Chemically initiated polymerization

Not much effort has been made, except for the Tafel studies, to establish the empirical kinetics and models of interfacial reactions to obtain thick polymeric films (>100 nm) of industrial interest from different monomers. However, this is much more than the few kinetic studies performed until now to understand the mechanism of chemically initiated polymerization. Electrochemical models still have an advantage in obtaining priority in the industrial production of tailored materials. 

The most important of these in chemically initiated polymerizations are the transfer reactions with solvent, rate Rs, and rate-constant ks, and with monomer, rate Rm, and rate-constant km. Solvent transfer was shown to be important by Ueno etal. (1966c) for the polymerization of styrene in toluene, and it will be discussed below. The chemistry of the transfer with an aromatic compound ArH, discovered by Plesch et al. (Plesch 1953 Brackman Plesch 1958 Penfold Plesch 1961), can be represented as... 

The second part of the theory, which is a logical consequence of the first, is that monomers that have more than one basic site, e.g., an aromatic ring or an oxygen atom, can form more than one type of complex with the carbenium ion this idea was first proposed by Plesch (1990) in the context of chemically initiated polymerizations. It helps to explain why aryl alkenes and alkyl vinyl ethers polymerize more slowly than isobutene and cyclopentadiene. The reason is that all the complexes formed by the alkyl alkenes are propagators, whereas for the aryl alkenes and vinyl ethers only a fraction of the population of complexes can propagate. 

In contrast to chemically initiated polymerization, all three monomers polymerized with essentially the same rates, giving the same product distribution. It was concluded that propagation and ring formation proceed on the same type of active species ... 

The most startling feature of these results is the magnitude of fep. Once ain these values are many orders of magnitude in excess of most of those determined from chemically initiated polymerizations. Even accepting that some of these estimations are upper limits, e.g. cyclopentadiene, the differences are enormous when a comparison is made with previous information. 

A similar though less comprehensive comparison is possible for the polymerization of styrene. Here much more data is available from chemically initiated polymerizations, but unfortunately none of these is conclusive with respect to the nature of the active centre (Table 6). 

Substitution of the appropriate rate values yields a ratio of about 10, extremely close to that obtained from chemically initiated polymerization. 

Recently, some kinetic coefficients have been determined in electro-chemically initiated polymerizations [144]. Trioxane, dissolved in three different solvents (acetonitrile, benzonitrile, and nitrobenzene) with tetra-butylammonium perchlorate as background electrolyte, was polymerized in the anodic compartment. Definite solvent dependence was shown. When the current was turned off, polymerization in acetonitrile ceased in... 

CPs can be synthesized by chemically initiated polymerization, vapor-phase methods, photochemical polymerization or by electrochemical polymerization where the choice of polymerization method depends to a large extent on the final application in mind [2, 3]. Chemical polymerization usually produces powdery materials and is the preferred technique for large-scale production of CPs. The major disadvantages of chemical polymerization are the limited choice of suitable oxidants, introduction of impurities with the oxidant and difficulties to control the oxidation strength... 

Fig. 4.1 Variation of maximum styrene conversion in homogeneous fluid phase, as a function of CO2 content for chemically initiated polymerizations at 80°C and two pressures, 30 and 50 MPa, in the presence of 1 mol% CBr4 [5, 6). The number average molecular weight of the resulting polymers is 10 g mol". ...

Fig. 4.9 Conversion dependence of (k,) for DMA and MMA homopolymerizations in bulk and with 40 wt% CO2 at 80°C and 30 MPa. The data are derived from chemically initiated polymerizations.

Our preceding kinetic studies into the aqueous-phase polymerization of AA with propionic add being present and also into the polymerization of 2-acrylamido-2-methylpropane sulfonic acid (AMPS) up to high degrees of monomer conversion provided some indication that kp depends on the total concentration of carboxylic groups, which may be part of the monomer, the polymer, or a carboxylic acid cosolvent, rather than only on monomer concentration. The PLP-SEC-derived kp values for AA were lowered upon increasing the concentration of propionic acid and kp for AMPS, as obtained from a combination of the single pulse (SP)-PLP technique with chemically initiated polymerization, appeared to be independent of monomer conversion. ... 

Estimated Propagation Rate Constants and the Activation Energies by the Radiation and Chemically Initiated Polymerization of Ethyl and Isopropyl Vinyl Ethers at 30°C. 

Besides the electrochemical method, other methods such as the chemically initiated polymerization method, the vapor-phase methods, and the Langmuir-Blodgett method have been devised for the preparation of conducting polymers. In the chemical method, a chemical oxidant such as ferric chloride initiates the polymerization. Polymerization usually results in a powdery product and the resultant powder is then compressed into a pellet or dissolved in a suitable solvent such as methylene chloride or chloroform and spin coated on a substrate. In the vapor-phase technique, which is a variation of the chemical method, a monomer film is vacuum sublimed onto a glass substrate and then exposed to a solution or vapor of ferric chloride for oxidative polymerization. Films thus prepared are usually thick, of the order of micrometers. For some applications in which ultrathin and very uniform films are required, the Langmuir-Blodgelt technique for depositing monolayers has also been tried successfully [5]. 

Natural history (and other) specimens were encapsulated in PMMA (Hibben, 1937 Organ, 1963) or PEMA (Puckett, 1940) blocks by both ultraviolet and chemically initiated polymerization. PMMA/MMA slurries have been used for restoration of glass (Technovit 4004a Kulzer) (Jackson, 1983), Plastogen G (Schmidt) and other materials. The two parts are a PMMA powder that contains an initiator and MMA hquid. The slurry sets to a translucent solid. The initial viscosity of the slurry, rate of setting and total shrinkage depend on the proportion of powder added. 

Practically, all monomers capable of undergoing free-radical polymerization can be polymerized upon irradiation with y- or X-rays. An initiator is not needed because free radicals generated by radiolysis of the monomer can initiate the polymerization. The generation of initiator radicals is temperature-independent and, therefore, the activation energy Tpoi of the overall rate of polymerization Vpoi, is lower than in chemically initiated polymerizations. 

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