Free radical polymerization mechanism/kinetics

Our acrylic polymerization model was developed to meet the need for solving these problems. Kinetics used are based on fairly well accepted and standard free radical polymerization mechanisms. 

The first two appear to be catalyses occurring within the coordination shell of the metal, but little has yet been learned of their mechanisms. The latter is a free radical polymerization, the kinetics of which have been examined in some detail. Other metal carbonyls are effective to some extent in all three cases. 

Polymers are economically important and many chemical engineers are involved with some aspect of polymer manufacturing during their careers. Polymerization reactions raise interesting kinetic issues because iof the long chains that are produced. Consider free-radical polymerization reaction kinetics as an illustrative example. A simple polymer-ization mechanism is represented fay the following set of elementary reactions. 

The styrene-MA CTC, with a 1 1 stoichiometry, appeared not to be very solvent dependent (Table 10.21). Various solvent-MA charge-transfer complexes also exhibit 1 1 stoichiometry. The addition of diphenylpicryl-hydrazyl, at several concentrations, gave overall kinetics consistent with a straight-forward free-radical polymerization mechanism. 

The mechanism of radiation polymerization reaction in the liquid phase can be determined by examination its kinetics. Namely, if the polymerization rate is directly proportional to the square root of the absorbed radiation dose by monomer and the resulting polymer molecular weight is inversely proportional to this value it shows the free radical polymerization mechanism ... 

The three-step mechanism for free-radical polymerization represented by reactions (6.A)-(6.C) does not tell the whole story. Another type of free-radical reaction, called chain transfer, may also occur. This is unfortunate in the sense that it complicates the neat picture presented until now. On the other hand, this additional reaction can be turned into an asset in actual polymer practice. One of the consequences of chain transfer reactions is a lowering of the kinetic chain length and hence the molecular weight of the polymer without necessarily affecting the rate of polymerization. 

Bamford and coworkers [24] also investigated the kinetics and mechanism of free radical polymerization of bulk MMA photoinitiated by Mn2(CO)io or Re2(CO)io in the presence of a series of fluoro-olefms such as ... 

Studies in the photoinitiation of polymerization by transition metal chelates probably stem from the original observations of Bamford and Ferrar [33]. These workers have shown that Mn(III) tris-(acety]acetonate) (Mn(a-cac)3) and Mn (III) tris-(l,l,l-trifluoroacetyl acetonate) (Mn(facac)3) can photosensitize the free radical polymerization of MMA and styrene (in bulk and in solution) when irradiated with light of A = 365 at 25°C and also abstract hydrogen atom from hydrocarbon solvents in the absence of monomer. The initiation of polymerization is not dependant on the nature of the monomer and the rate of photodecomposition of Mn(acac)3 exceeds the rate of initiation and the initiation species is the acac radical. The mechanism shown in Scheme (14) is proposed according to the kinetics and spectral observations ... 

Free radical polymerization of MMA is a well understood process. The kinetic mechanism neglecting the chain transfer reactions is given as follows (Odian (1970), Rudin (1982)). 

Eqs. (3), (4), (5), and (7) describe the mechanism of an initiated free radical polymerization in a form amenable to general kinetic treatment. The rate of initiation of chain radicals according to Eqs. (3) and (4) may be written... 

R.A. Hutchinson, M.C. Grady and WJ. Simonsick, Preprints International Symposium on Free Radical Polymerization Kinetics and Mechanism, II Ciocco, Italy (2001). 

Benoit D, Grimaldi S, Robin S et al. (2000) Kinetics and mechanism of controlled free-radical polymerization of styrene and n-butyl acrylate in the presence of an acyclic beta-phosphonylated nitroxide. J Am Chem Soc 122 5929-5939... 

Photoinitiated free radical polymerization is a typical chain reaction. Oster and Nang (8) and Ledwith (9) have described the kinetics and the mechanisms for such photopolymerization reactions. The rate of polymerization depends on the intensity of incident light (/ ), the quantum yield for production of radicals ( ), the molar extinction coefficient of the initiator at the wavelength employed ( ), the initiator concentration [5], and the path length (/) of the light through the sample. Assuming the usual radical termination processes at steady state, the rate of photopolymerization is often approximated by... 

These investigations have demonstrated the successful application of cyclodex-trins in polymer synthesis in aqueous solutions via free radical polymerization or via a oxidative recombination mechanism. Some special aspects of cyclodextrins were found concerning the kinetics, chain transfer reaction, and copolymerization parameters [63],... 

Paramagnetic centers containing a sulfur atom in different oxidation states, (=Si-0)3Si-0-S = O, (=Si-0)3Si-0-S 02, (=Si-0)3Si-0-S02-0, and (=Si-0)3Si-0-S02-0-0, were obtained in Ref. [118]. Their radio-spectroscopic parameters were determined, and the mechanism of free radical oxidation of S02 molecules in this system was established. The mechanism of the initial steps of free radical polymerization and copolymerization of hydrogen- and fluorine-substituted unsaturated hydrocarbons was studied in Ref. [117]. The pathways were found and the kinetic parameters were determined for reactions of intramolecular H(D) atom transfer between r (CH3, CD3, CH2-CH3) and r (CH2-CH2, CD2-CD2), in the structure of (=Si-0)2Si(r)(rI) [120]. 

In conventional free radical polymerization, the initiation, propagation, and termination are kinetically coupled. Consequently, the increase of initiation rate increases the overall polymerization rate but reduces the degree of polymerization. In contrast to this situation (kinetically coupled initiation, propagation, and termination), the formation of chemically reactive species is not the initiation of a subsequent polymerization. Under such an activation/deactivation decoupled reaction system, the mechanism for how chemically reactive species are created and how these species react to form solid material deposition cannot be viewed in analogy to polymerization. 

In the chain growth free radical polymerization of a vinyl monomer (conventional polymerization), the growth reaction is the repeated reaction of a free radical with numbers of monomer molecules. According to the termination by recombination of growing chains, 2 free radicals and 1000 monomer molecules leads to a polymer with the degree of polymerization of 1000. In contrast to this situation, the growth and deposition mechanisms of plasma polymerization as well as of parylene polymerization could be represented by recombinations of 1000 free radicals (some of them are diradicals) to form the three-dimensional network deposit via 1000 kinetic... 

It has been generally assumed that the polymer is formed by a free-radical chain which is initiated by H atoms or alkyl radicals. While this mechanism may be true for the mercury-photosensitized polymerization, recent results on the direct photopolymerization suggest that the two systems may be very similar and that a free-radical chain mechanism is not tenable for the latter case. It is probably most logical to examine this system by first studying the polymer that is formed, and then establishing the kinetics and mechanism of its formation. 

The mechanisms of these reactions are not completely established, but it has been suggested from the kinetic relationships, the effect of zinc and aluminium alkyls on molecular weight and polymer structure, that a coordinated anionic mechanism is probable in hydrocarbon solvents [215]. Similar conclusions have been drawn for the Cr(acac)3/AlEt3 system. For acetonitrile solution, however, the evidence is less certain and simultaneous coordinate anionic and free radical polymerization may occur. 

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