Polymerization, chain growth and

Most types of polymerizations can be performed in liquid and supercritical C02. The two major types of polymerizations, chain-growth and step-growth, have been demonstrated in C02. Reviews in the literature (Canelas and DeSimone, 1997b Kendall et al., 1999) have described numerous polymerizations in C02, many of which will not be discussed in this chapter. Since only amorphous or low-melting fluoropolymers and silicones show appreciable solubility at relatively mild temperatures and pressures (T< 100 °C, P<400 bar), only these two classes of polymers can be synthesized by a homogeneous polymerization in C02. All other types of polymers, including semicrystalline fluoropolymers and lipophilic or hydrophilic polymers, must be made by heterogeneous methods, such as precipitation, dispersion, emulsion, and suspension, since the polymers are insoluble in C02 (when T< 100 °C and P<400 bar). Some semicrystalline fluoropolymers and hydrocarbon polymers can be dissolved at more extreme temperatures and pressures and are discussed in Chapter 7 of this book. 

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. 

Chain-Growth and Step-Growth Polymerizations Some Comparisons... 

Block copolymer synthesis from living polymerization is typically carried out in batch or semi-batch processes. In the simplest case, one monomer is added, and polymerization is carried out to complete conversion, then the process is repeated with a second monomer. In batch copolymerizations, simultaneous polymerization of two or more monomers is often complicated by the different reactivities of the two monomers. This preferential monomer consumption can create a composition drift during chain growth and therefore a tapered copolymer composition. 

Chapters 5 through 7 deal with polymers formed from chain-growth polymerization. Chain-growth polymerization is also called addition polymerization and is based on free radical, cationic, anionic, and coordination reactions where a single initiating species causes the growth of a polymer chain. 

Figure 5.3 Schematic diagram of the progress of polymerization of a polysilane, showing defect-controlled chain growth and termination. Reprinted with permission from reference 20. Copyright 2003 American Chemical Society.

Thus, we project that hydrogenation of carbidic carbon to form CH species, polymerization of the CH species leading to C-C chain growth, and chain termination leading to the products (followed by their desorption) will occur on carbided iron surfaces where the reaction energetics resembles that on metallic Pd or Pt surfaces. Table XIII clearly shows that the activation barriers for all processes of recombination and desorption are much smaller on Pt than on Fe. Moreover, from Table XIII it follows that on a pure Fe surface such as Fe(110), the desorption energies for... 

An essential feature of a strictly living polymerization is the absence of transfer reactions [652]. This requirement was found to be valid for the polymerization of IP catalyzed by NdCl3 TBP/TIBA in hexane. The lack of chain termination reactions, the second requirement for a living polymerization [652], was confirmed by the application of a mathematical model to the experimental data [279]. Bruzzone et al. realized that transfer reactions occur in the polymerization of BD. In spite of this observation the pseudo-living character of the polymerization was assigned to the superposition of chain growth and chain transfer both of which exhibit a different dependence on monomer conversion [87]. 

Polymerization of ethylene oxide with stannic chloride also leads to a mixture of polymer and dioxane (2). However, in contrast with the borontrifluoride initiated reaction the polymers can reach a molecular weight of up to 20.000 and chain growth and dioxane formation seem to be parallel with each consuming about one half of the monomer. 

Explain the differences between addition and condensation polymers. Show the differences between the chain-growth and step-growth mechanisms of polymerization. 

Chain-Growth Polymerizations. Chain-growth polymerizations are very important to many commercially successful epoxy structural adhesives. They can be extremely rapid and contribute to the fast cure times needed for high productivity in many manufacturing operations. 

Figure 19 Structural development model during polymerization including chain growth and crystallization at Tpo y = 20 °C (A) and 70 °C (B). Panel (C) indicates the intermediate region B sandwiched between crystalline A and amorphous C phases.

We are now going to discuss the two major types of polymerization systems, step growth and chain growth, and show what the difference implies for their control. 

Homogeneous polyaUcene catalysis has progressed to the point where metals not generally associated with coordination polymerization can now be made to promote olefin chain growth and metals long ago associated with polyalkene catalysis have been given new life. 

We turn our attention now to chain-growth polymerizations. The reader should recall that the Features which distinguish chain-growth and step-growth polymerizations were summarized in Section 5.2. The present chapter is devoted to the basic principles of chain polymerizations in which the active centers are free radicals. Chain-growth reactions with active centers having ionic character are reviewed in Chapter 9. 

The reactions that limit chain growth and initiate polymerization have not been defined. Is polymerization terminated by impurities Does solvent participate in transfer Does polymerization start by the reaction with a monomer or with impurities in the system These questions must be answered. For example, we noticed that successful formation of the monomodal high-molecular-weight polysilane requires the addition of a few drops of monomer to sonicated sodium dispersion prior to the addition of the main part of disubstituted dichlorosilane. 

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