Polymers reaction kinetics

It is most fortunate for the development of polymer science that these imagined complications have turned out to be almost wholly illusory. As will be brought out in the course of this chapter, the influence of molecular size and complexity on chemical reactivity may be disregarded in very nearly all polymer reactions. If this were not the case, application of the principles of reaction kinetics to polymerization and polymer degradation reactions would be difficult, and might be so complicated as to be fruitless. Not only would polymer reaction kinetics... 

The large number of reported studies indicates that mass spectrometry is growing in importance as a valuable technique for the analysis and characterization of synthetic polymers. While publications relating to synthetic polymer/ copolymer characterization seem to favor MALDI over other mass spectrometry methods, ESI is also used consistently, in numerous studies, to follow polymer reaction kinetics and to study polymer chain growth on active catalyst substrates. 

The successful preparation of polymers is achieved only if tire macromolecules are stable. Polymers are often prepared in solution where entropy destabilizes large molecular assemblies. Therefore, monomers have to be strongly bonded togetlier. These links are best realized by covalent bonds. Moreover, reaction kinetics favourable to polymeric materials must be fast, so tliat high-molecular-weight materials can be produced in a reasonable time. The polymerization reaction must also be fast compared to side reactions tliat often hinder or preclude tire fonnation of the desired product. 

Poll Fl-U, Arzt M and Wickleder K-FI 1976 Reaction kinetics in the polymerization of thin films on the electrodes of a glow-discharge gap Eur. Polym. J 12 505-12... 

The use of water as a co-catalyst in Ziegler-type polymerizations was first introduced in 1962 (47). The reaction kinetics and crystallinity of the resulting polymers measured by x-ray scattering has been studied (48—51). 

The use of light olefins, diolefins, and aromatic-based monomers for producing commercial polymers is dealt with in the last two chapters. Chapter 11 reviews the chemistry involved in the synthesis of polymers, their classification, and their general properties. This book does not discuss the kinetics of polymer reactions. More specialized polymer chemistry texts may be consulted for this purpose. 

Starnes et al.hl have also suggested that the head adduct may undergo p-scission to eliminate a chlorine atom which in turn adds VC to initiate a new polymer chain. Kinetic data suggest that the chlorine atom does not have discrete existence. This addition-elimination process is proposed to he the principal mechanism for transfer to monomer during VC polymerization and it accounts for the reaction being much more important than in other polymerizations. The reaction gives rise to terminal chloroallyl and 1,2-dichlorocthyl groups as shown in Scheme 4.8. 

Tethering may be a reversible or an irreversible process. Irreversible grafting is typically accomplished by chemical bonding. The number of grafted chains is controlled by the number of grafting sites and their functionality, and then ultimately by the extent of the chemical reaction. The reaction kinetics may reflect the potential barrier confronting reactive chains which try to penetrate the tethered layer. Reversible grafting is accomplished via the self-assembly of polymeric surfactants and end-functionalized polymers [59]. In this case, the surface density and all other characteristic dimensions of the structure are controlled by thermodynamic equilibrium, albeit with possible kinetic effects. In this instance, the equilibrium condition involves the penalties due to the deformation of tethered chains. 

With such modeling efforts, coupled with some small-scale tests, we can assess the hazards of a polymer reaction by knowing certain physical, chemical and reaction kinetic parameters. 

In conclusion, we have reviewed how our kinetic model did simulate the experiments for the thermally-initiated styrene polymerization. The results of our kinetic model compared closely with some published isothermal experiments on thermally-initiated styrene and on styrene and MMA using initiators. These experiments and other modeling efforts have provided us with useful guidelines in analyzing more complex systems. With such modeling efforts, we can assess the hazards of a polymer reaction system at various tempera-atures and initiator concentrations by knowing certain physical, chemical and kinetic parameters. 

The kinetics of reactions of bifunctional compounds are especially significant in relation to polymer reactions. The esterification rate constants shown in the third column (B) of Table V for the homologous series of dibasic acids do not differ greatly from those for the monobasic acids. These differences vanish as the length of chain separating the carboxyl groups increases. 

