Polymerization, CAN-initiated

Organic peroxides are used extensively for the curing of unsaturated polyester resins and the polymerization of monomers having vinyl unsaturation. The —O—O— bond is split into free radicals which can initiate polymerization or cross-linking of various monomers or polymers. 

Block copolymers are synthesized by a variety of methods (45,46) most important are sequential polymeriza tion and step growth. In sequential polymerization, a polymer (A) is first synthesized in such a way that it contains at least one group per molecule that can initiate polymerization of another monomer B. 

The proposed polymerization mechanism is shown in Scheme 9.12. Thermal decomposition of the hexasubstituted ethane derivative yields hindered tertiary radicals that can initiate polymerization or combine with propagating species (primary radical termination) to form an oligomeric macroinitiator. The addition of the diphenylalkyl radicals to monomer is slow (e.g. k[ for 34 is reported as KT M"1 s l at 80 °C84) and the polymerization is characterized by an inhibition period during which the initiator is consumed and an oligomeric macroinitiator is formed. The bond to the Cl I formed by addition to monomer is comparatively thermally stable. 

Anionic polymerization of 1,3-disilacyclobutanes also is possible. Solid KOH and alkali metal silanolates were mentioned as being effective by Russian authors [18, 19. 20]. However, alkyllithiums, which can initiate polymerization of silacyclobutanes (eq. 8) [21], do not initiate polymerization of 1,3-disilacyclobutanes [18, 22]. The problem is one of steric hindrance. 

To develop thermal iniferters which act at a lower temperature, Otsu and Tazaki proposed a redox iniferter system [153,154]. For example, reduced nickel (Ni°) reacts with organic halides (R-X) such as benzyl chloride to form a radical, which can initiate polymerization (Eqs. 33-35) ... 

DSC can be used effectively in the isothermal mode as well. In this case, the container with the sample is inserted into the DSC preheated to the desired test temperature. This type of experiment should be performed to examine systems for induction periods that occur with autocatalytic reactions and with inhibitor depletion reactions. (Reactions with induction periods can give misleading results in the DSC operated with increasing temperature scans.) Autocatalytic reactions are those whose rates are proportional to the concentration of one or more of the reaction products. Some hydroperoxides and peroxy esters exhibit autocatalytic decomposition. Inhibitor depletion can be a serious problem with certain vinyl monomers, such as styrene and acrylic acid, that can initiate polymerization at ambient temperatures and then selfheat into runaways. Isothermal DSC tests can be used to determine a time to runaway that is related to the inhibitor concentration. 

The hydroxyl radicals may abstract hydrogen which gives a radical on the substrate (P-) that can initiate polymerization by adding a vinyl monomer (M) ... 

Oxidative Polymerization Reactions. Clays can initiate polymerization of unsaturated compounds through free radical mechanisms. A free radical R", which may be formed by loss of a proton and electron transfer from the organic compound to the Lewis acid site of the clay or, alternatively, a free radical cation, R+, which may be formed by electron transfer of an electron from the organic compound to the Lewis acid site of the clay, can attack a double bond or an aromatic ring in the same manner as an electrophile. The intermediate formed is relatively stable because of resonance, but can react with another aromatic ring to form a larger, but chemically very similar, species. Repetition of the process can produce oligomers (dimers, trimers) and, eventually, polymers. 

Fig. 3. Schematic description of self-condensing vinyl polymerization used for the synthesis of of hyperbranched polymers based on vinyl monomers as presented by Frechet [52] -(represents a reactive site which can initiate polymerization)...

The use of a cationogen such as an alkyl or acyl halide with a Lewis acid generates car-bocations and acylium ions, either in situ or as isolable salts, which can initiate polymerization, for example... 

Now the question arises whether the TA can initiate polymerization of epoxy compounds in the absence of any proton-donor compounds. There is no clear solution to this problem. Some investigators 136,, 63) believe that the TA itself is capable of opening the epoxy ring to give a zwitter-ion ... 

Ultraviolet light - A form of energy occupying a position in the spectrum of sunlight beyond the violet, and having wavelengths of less than four micrometers which is the limit of visible light. UV (ultraviolet) rays are very active chemically, exhibit bactericidal action, and cause many substances to fluoresce. UV rays accelerate deterioration of rubber parts exposed to them and can initiate polymerization. 

The problems inherent to these two processes are not only the production of corrosive salts, but also the possibility of product contamination by 2-chloroethylamine [689-98-5], as starting material or intermediate. This substance can initiate polymerization of ethyleneimine with the elimination of HC1. 

