Cationic coordination polymerization activated monomer

Chain gro tvth polymerization begins when a reactive species and a monomer react to form an active site. There are four principal mechanisms of chain growth polymerization free radical, anionic, cationic, and coordination polymerization. The names of the first three refer to the chemical nature of the active group at the growing end of the monomer. The last type, coordination polymerization, encompasses reactions in which polymers are manufactured in the presence of a catalyst. Coordination polymerization may occur via a free radical, anionic, or cationic reaction. The catalyst acts to increase the speed of the reaction and to provide improved control of the process. 

Kinetics of Addition Polymerization. As the name suggests, addition polymerizations proceed by the addition of many monomer units to a single active center on the growing polymer chain. Though there are many types of active centers, and thus many types of addition polymerizations, such as anionic, cationic, and coordination polymerizations, the most common active center is a radical, usually formed at... 

Most data were obtained from copolymerization studies. The copolymerization parameter r (see Chap. 5, Sect. 5.2) is the rate constant ratio for the addition of two different monomers to the same active centre. The inverse values of r j determined for the copolymerization of a series of monomers with the monomer M, define the relative reactivities of these monomers with the active centre from the first monomer, M°,. Thus it is possible to order monomers according to their reactivities in radical, anionic, cationic and coordination polymerizations from the tabulated values of copolymerization parameters [101-103]. 

Cationic organozinc compounds are expected to be good catalysts for ring opening polymerization reactions of epoxides and lactones because the enhanced Lewis acidity (see Lewis Acids Bases) of the zinc center favors its coordination to the monomer. For example, Walker and coworkers have found that the cationic zinc substituted cyclopentadienyl complex [3,5-Me2C6H3CH2CMe2C5H4Zn(TMEDA)]+ [EtB(C6F5)3] is an active initiator species for the polymerization of cyclohexene oxide and e-caprolactone. ... 

Up till now, the predominant and, it should be mentioned, successfully solved problems have been related to the determination of the nature (cationic, free-radical or anionic) and the structure of the active center of the growing polymer chain represented by an asterisk in Scheme 1. However, the investigation of the process of the direct insertion of the monomer in the polymer chain, i.e. everything represented in Scheme 1 by an arrow - was considered to be of secondary importance, with the exception of anionic coordination polymerization. It is usually a priori assumed that this is an elementary single-stage activation transition in the literal sense without any peculiar features, and if these features even exist, they are completely predetermined by (Fig. 1). 

Figure 5.9 outlines the steps for the chain polyaddition mechanism involved in the coordination polymerizations for any kind of active species initiated through different cocatalysts. The counteranion species was suppressed for practical representation of the active site. Once the cationic species is created, it starts the growth of the polymeric chain through continuous addition of monomer. The propagation step is forward described in Figure 5.9 according to the most accepted reaction cycle proposed by Cossee and Arlman, which is known as the Cossee-Arlman mechanism [51]. 

Several reaction mechanisms were proposed. One suggested pathway for propylene oxide polymerization pictures an initial coordination of the monomer with a cationically active center ... 

Styrene is one of those monomers that lends itself to polymerization by free-radical, cationic, anionic, and coordination mechanisms. This is due to several reasons. One is resonance stabilization of the reactive polystyryl species in the transition state that lowers the activation energy of the propagation reaction. Another is the low polarity of the monomer. This facilitates attack by free radicals, differently charged ions, and metal complexes. In addition, no side reactions that occur in ionic polymerizations of monomers with functional groups,are possible. Styrene pol erizes in the dark by a free-radical mechanism more slowly than it does in the presence of light. Also, styrene formed in the dark is reported to have a greater amount of syndiotactic placement. The amount of branching in the polymer prepared by a free-radical mechanism increases with temperature. This... 