The combined results of kinetic studies on condensation polymerization reactions and on the degradation of various polymers by reactions which bring about chain scission demonstrate quite clearly that the chemical reactivity of a functional group does not ordinarily depend on the size of the molecule to which it is attached. Exceptions occur only when the chain is so short as to allow the specific effect of one end group on the reactivity of the other to be appreciable. Evidence from a third type of polymer reaction, namely, that in which the lateral substituents of the polymer chain undergo reaction without alteration in the degree of polymerization, also support this conclusion. The velocity of saponification of polyvinyl acetate, for example, is very nearly the same as that for ethyl acetate under the same conditions. ... 

Hamielec, A.E., Introduction to Polymerization Kinetics , Polymer Reaction Engineering - Intensive Short Course on Polymer Production echnology , McMaster University, Hamilton, Ontario, Canada, June 1977. 

Thus propagation must be much faster than isomerization, and the product will be determined by thermodynamics, rather than by reaction kinetics. The net results of the two processes may be quite similar, however, in that polymers of unexpected structures may be obtained, and copolymers may be prepared by polymerization of a single monomer. 

Reaction Kinetics of Functional Groups Attached to a Swollen Polymer Gel... 

In the synthesis of polypeptides with biological activity on a crosslinked polymer support as pioneered by Merrifield (1 2) a strict control of the amino acid sequence requires that each of the consecutive reactions should go virtually to completion. Thus, for the preparation of a polypeptide with 60 amino acid residues, even an average conversion of 99% would contaminate the product with an unacceptable amount of "defect chains". Yet, it has been observed (13) that with a large excess of an amino acid reagent —Tn the solution reacting with a polymer-bound polypeptide, the reaction kinetics deviate significantly from the expected exponential approach to quantitative conversion, indicating that the reactive sites on the polymer are not equally reactive. 

Polymer properties, influence of ions, 258 Polymer surface reactions, kinetics, 322-323 Polymer transformation reactions configurational effect, 38 conformational effects, 38 hydrolysis of polyfmethyl methacrylate), 38 neighboring groups, 37-38 quaternization of poly(4-vinyl pyridine), 37-38 Polymerization, siloxanes, 239... 

Thermogravimetric analysis (TGA) has often been used to determine pyrolysis rates and activation energies (Ea). The technique is relatively fast, simple and convenient, and many experimental variables can be quickly examined. However for cellulose, as with most polymers, the kinetics of mass loss can be extremely complex (8 ) and isothermal experiments are often needed to separate and identify temperature effects (9. Also, the rate of mass loss should not be assumed to be related to the pyrolysis kinetic rate ( 6 ) since multiple competing reactions which result in different mass losses occur. Finally, kinetic rate values obtained from TGA can be dependent on the technique used to analyze the data. 

Radical polymerizations have three important reaction steps in common chain initiation, chain propagation, and chain termination. For the termination of chain radicals several mechanisms are possible. Since the lifetime of a radical is usually less than 1 s, radicals are continuously generated and terminated. Each propagating radical can add a finite number of monomers between its initiation and termination. If a divinyl monomer is in the monomer mixture, the reaction kinetics changes drastically. In this case, a dead polymer chain may grow again as a macroradical, when its pendant vinyl groups react with radicals, and the size of the macromolecule increases until it extends over the whole available volume. 

The temperature reached by a monomer undergoing photopolymerization plays a key role on the reaction kinetics, in particular on the ultimate degree of conversion and therefore on the physico-chemical properties of the UV-cured polymer. It is strongly dependent on the formulation reactivity, the film thickness, as well as on the light intensity. 

Shi, Y. and Jabarin, S. A., Transesterification reaction kinetics of poly(ethylene terephthalate)/poly(ethylene 2,6-naphthalate) blends, J. Appl. Polym. Sci., 80, 2422-2436 (2001). 

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