Free-radical polymers kinetics

The termination of chain growth can also occur both in the gas phase and at the polymer surface. In the gas phase, free radicals are lost by reaction with both hydrogen atoms and other free radicals. The kinetics of these processes are given by... 

FRP leads to the formation of statistical copolymers, where the arrangement of monomers within the chains is dictated purely by kinetic factors. The most common treatment of free-radical copolymerization kinetics assumes that radical reactivity depends only on the identity of the terminal unit on the growing chain. The assumption provides a good representation of polymer composition and sequence distribution, but not necessarily polymerization rate, as discussed later. This terminal model is widely used to model free-radical copolymerization according to the set of mechanisms in Scheme 3.11. 

Saverio Russo is a senior professor of industrial chemistry at Genoa University, Italy. He has been, and still is, project leader of several research programs supported by the European Union, Italian Ministry of University, and Chemical Companies. He has been working for more than 40 yeare in the field of macromolecular science and technology, mainly on advanced polymeric materials synthesis, characterization and applications. Polyamide 6 by the anionic routes has been one of the major topics of his research. He is author of more than 250 scientific publications, mostly in international journals, and six patents. Prof Russo has been member of the Scientific Committee of INSTM (Interuniversity Consortium of Materiab Science and Technology) and director of its S Hon on Functional and Structural Polymeric Materials. He was the co- itor of four volumes of Comprehensive Polymer Science, Peigamon, 1989 and two supplement volumes (1992 and 1996). He was the organizer and co-chairman of two lUPAC Symposia of Free Radical Polymerization Kinetics and Mechanism, in 1987 and 1996. 

Naphthalene excitation by UV-light in glassy cellulose triacetate films induces formation of polymeric free radicals, polymer chain break and addition of naphthalene fractures to macromolecules. In the absence of oxygen, the naphthalene consumption rate increases in direct proportion to UV-light intensity, whereas the rate of polymeric chain breaks varies in proportion to quadratic intensity. The process is initiated by singly excited triplet naphthalene molecules localized in the structural matrix zones, where they are incapable of irradiative deactivation. The features of phototransformation are explained in the framework of the hetero-nanophase kinetic model of the process, taking into account the radiationless translation of triplet excitation energy to naphthalene molecules present in the zones, where sensitization is absent and only triplet state deactivation is performed. 

Polyethylene (Section 6 21) A polymer of ethylene Polymer (Section 6 21) Large molecule formed by the repeti tive combination of many smaller molecules (monomers) Polymerase chain reaction (Section 28 16) A laboratory method for making multiple copies of DNA Polymerization (Section 6 21) Process by which a polymer is prepared The principal processes include free radical cationic coordination and condensation polymerization Polypeptide (Section 27 1) A polymer made up of many (more than eight to ten) amino acid residues Polypropylene (Section 6 21) A polymer of propene Polysaccharide (Sections 25 1 and 25 15) A carbohydrate that yields many monosacchande units on hydrolysis Potential energy (Section 2 18) The energy a system has ex elusive of Its kinetic energy... 

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. 

Liquid-phase chlorination of butadiene in hydroxyhc or other polar solvents can be quite compHcated in kinetics and lead to extensive formation of by-products that involve the solvent. In nonpolar solvents the reaction can be either free radical or polar in nature (20). The free-radical process results in excessive losses to tetrachlorobutanes if near-stoichiometric ratios of reactants ate used or polymer if excess of butadiene is used. The "ionic" reaction, if a small amount of air is used to inhibit free radicals, can be quite slow in a highly purified system but is accelerated by small traces of practically any polar impurity. Pyridine, dipolar aptotic solvents, and oil-soluble ammonium chlorides have been used to improve the reaction (21). As a commercial process, the use of a solvent requites that the products must be separated from solvent as well as from each other and the excess butadiene which is used, but high yields of the desired products can be obtained without formation of polymer at higher butadiene to chlorine ratio. 

The block copolymer produced by Bamford s metal carbonyl/halide-terminated polymers photoinitiating systems are, therefore, more versatile than those based on anionic polymerization, since a wide range of monomers may be incorporated into the block. Although the mean block length is controllable through the parameters that normally determine the mean kinetic chain length in a free radical polymerization, the molecular weight distributions are, of course, much broader than with ionic polymerization and the polymers are, therefore, less well defined,... 

