Mechanisms affecting polymerization

Chapter 3 describes basic mechanical testing procedures. This chapter describes various structural-related factors which affect polymeric mechanical properties. 

The activated phenols are C-0 coupled each other. The dimer thus formed is activated by a similar mechanism, and polymerization occurs. The effects of the amine ligand (L) are to improve the solubility and stability of the Cu ion, to affect the stability of the substrate-coordinated complex, and to control the redox potential of the Cu ion. The Cu-complex catalyst not only enhances the rate of polymerization, but it also has an important effect on the coupling reaction. 

Two frequently asked questions are (1) How do the metal-metal bonded species affect the kinetics and mechanisms of polymerization compared to... 

This paper examines some factors which affect not only the overall activity, but also the rate of termination of polyethylene chains growing on the Phillips Cr/silica polymerization catalyst. Although the theme of this symposium is not the termination but the initiation of polymer chains, the two aims are not inconsistent because on the Phillips catalyst the initiation and termination reactions probably occur together. They are both part of a continuous mechanism of polymerization. One possibility, proposed by Hogan, is shown below. The shift of a beta hydride simultaneously terminates one live chain while initiating another ... 

Because it is the extraordinarily large size of the macromolecules which leads to their unusual properties, it would be most sensible to classify polymerization reactions in accordance with the way in which they affect the molecular size and size distribution of the final product, i.e., in terms of the mechanism of polymerization. On this basis, there appear to be only two basic processes whereby macromolecules are synthesized (Zhang et al., 2012 Penczek and Premia, 2012 Moore, 1978 Saunders and Dobinson, 1976 Odian, 2004b Penczek, 2002 Jenkins et al., 1996) (1) step-growth polymerization (polycondensation and polyaddition) and (2) chain-growth (chain) polymerization. 

The mechanism of polymerization in ternary and quaternary oil-in-water microemulsions has become understood only in recent years. The onset of turbidity upon polymerization and the lack of stability with time observed by most authors, particularly for MM A monomer, is likely the reason for the slow progress in the comprehension of the mechanism of O/W systems. Only slight changes in the formulation are sufficient to significantly affect the polymerization process and to induce particle coagulation at any stage of the reaction. This may explain the disparity in the kinetic data reported by some authors for very similar systems. With this remark in mind, one can, however, conclude that the scheme that is now well accepted is that of a continuous particle nucleation mechanism as in the case of inverse systems. This view is supported by several features. 

Hydroxy-terminated liquid polybutadienes are prepared for reactions with diisocyanates to form elastomeric polyurethanes (see Chapter 6). Such materials can be prepared by anionic polymerizations as living polymers and then quenched at the appropriate molecular weight. These polybutadienes can also be formed by a free-radical mechanism. The microstructures of the two products differ, however, and this may affect the properties of the finished products. To form hydroxy terminated polymers by a free-radical mechanism, the polymerization reactions may be initiated by hydroxy radicals from hydrogen peroxide. 

The mechanism affecting the appearance and development of crosslinked polymer particles during suspension polymerization may be described usually by different schemes, depending on the monomer systems used. Starting from the suspension of water-insoluble monomers, the formation mechanism of the crosslinked polymers might be schematically depicted such as in Figure 3.1. 

The catalytic cycle is a convenient graphical way to describe the central role played by the active site in the mechanism of polymerization. Changes in the nature of the active site will affect the catalytic mechanism and consequently the activity and the selectivity of the polymerization. Changes in the polymerization reactor conditions, such as temperature and monomer concentration, play a vital role in the catalyst mechanism because they affect the rate constants of each of these steps. Figure 8.13 shows a catalyst cycle for olefin polymerization with coordination catalysts. 

In contrast, in coordination polymerization chain growth and termination take place by insertion of the monomer or chain-transfer agent into a metal-carhon bond, as proposed by the Cossee mechanism. Consequently, electrical and steric effects around the active site affect polymerization kinetics as much as does the monomer type. The mechanisms of free-radical and coordination polymerization are contrasted in Figure 8.19. 

Here [M]g denotes the equilibrium concentration of the monomer and K the equilibrium constant of propagation. The equilibrium concentration of the monomer depends on the system, i.e., on the nature of monomer, of solvent and temperature, but its value is not affected by the mechanism of polymerization. Determination of [M]g over a temperature range allows us to calculate AH and AS of propagation, and its dependence on polymer concentration and the nature of solvent provides information on the solvent-polymer interaction. A thorough discussion of these topics is reported elsewhere However, it should be stressed that the above thermodynamic ramifications apply to high-molecular weight polymers and the treatment has to be modified when one deals with oligomers. ... 

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. 

Oxidizers. The characteristics of the oxidizer affect the baUistic and mechanical properties of a composite propellant as well as the processibihty. Oxidizers are selected to provide the best combination of available oxygen, high density, low heat of formation, and maximum gas volume in reaction with binders. Increases in oxidizer content increase the density, the adiabatic flame temperature, and the specific impulse of a propellant up to a maximum. The most commonly used inorganic oxidizer in both composite and nitroceUulose-based rocket propellant is ammonium perchlorate. The primary combustion products of an ammonium perchlorate propellant and a polymeric binder containing C, H, and O are CO2, H2, O2, and HCl. Ammonium nitrate has been used in slow burning propellants, and where a smokeless exhaust is requited. Nitramines such as RDX and HMX have also been used where maximum energy is essential. 

The quahty of the water used in emulsion polymerization has long been known to affect the manufacture of ESBR. Water hardness and other ionic content can direcdy affect the chemical and mechanical stabiUty of the polymer emulsion (latex). Poor latex stabiUty results in the formation of coagulum in the polymerization stage as well as other parts of the latex handling system. 

Many different combinations of surfactant and protective coUoid are used in emulsion polymerizations of vinyl acetate as stabilizers. The properties of the emulsion and the polymeric film depend to a large extent on the identity and quantity of the stabilizers. The choice of stabilizer affects the mean and distribution of particle size which affects the rheology and film formation. The stabilizer system also impacts the stabiUty of the emulsion to mechanical shear, temperature change, and compounding. Characteristics of the coalesced resin affected by the stabilizer include tack, smoothness, opacity, water resistance, and film strength (41,42). 

---