Ionic initiators are much more specific toward the type of monomer and can initiate polymerization in some cases where radical initiation is ineffective (cyclic, vinyl ether, or allylic monomers). Monomers suitable for anionic initiation are those with an electron-withdrawing substituent attached to the double bond (phenyl, carbonyl, etc.). The reaction consists... 

The large difference in reactivity ratio between L-LA and DXO makes it difficult to synthesize a tri-block copolymer, but the task can be carried out by using a cyclic tin alkoxide as initiator. The DXO macrocycle can initiate polymerization of l-LA and by ring expansion polymerization the two side blocks of l-LA are formed simultaneously. Scheme 7 shows the reaction pathway for the synthesis of tri-block copolymers from l-LA and DXO. 

Vinyl chloride is generally transported via pipeline, and in railroad tank cars and tanker ships. Containers of vinyl chloride must be labeled "vinyl chloride," "extremely flammable gas under pressure," and "cancer-suspect agent" (146). Because hazardous peroxides can form on standing in air, especially in the presence of iron impurities, vinyl chloride should be handled and transported under an inert atmosphere. The presence of peroxide from vinyl chloride and air can initiate polymerization of stored vinyl chloride however, stabilizer can be added to prevent polymerization. Inhibitors such as hydroquinone [123-31-9] are often added, particularly when shipping long distances in warmer climates. 

It is evident that the tendency for free radical polymerization is a function of the monomer. Natta (287) stated that these radicals can initiate polymerization of styrene and diolefins to high molecular weight products but not that of aliphatic alpha olefins. North (339) showed that the decomposition of phenyl tri-isopropoxy titanium initiates radical polymerization of styrene but not of ethylene. 

Both radicals, [R—N=N] and R, can initiate polymerization [47]. In the system Cu2-hydrazine hydrate, the reaction... 

Not all the radicals generated by the dissociation of the initiator can initiate polymerization. The effectiveness of initiation,/, is defined by the ratio of the initiation rates and initiator decomposition... 

The dissociation of bases, acids and salts to ions in melts and suitable solvents has been known for a long time. In order that anions formed in this way can initiate polymerization, stringent conditions must be met. The bond between the anion and the monomer must not be solvolyzed by the medium solvation of the anion must not prevent penetration of the monomer the acidity of the monomer must be higher than that of the eventual solvent the nucleophilic properties of the anion must not be reduced by resonance below a certain lower limit, the level of which depends on monomer acidity the medium must not contain an acceptor which could neutralize the anion. For this reason, strongly polar solvents are almost completely excluded as polymerization medial In non-polar media, the formation of anions by dissocia-... 

The resulting complexes can initiate polymerization [186] of vinyl and heterocyclic [187] monomers. 

Mutual donor-acceptor interaction is affected by the changes of electron density on the atoms and bonds of interacting particles. The formation of a donor-acceptor bond between molecules is caused by electron transfer from the highest occupied orbital of the donor to the lowest unoccupied orbital of the acceptor molecule. The properties and structure of the complex depend (inter alia) on the type of the participating orbitals. Usually the complex is of zwitterion character, easily decomposing under the influence of external forces to sets of atoms with properties differing from those of the original donor or acceptor. These zwitterions or their decomposition products can initiate polymerizations. 

Complexes of monomers with acceptors can initiate polymerizations of the respective monomers even without photoexcitation. The properties of DA complexes and the conditions of the thermal activation are determined by the type of donor (monomer), acceptor, and solvent [300,303]. Ion radicals from the monomer, dipolar intermediates (zwitterions), as well as the relatively... 

There are a number of factors that determine whether a protonic acid can initiate polymerization of alkenes. Their acidity (pKa), and therefore the basicity of the resulting counteranion, determines the efficiency of initiation. Although reliable pKa values of acids stronger than sulfuric or hy-droiodic (pKa < -9) are difficult to obtain in aqueous solutions due to their nearly complete dissociation, the pKa values of acetic acid (4.75) and trichloroacetic acid (0.7) in water provide useful references. Conductometric and potentiometric estimates of the pK values of selected protonic acids in various organic solvents are summarized in Table 11 in descending acid strength. These values are not very precise, however, because the amount of moisture in each system was not monitored precisely. 

M. Szwarc et al., tbid. 78, 2656 (1956), and N. D. Scott, U.S. Patent 2,181,771 (1939), have shown that these aromatic anions can initiate polymerization of olefins by a mechanism of electron transfer to form a radical-anion, followed by dimerization of two radical-anions to form di-anions which can propagate indefinitely ... 

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