Olefinic monomers chain-growth polymerization

Plasma can be utilized in the polymerization of monomer liquid. In this case, no substrate is employed, and monomers are typically organic compounds with an olefinic double bond (monomer for chain growth polymerizations). In a typical case, the vapor phase of a monomer liquid in a sealed tube is used to create plasma. The duration of plasma is generally very short (on the order of a few seconds). After plasma exposure, the tube is shaken to mix plasma-induced reactive species with the monomer and is kept at a constant temperature (polymerization temperature) for a prolonged period. 

Because of the unique growth mechanism of material formation, the monomer for plasma polymerization (luminous chemical vapor deposition, LCVD) does not require specific chemical structure. The monomer for the free radical chain growth polymerization, e.g., vinyl polymerization, requires an olefinic double bond or a triple bond. For instance, styrene is a monomer but ethylbenzene is not. In LCVD, both styrene and ethylbenzene polymerize, and their deposition rates are by and large the same. Table 7.1 shows the comparison of deposition rate of vinyl compounds and corresponding saturated vinyl compounds. 

In the previous chapter, the synthesis of polymers by step polymerization and the kinetics of the process were considered. We turn our attention now to chain-growth polymerizations. The reader should recall that the features.that distinguish step-growth and chain-growth polymerizations are summarized in Table 5.1. A large number of different class of unsaturated monomers, such as ethylene (CH2=CH2, the simplest olefin), a-olefins (CH2=CHR, where R is an alkyl group), vinyl compounds (CH2=CHX, where X = Cl, Br, I, alkoxy, CN, COOH, COOR, CeHs, etc., atoms or groups), and... 

Chain growth polymerization has the characteristic of having an intermediate within the process that cannot be isolated [5], The intermediate can be a metal complex, a free radical, or an ion. These intermediates are transient to the process. The terms vinyl, olefin, and addition polymerization have been associated with this process [13], Monomer units add to a chain very rapidly once it has been initiated. Initiation is the creation of an active center such as a free radical or carbanion [13], An example is the thermal decomposition of benzoyl peroxide shown in Figure 3.4. To propagate the chain, an additional monomer is added at a very rapid rate as monomer concentration is reduced. Figure 3.5 shows the propagation of polystyrene. 

The monomers used most commonly in chain-growth polymerization are ethylene (ethene) and substituted ethylenes. In the chemical industry, monosubstituted ethylenes are known as alpha olefins. Polymers formed from ethylene or substituted ethylenes are called vinyl polymers. Some of the many vinyl polymers synthesized by chain-growth polymerization are listed in Table 28.1. 

When a transition metal alkyl or a metal hydride reacts with olefin molecules to undergo successive insertions, chain growth of a polymer attached to the transition metal takes place. If -hydrogen elimination occurs from the polymer chain, a transition metal hydride coordinated with the olefin derived from the polymer chain will be produced. By displacement of the coordinated olefin from the transition metal by the other monomer olefin, the polymer with an unsaturated terminal bond is liberated with generation of a transition metal hydride coordinated with the olefin. New chain growth will follow from the hydride, with the net result of control of the molecular weight without termination of the polymerization process. The process is in fact a chain transfer process. 

Solid-state polymerization n. A chain-growth polymerization initiated by exposing to ionizing radiation a crystalline monomeric substance. A large number of olefinic and cyclic solid monomers have been so polymerized, the crystalline monomer converting directly to the polymer with no obvious change in appearance of the solid. Odian GC (2004) Principles of polymerization. John Wiley and Sons Inc., New York. 

Chain-growth polymerization has been used to prepare a number of olefin-functionalized metal-containing monomers. For example, a munber of articles have detailed the polymerization of organometaUic olefinic monomers such as 82-84. Depending on the nature of the double bond and the metallic group, radical, cationic, and anionic initiators can be used to polymerize these monomers. 