Recently, Baird and co-workers have reported (75) examples of polymerizations by a simple mono-Cp titanium complex, (C5(CH3)5)Ti(CH3)3 activated with a Lewis acid (B(C6F5)3) that not only copolymerizes ethylene and a-olefins but also induces polymerization of monomers normally associated with cationic polymerization such as isobutylene and vinyl ethers. Shaffer and Ashbaugh foimd (76) that for isobutylene and a-methylstyrene, the metal complex is an initiator rather than a catalyst (if it even participates at all), but that a transition from cationic to coordination polymerization occurs in styrene polymerization as temperature is raised. Even if it merely functions as an initiator, however, these investigations have revealed new polymerization systems based on anions such as [RB(C6F5)3l (R = alkyl, CeFs) that are less prone to side reactions tending to limit the MW and degree of polymerization of monomers like isobutylene at moderate temperatures (T > -80°C). 

A boron bridge offers more functionality than silicon or carbon bridges to the chemistry of group 4 ansa-metallocene complexes because it can reversibly bind a variety of Lewis basic moieties that are designed to influence the electronic and stereochemical properties of the complex. By coordinating an anionic Lewis base, the boron can potentially serve as an internal, counteranion to the catalytically active, cationic group 4 metal alkyl, as in the hypothetical zwitterionic complex shown in Figure 5.1. The polymerization activity of the zwitterionic catalyst should benefit from the remote location of the counteranion from the active site, where it will not compete with the alkene monomer for a coordination site on the metal. 

One of the key technologies needed to make cyclic conjugated diene polymers useful is an expansion of monomer availability. Presently, neither 1,3-cycloheptadiene nor 1,3-cyclooctadiene has been coordinatively polymerized, even with highly active cationic Ni complexes. The polymerization of functionalized CHDs is, so far, limited to ANiTFA. In order to provide processability and functionality to cyclic conjugated diene polymers, these problems must be overcome. The progress of transition metal-catalyzed polymerization may make this possible in the near future. 

In chain reactions the different types of monomers can be added subsequently to an active chain end. The most important techniques here are sequential living polymerization techniques, such as anionic or cationic polymerization. Certain metallocenes can be used in coordination polymerization of olefins leading to stereo block copolymers, like polypropylene where crystalline and amorphous blocks alternate with each other due to the change of tacticity along the chain [34]. In comparison to living polymerization techniques, free radical and coordination polymerization lead to rather polydisperse materials in terms of the number of blocks and their degree of polymerization. 

Epoxide polymerization can be described in terms of three different mechanisms (1) anionic (base-catalyzed), (2) cationic (acid-catalyzed), and (3) coordinate. The third actually combines features of the first two extremes, since it involves coordination of the monomer oxygen at a Lewis acid catalyst site (L), followed by attack on the thus activated monomer by an alkoxide already bound to the site. 

In this chapter, the anionic and related nudeophiUc polymerizations of epoxides are reviewed. The elementary mecharrisms involved in the presence of different initiators and catalysts and the main sjmthetic strategies developed for the preparation of epoxide homopolymers and copolymers are described. In the second section, the anionic polymerization of epoxides involving alkali metal derivatives is described. The use of orgarric derivatives as counterions or catalysts is presented in the third section. The fourth section is devoted to epoxide-coordinated polymerization. Finally, in the last sertion, monomer-activated epoxide polymerization is described. The cationic polymerization of epoxides is described in another chapter. 

An active catalyst site requires a metal-carbon bond that may have existed in the pre-catalyst, may have been formed upon initial activation by cocatalyst (via ligand exchange), or may exist because of a previous migratory insertion event. In most cases, the starting precursor of the catalyst is a metallocene dichloride (dichlorides are usrraUy the most artive precursors for coordination polymerization) complex, which obtains a vacant site as a consequence of reaction with cocatalyst (see Section 3.21.3.1 below). In the case of metallocene activation by MAO, the produced active center is a strongly Lewis acidic cationic metal complex stabilized by a bulky MAO anion the transition metal bears a vacant coordination site ready for complexation of the olefinic monomer (Figure 4(a)). 

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