24). We have observed a similar effect on vinyl ether based coatings. 

Ionic Polymerization Ionic chain polymerizations can also be initiated with the aid of Vis/UV light although, as in the case of free-radical photopolymerization, the light serves only as an initiating tool. Most studies on ionic photopolymerization have focused on cationic polymerization, which proceeds rapidly and is not inhibited by oxygen [18,21-24]. Moreover, cationic photopolymerization is apt to polymerize monomers such as vinyl ethers, oxiranes (epoxides), and other heterocyclic compounds that do not polymerize via a free-radical mechanism (Table 3.9). 

Solomon (3, h, 5.) reported that various clays inhibited or retarded free radical reactions such as thermal and peroxide-initiated polymerization of methyl methacrylate and styrene, peroxide-initiated styrene-unsaturated polyester copolymerization, as well as sulfur vulcanization of styrene-butadiene copolymer rubber. The proposed mechanism for inhibition involved deactivation of free radicals by a one-electron transfer to octahedral aluminum sites on the clay, resulting in a conversion of the free radical, i.e. catalyst radical or chain radical, to a cation which is inactive in these radical initiated and/or propagated reactions. 

Inhibitors of proteases have also been developed from polymers. These molecules are as simple as a polymer chain, or much more complex. The mechanisms for protease inhibition are variable depending upon the type of molecule used specific protease inhibitors are available for conjugation to a polymer while other polymeric inhibitors inhibit all divalent cation dependent proteases. The development of these inhibitory polymers and polymer conjugates greatly increase the possibility to protect proteins from degradation in the gastrointestinal tract. 

The effect of water is to be explained. Cationic polymerization of VCZ is generally not inhibited by water. The monomer is very basic and can well compete for the carbonium ion with water. Since the polymerization is readily initiated by a proton, water acts as chain transfer agent rather than inhibitor. Although the reactivity of the carbonium ion depends certainly on the nature of the counter-ion, as will be discussed later, water seems to act as an efficient chain transfer agent, at least in the present system. The Ion-radical might consequently be converted to a proton so that the cationic propagation could even be promoted in the presence of water. 

The results show that the presence of bulky substituent on a polymer chain may effectively inhibit the termination proceeding by this mechanism. The results presented at this point may be summarized as follows chain transfer to polymer is a general feature of cationic ring-opening polymerization although for different systems the contribution of this reaction may vary only in some systems this process results in termination (These systems involve, e.g., cyclic amines (3- and 4-membered) and cyclic sulfides (3- and 4-membered) and the contribution of the reaction is reduced for substituted chains. 

The main advantages of cationic photoinitiators is that they have high reaction rates and require a low energy. They can operate at a low temperature, they are not inhibited by oxygen, they do not promote the polymerization of epoxy groups in the dark, and they are often stable at elevated temperatures. Some disadvantages exist that is, inhibition by bases, chain-transfer reaction by water, and the presence of acids in cured products. 

With formic and carbonic acid as potential chain transfer agents, termination would involve the proton of the undissociated acid, but HCOO" and HOCOO were very poor nucleophiles and did not reinitiate the polymerization. This amounted to an overall inhibition or retardation of the formaldehyde polymerization. 2 mole % of COj inhibited formaldehyde polymerization completely [29], as did other inorganic or organic acids which gave poorly nucleophilic anions. Higher organic acid concentration may cause cationic polymerization. 

The nature of the active sites is open to discussion. Toby et al. chose to follow a possible earlier suggestion and used the addition of formaldehyde to a neutral polymer chain as the propagation mechanism. Because of a slight inhibition of the polymerization by oxygen, a radical mechanism was not completely ruled out. Looking at the formaldehyde polymerization as a whole and accepting some of the observations of Toby et al., it must be concluded that their formaldehyde polymerization was a cationic polymerization. Active centres, or active sites were actually oxonium... 

The polymerization of MAH does not occur under normal conditions but is readily initiated under gamma or ultraviolet radiation and by the use of radical catalysts at high concentrations or having a short half life at the reaction temperature. The radical initiated homopolymerization is promoted by the presence of photosensitizers in the absence of light 2, 2 ). It has been proposed that under these conditions MAH undergoes excitation and the excited monomer, actually an excited dimer or charge transfer complex, polymerizes. The participation of the excimer or excited complex and the cationic character of the propagating chain has been confirmed by the total inhibition of MAH polymerization in the presence of small amounts of dimethyIformamide which has no effect on the polymerization of "reactive acrylic monomers ( ). 

Here we can draw an analogy with the equilibrium dissociation reaction, when the association rate constant in equilibrium is not limited by diffusion, regardless of the viscosity of the medium. In our opinion, this question requires at present a theoretical and experimental investigation. It is customary to assume that radical polymerization is characterized by a rather intensive chain termination reaction and a short time for the propagation of one chain, as compared to the time of polymerization. The existence of continuous processes ( living polymers) has been ascertained for anionic9 and cationic polymerization10, where there is no bimolecular interaction of active centers with one another. Let us now examine certain radical polymerization processes in which the chain termination reactions are considerably inhibited or almost excluded. 

An important characteristic of ionic polymerization is that the propagation rate coefficients are several orders of magnitude higher than for free-radical polymerization. In the equation fct[X] is the bimolecular termination rate coefficient multiplied by the impurity concentration. This equation shows that the rate of polymerization is proportional to the first power of initiation rate, i.e., to the first power of dose rate. Water is a common chain breaker of cationic polymerization since it replaces the cation by a hydroxonium ion. As a proton donor it also inhibits the anionic polymerization... 

The reaction mechanism of oxidative polymerization of aniline has been a big controversy (Fig. 7). The dimerization step is generally proposed as (i), in which aniline is one-electron-oxidized to a cation radical, followed by coupling of two molecules of the cation radical to a dimer. The subsequent steps of chain extension are under discussion routes involving coupling of cation radicals such as (ii)-(iv) (137-140) and routes via electrophilic attack of a two-electron-oxidized quinodal diiminium ion (v) or nitrenium ion (vi) (141,142) have been proposed. The addition of electron-rich arenes does not inhibit the polymerization, and therefore the route through the nitrenium ion (vi) seems to be rejected (137). 

The aryl radicals produced from the photolysis of the diaryliodonium salt abstract a hydrogen atom from THF producing the aryl hydrocarbon and the THF radical. The THF radical is further oxidized by the diaryliodonium salt, resulting in the formation of a stabilized THF cation which may initiate cationic polymerization. Simultaneously, the aryl radical which is the principal chain carrier is regenerated. Because such free radical chain reactions are inhibited by oxygen, this process is most efficiently carried out in the absence of air. 

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