Initiator, anionic free radical

Addition polymerization is employed primarily with substituted or unsuhstituted olefins and conjugated diolefins. Addition polymerization initiators are free radicals, anions, cations, and coordination compounds. In addition polymerization, a chain grows simply hy adding monomer molecules to a propagating chain. The first step is to add a free radical, a cationic or an anionic initiator (I ) to the monomer. For example, in ethylene polymerization (with a special catalyst), the chain grows hy attaching the ethylene units one after another until the polymer terminates. This type of addition produces a linear polymer ... 

Traditional polymerizations usually involve AB-type monomers based on substituted ethylenes, strained small ring compounds using chain reactions that may be initiated by free radical, anionic or cationic initiators [20]. Alternatively, AB-type monomers may be used in polycondensation reactions. 

The kinetic chain reaction typically consists of three steps (1) initiation, (2) propagation, and (3) termination. The initiators for free radical, anionic, and cationic polymerizations... 

In addition to the dark oxidation of S(IV) on surfaces, there may be photochemically induced processes as well. For example, irradiation of aqueous suspensions of solid a-Fe203 (hematite) containing S(IV) with light of A > 295 nm resulted in the production of Fe(II) in solution (Faust and Hoffmann, 1986 Faust et al., 1989 Hoffmann et al., 1995). This reductive dissolution of the hematite has been attributed to the absorption of light by surface Fe(III)-S(IV) complexes, which leads to the generation of electron-hole pairs, followed by an electron transfer in which the adsorbed S(IV) is oxidized to the SO-p radical anion. This initiates the free radical chemistry described earlier. 

This time-constant rate is proportional to the a-TiCU amount which proves that, at least formally, the over-all polymerization process is really a catal3rtic one, with regard to the a-TiCh. The catalytic behavior of -TiCU is, in any case, connected with the existence on its surface of metal-lorganic complexes which act in the polymerization only if a-TiCU is present. This makes stereospecific polymerization processes (of coordinated anionic nature) very different from the better known polymerization processes, initiated with free radicals. In the latter process, the initiator is not a true catalyst, since it decomposes during the reaction, forming radicals which are bound to the dead polymer on the contrary, in the case of stereospecific polymerization, each molecule of polymer, at the end of its growing period, can be removed from the active center on the solid surface of the catalyst which maintains its initial activity. 

The most important recent development in this area has been the use of various single-electron reductants to initiate the free radical chain process. Such reduc-tants have been most commonly metals or anionic species, and such processes have been used either to initiate addition processes or substitution (SRN1) processes... 

In non-aqueous media (DMF, DMSO, acetonitrile), purine and some of its 6-sub-stituted derivatives (adenine, 6-methylpurine, 6-methylaminopurine, 6-methoxypurine, 6-dimethylaminopurine) undergo an initial le reduction to form the corresponding anionic free radical, followed by dimerization with rate constantsof 103—10s lmol l l64. The free radical nature of the le reduction product was independently established by ESR spectroscopy l69>. This behaviour is to be contrasted with the 2e and 4e reductions undergone by these compounds in aqueous media. s6-99-155. ... 

Since polystyrene is one of the oldest commercial polymers with over 9 million tonnes/yr of sales, there have been thousands of patents issued covering all aspects of its manufacture and property enhancement. The styrene monomer readily polymerizes to polystyrene either thermally or with free-radical initiators (see Chapter 6 on free-radical polymerization and Chapter 8 on nitroxide-mediated polymerization). Commercial processes for the manufacture of polystyrene are described in Chapter 3 while process modelling and optimization of styrene polymerization is examined in Chapter 5. Styrene also can be polymerized via anionic and Ziegler-Natta chemistries using organometallic initiators. Using free radical and anionic polymerization chemistries, the... 

The active site in chain-growth polymerizations can be an ion instead of a free-radical. Ionic reactions are much more sensitive than free-radical processes to the effects of solvent, temperature, and adventitious impurities. Successful ionic polymerizations must be carried out much more carefully than normal free-radical syntheses. Consequently, a given polymeric structure will ordinarily not be produced by ionic initiation if a satisfactory product can be made by less expensive free-radical processes. Styrene polymerization can be initiated with free radicals or appropriate anions or cations. Commercial atactic styrene polymers are, however, all almost free-radical products. Particular anionic processes are used to make research-grade polystyrenes with exceptionally narrow molecular weight distributions and the syndiotactic polymer is produced by metallocene catalysis. Cationic polymerization of styrene is not a commercial process. 

