Thermal polymerization Chain initiation

Additives of the stearates of iron (IS), copper (CpS), cobalt (CbS), zinc (ZS), and lead (LS) within a certain concentration range were found to increase the polymerization rate of styrene and methylmethacrylate (MMA) in comparison with thermal polymerization. By initiating activity, they can be arranged as LS < CbS < ZS < IS < CpS. The decreases in the effective activation energy, the activation energy of the initiating reaction, and the kinetic reaction order with respect to monomer point to the monomer s active participation in chain initiation. IR spectroscopy data show that an intermediate monomer-stearate complex is formed and then decomposed into active radicals to initiate polymerization. The benzoyl peroxide (BP)-IS (or CpS) systems can be nsed for effective polymerization initiation. Concentration inversion of the catalytic properties of... 

Reciprocal degrees of polymerization of polystyrenes prepared by thermal polymerization at 100°C in hydrocarbon solvents are plotted against [>8]/[itf] in Fig. 16. Conversions were sufficiently low to permit the assumption of constancy in this ratio, which is taken equal to its initial value. The linearity of plots such as these, including those for numerous other monomer-solvent pairs which have been investigated, affords the best confirmation for the widespread occurrence of chain transfer and for the bimolecular mechanisms assumed. It is... 

The inhibitors more commonly used are molecules which in one way or another react with active chain radicals to yield product radicals of low reactivity. The classic example is benzoquinone. As little as 0.01 percent causes virtual total suppression of polymerization of styrene or other monomers. This is true of both thermal and initiated polymerizations. Results of Foord for the inhibition of thermal polymerization of styrene by benzoquinone are shown in Fig. 22. The... 

In the study of polymerizations of the first vesicular system it has been learned that thermal polymerization in an aqueous medium was not possible because of the hydrolysis of the acetal group. UV irradiation seemed the only practical initial way to perform the polymerization. However, it is difficult to follow and manipulate the polymerization, and therefore it is not possible to control the molecular weight of the polymer chain in the vesicular system. 

The monomers commonly used for the preparation of polymer monoliths are either hydrophobic, for example, styrene/divinylbenzene and alkyl methacrylates, or hydrophilic, for example, acrylamides. The polymerization is usually accomplished by radical chain mechanisms with thermal or photochemical initiation, as detailed in the reviews (Eeltink et al., 2004 Svec, 2004a and b). Internal structures of polymer monoliths are described to be corpuscular rather than spongy this means through-pores were found to be interstices of agglomerated globular skeletons as shown in Fig. 7.1 (Ivanov et al., 2003). Porosity is presumably predetermined by the preparation... 

Radical Polymerization. Radical chain polymerization involves initiation, propagation, and termination. Consider the polymerization of ethylene. Initiation typically involves thermal homolysis of an initiator such as benzoyl peroxide... 

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. 

The radiolysis of olefinic monomers results in the formation of cations, anions, and free radicals as described above. It is then possible for these species to initiate chain polymerizations. Whether a polymerization is initiated by the radicals, cations, or anions depends on the monomer and reaction conditions. Most radiation polymerizations are radical polymerizations, especially at higher temperatures where ionic species are not stable and dissociate to yield radicals. Radiolytic initiation can also be achieved using initiators, like those used in thermally initiated and photoinitiated polymerizations, which undergo decomposition on irradiation. 

For any specific type of initiation (i.e., radical, cationic, or anionic) the monomer reactivity ratios and therefore the copolymer composition equation are independent of many reaction parameters. Since termination and initiation rate constants are not involved, the copolymer composition is independent of differences in the rates of initiation and termination or of the absence or presence of inhibitors or chain-transfer agents. Under a wide range of conditions the copolymer composition is independent of the degree of polymerization. The only limitation on this generalization is that the copolymer be a high polymer. Further, the particular initiation system used in a radical copolymerization has no effect on copolymer composition. The same copolymer composition is obtained irrespective of whether initiation occurs by the thermal homolysis of initiators such as AIBN or peroxides, redox, photolysis, or radiolysis. Solvent effects on copolymer composition are found in some radical copolymerizations (Sec. 6-3a). Ionic copolymerizations usually show significant effects of solvent as well as counterion on copolymer composition (Sec. 6-4). 