The transition metal-catalyzed polymerization of olefins yields high molecular weight polymers as the result of the successive insertion of monomer into the metal-carbon bond of the growing polymer chain. This chain growth is... 

It is now clear that, when propagation centers are formed, olefin polymerization by all solid catalysts (including the Phillips Petroleum catalyst from chromium deposited on oxides, and the Standard Oil catalyst of molybdenum oxide on aluminum oxide) essentially follows the same mechanism chain growth through monomer insertion into the transition-metal-carbon bond, with precoordination of the monomer. Interestingly,... 

ADMET is a step growth polymerization in which all double bonds present can react in secondary metathesis events. However, olefin metathesis can be performed in a very selective manner by correct choice of the olefinic partner, and thus, the ADMET of a,co-dienes containing two different olefins (one of which has low homodimerization tendency) can lead to a head-to-tail ADMET polymerization. In this regard, terminal double bonds have been classified as Type I olefins (fast homodimerization) and acrylates as Type II (unlikely homodimerization), and it has been shown that CM reactions between Types I and II olefins take place with high CM selectivity [142], This has been applied in the ADMET of a monomer derived from 10-undecenol containing an acrylate and a terminal double bond (undec-10-en-l-yl acrylate) [143]. Thus, the ADMET of undec-10-en-l-yl acrylate in the presence of 0.5 mol% of C5 at 40°C provided a polymer with 97% of CM selectivity. The high selectivity of this reaction was used for the synthesis of block copolymers and star-shaped polymers using mono- and multifunctional acrylates as selective chain stoppers. 

Qince the discovery (6) of supported chromium oxide catalysts for polymerization and copolymerization of olefins, many fundamental studies of these systems have been reported. Early studies by Topchiev et al. (18) deal with the effects of catalyst and reaction variables on the over-all kinetics. More recent studies stress the nature of the catalytically active species (1, 2, 9,13, 14,16, 19). Using ESR techniques, evidence is developed which indicates that the active species are Cr ions in tetrahedral environment. Other recent work presents a more detailed look at the reaction kinetics. For example, Yermakov and co-workers (12) provide evidence which suggests that chain termination in the polymerization of ethylene on the catalyst surface takes place predominantly by transfer with monomer, and Clark and Bailey (3, 4) give evidence that chain growth occurs through a Langmuir-Hinshelwood mechanism. 

In addition to the configurational isomerism encountered in polymers derived from asymmetric olefins, geometric isomerism is obtained when conjugated dienes are polymerized, e.g., (CH2=CX—CH=CH2). Chain growth from monomers of this type can proceed in a number of ways, illustrated conveniently by 2-methyl-1,3-butadiene (isoprene). Addition can take place either through a 1,2-mechanism or a 3,4-mech-anism, both of which could lead to isotactic, syndiotactic, or atactic structures, or by a 1,4-mode leaving the site of unsaturation in the chain. 

Evidence has now been presented that indicates that the above compound behaves as a carbocationic polymerization initiator for styrene, W-vinylcarbazole, vinyl ethers, and isobutylene. The mechanism of initiation and polymerization of these monomers by such metallocene complexes is still being investigated. It was suggested by Wang et al. [53], that the mechanism of carbocationic polymerization of such olefins by the above complex would involve coordination of the olefins, as shown below, in a nonclassical p -fashion, with the metal-olefin. This interaction is stabilized by a complementary borate-olefin interaction. The next step in the polymerization process by this mechanism, then involves attack on the carbocationic centers of the metal ions-activated olefin molecules by secondary olefin monomers, followed by chain growth [53] ... 

A plausible mechanism for this catalytic oligomerization is shown in Scheme 22.14. Chain growth occurs by olefin insertion, and chain termination occurs by 3-hydride elimination or chain transfer to monomer. These mechanisms parallel the mechanisms for olefin polymerization, but the relative rate for chain growth versus chain transfer is smaller than that for polymerization. These relative rates, and therefore tlie average chain length, are influenced by added ligands such as tertiary phosphines. 

---