Polymers in Schemes 12 and 13 were the first examples of the preparation of pyridinium and iminopyridinium ylide polymers. One of the more recent contributions of Kondo and his colleagues [16] deals with the sensitization effect of l-ethoxycarbonyliminopyridinium ylide (IPYY) (Scheme 14) on the photopolymerization of vinyl monomers. Only acrylic monomers such as MMA and methyl acrylate (MA) were photoinitiated by IPYY, while vinylacetate (VA), acrylonitrile (AN), and styrene were unaffected by the initiator used. A free radical mechanism was confirmed by a kinetic study. The complex of IPYY and MMA was defined as an exciplex that served as a precursor of the initiating radical. This ylide is unique in being stabilized by the participation of a... 

Well before the advent of modern analytical instruments, it was demonstrated by chemical techniques that shear-induced polymer degradation occurred by homoly-tic bond scission. The presence of free radicals was detected photometrically after chemical reaction with a strong UV-absorbing radical scavenger like DPPH, or by analysis of the stable products formed from subsequent reactions of the generated radicals. The apparition of time-resolved ESR spectroscopy in the 1950s permitted identification of the structure of the macroradicals and elucidation of the kinetics and mechanisms of its formation and decay [15]. 

Radiolytic ethylene destruction occurs with a yield of ca. 20 molecules consumed/100 e.v. (36, 48). Products containing up to six carbons account for ca. 60% of that amount, and can be ascribed to free radical reactions, molecular detachments, and low order ion-molecule reactions (32). This leaves only eight molecules/100 e.v. which may have formed ethylene polymer, corresponding to a chain length of only 2.1 molecules/ ion. Even if we assumed that ethylene destruction were entirely the result of ionic polymerization, only about five ethylene molecules would be involved per ion pair. The absence of ionic polymerization can also be demonstrated by the results of the gamma ray initiated polymerization of ethylene, whose kinetics can be completely explained on the basis of conventional free radical reactions and known rate constants for these processes (32). An increase above the expected rates occurs only at pressures in excess of ca. 20 atmospheres (10). The virtual absence of ionic polymerization can be regarded as one of the most surprising aspects of the radiation chemistry of ethylene. 

The oxidation of polymers is most commonly depicted in terms of the kinetic scheme developed by BoUand [14]. The scheme is summarized in Figure 15.1. The key to the process is the initial formation of a free-radical species. At high temperatures and at large shear forces, it is likely that free-radical formation takes place by cleavage of C-C and C-H bonds. 

A series of simulations were performed to study the effect of variables such as initiator concentration, initiator half-life and activation energy on the optimum temperature and optimum time. It was assumed that initially the polymerization mixture contained S volume percent monomer, the rest of the mixture being solvent and polymer formed earlier. It was required to reduce the monomer concentration from S volume percent to 0.S volume percent in the minimum possible time. The kinetic and tbeimodyamnic parameters used are similar to those of free radical polymerization of MMA. The parameter values are given in Appendix B. 

Advanced computational models are also developed to understand the formation of polymer microstructure and polymer morphology. Nonuniform compositional distribution in olefin copolymers can affect the chain solubility of highly crystalline polymers. When such compositional nonuniformity is present, hydrodynamic volume distribution measured by size exclusion chromatography does not match the exact copolymer molecular weight distribution. Therefore, it is necessary to calculate the hydrodynamic volume distribution from a copolymer kinetic model and to relate it to the copolymer molecular weight distribution. The finite molecular weight moment techniques that were developed for free radical homo- and co-polymerization processes can be used for such calculations [1,14,15]. 

Most kinetic treatments of the photo-oxidation of solid polymers and their stabilization are based on the tacit assumption that the system behaves in the same way as a fluid liquid. Inherent in this approach is the assumption of a completely random distribution of all species such as free radicals, additives and oxidation products. In all cases this assumption may be erroneous and has important consequences which can explain inhibition by the relatively slow radical scavenging processes (reactions 7 and 9) discussed in the previous section. 

If we presume that oxidation of polymer PH takes place according to the Scheme 2 of Bolland-Gee mechanism, in which reactions 2, 3, 5, 6, 7, 8, 9 and 10 predominate, involving initiation, propagation and termination of the free radical process. The production of primary radicals by the reaction 1 is governed rapidly by decomposition of hydroperoxides. From the shape of kinetic runs it may be assumed that decomposition of hydroperoxides has the character of a bimole-cular reaction (reaction 5) even in a very short time after the start of experiment. 

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