In the thermal-catalytic method a peroxide catalyst is usually used to initiate the free radical chain reaction. The main disadvantages are the higher temperatures required for carrying out the polymerizations, the potential hazard of explosion on addition of catalyst to the monomer, and disposal of excess catalyzed monomer after impregnating. Combinations of heat, radiation, and catalyst have been experimented with to reduce the radiation and catalyst requirements and to increase the rate of polymerization. In thermal polymerization a muffle furnace, infrared heating, and microwave heating can be used to provide the thermal energy. 

The participation of Diels-Alder type intermediates in polymerization was considered by Hill et ah (26) in 1939 as a result of the elucidation of the structures of the butadiene homopolymer and the butadiene-methyl methacrylate copolymer resulting from thermal polymerization in emulsion. The considerable amount of alternating 1,4 and 1,2 structures in the homopolymer and the predominantly 1,4 structure of the butadiene in the copolymer which contained more than 50% alternating units of butadiene and methyl methacrylate led to the proposal that the reaction proceeded through a Diels-Alder dimer complex or activated complex. Chain initiation involved a thermal reaction in which the activated com-... 

Vinyl monomers, such as styrene, methyl methacrylate, vinyl acetate, vinyl chloride or acrylonitrile are preferably polymerized by chain polymerization techniques initiated by free radicals. Suitable free radicals can be handily achieved from unstable chemicals like peroxides (benzoyl peroxide, dicumil peroxide) or di-azo reagents (e.g. 2,2 -azo-bis-isobutyronitrile, AIBN) which are dissolved in monomer and usually thermally decompose at temperature range of 40-120 °C. Alternatively, suitable radicals for polymerization can also be activated without addition of external initiators, by just applying ultraviolet light (wave length 200-350 nm) or ultrasound (15,33,34) onto monomer. 

Thermal decomposition of —N=N— groups in the presence of monomers (for example vinyl chloride) leads to the formation of block copolymers [107], When the decomposing —N=N— groups is bound to a polymeric chain as a substituent, graft copolymers are produced. An example of such an initiator may be the copolymer of styrene with (4-vinylphenylazo)-2-methylmalononitrile [108]... 

Eqs. (1) to (3) indicate that conversion studies under conditions where thermal polymerization prevails can only yield e and the product nyo, whereas the photon-induced reaction provides information on the product nq. To disentangle chain initiation and chain propagation effects an independent determination of the kinetic chain length is required. 

Of particular interest are changes in the chain length occurring in connection with the autocatalytic reaction acceleration in TS-6. Numerous thermal polymerization studies showed that the activation energy is EJl = 1.00 + 0.02 eV, independent of conversion. Consequently the autocatalytic reaction enhancement cannot be the result of an increase of the Boltzmann factor. Instead, an increase in the number of monomers consumed per primary chain initiation event has been postulated. Experimentally n(X = 0.5)/n(X = 0) = 200 is found... 

The essence of the energetic studies on TS and 4-BCMU is contained in Fig. 9. In TS formation of the chain initiating species -- a dimer — requires an energy of 1.0 eV. It can be supplied thermally or optically via monomer excitation. In the former case it is this chain initiation reaction that controls the thermal reactivity and its temperature-dependence. Chain initiation can also be produced optically at a yield of order 10 per absorbed UV-quantum. In this case it is chain propagation that determines the temperature dependence of the polymerization yield. However, the activation energy E" need not be and in general is not identical with the energy... 

Another example is the copper-catalyzed surface-initiated radical polymerization of MMA from S-7 at room temperature without addition of free initiator. The molecular weights and MWDs of the polymers were directly measured after removing the brushes from the surface. For example, the surface with 40 nm thickness had Mn of 68900 and MWDs of 1.45. A high graft density (180 A2/chain) and decreased surface roughens (0.54 nm) were observed. This method is free from solution and thermal polymerizations due to the absence of free initiators and a low polymerization temperature, which permits a simple washing step without Soxhlet extraction. 

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