The electron ejected from the monomer molecule attaches to the double bond of another monomer molecule, which leads to an anion-free radical. In essence, the irradiation produces cation, anion, and free radical, any of which can initiate the monomer unaffected by the irradiation. Which end of the initiator, i.e., cation, anion, or free radical, initiates the polymerization is dependent on the nature of the double bond (electrophilic or electrophobic) and the purity of the monomer [2]. In some monomers in extremely high purity, polymerization proceeds by all three polymerizations, i.e., cationic, anionic, and free radical, as depicted schematically in Figure 5.1. 

Mechanism and kinetics of cationic poiymerization initiation. Unlike free-radical and anionic polymerization, initiation in cationic polymerization employs a true catalyst that is restored at the end of the polymerization and does not become incorporated into the terminated polymer chain. Initiation of cationic polymerization is brought about by addition of an electrophile to a monomer molecule. TVpical compounds used for cationic polymerization include protonic acids (e.g., H2SO4, H3PO4), Lewis acids (e.g., AICI3, BF3, TiCl4, SnCl4), and stable carbenium-ion salts (e.g., triphenylmethyl halides, tropylium halides) ... 

Table 5. Reduction potentials ( red), excited triplet (singlet for NTAB and CTAB) energies, free energy changes (AGet) and quenching rate constants (A q, measured for n-butyltriphenylborate anion) of the light absorbing molecules used as initiators of free-radical polymerization.

Radical anions of geminal nitro compounds in DMF cleave rapidly to nitrite and a radical which initiate a free radical chain in which a v/c-dinitro compound is formed [43]. Radical anions of other a-substituted nitroalkanes may cleave either the nitro group or the other substituent 1-cyano-l-nitrocyclohexane thus splits off nitrite whereas 1-nitrocyclo-hexyl p-tolyl sulfone loses p-tolylsulfinate [44]. 

Manganese has a rich history within the field of organic synthesis (363, 364). For example, the permanganate anion has long been used as an oxidant to produce a variety of products (363). Manganese111 acetate also has been extensive explored over the years for the initiation of free radical reactions that lead to carbon-carbon bond formation. These topics have been reviewed and will not be presented further here (363, 364). Manganese chemistry, however, has made an impact in other areas as well, notably the asymmetric epoxidation of alkenes. 

In chain polymerization initiated by free radicals, as in the previous example, the reactive center, located at the growing end of the molecule, is a free radical. As mentioned previously, chain polymerizations may also be initiated by ionic systems. In such cases, the reactive center is ionic, i.e., a carbonium ion (in cationic initiation) or a carbanion (in anionic initiation). Regardless of the chain initiation mechanism—free radical, cationic, or anionic—once a reactive center is produced it adds many more molecules in a chain reaction and grows quite large extremely rapidly, usually within a few seconds or less. (However, the relative slowness of the initiation stage causes the overall rate of reaction to be slow and the conver-... 

As ionic polymerizations with stringent reaction conditions are more difficult to carry out than normal free-radical processes, the latter are invariably preferred where both free-radical and ionic initiations give a similar product. For example, commercial polystyrenes are all free-radical products, though styrene polymerization can be initiated with free radicals as well as with appropriate anions or cations. However, to make research grade polystyrenes with exceptionally narrow molecular-weight distributions and di-block or multi-block copolymers of styrene and other monomers, ionic processes are necessarily employed. 

It requires a generator of active centers (usually an initiator for free radicals, anions, or cations). 

It would be natural to ask here what conditions are needed for a chain to start growing. And how does the process stop A reaction like (3.1) cannot begin of its own accord. To start such a reaction the active center (it may be a free radical, cation or anion) should be produced first, for this purpose chemists are normally using the so-called initiators — special substances which can generate active species. In a simple example the initiators easily decompose and form free radicals, i.e. molecules containing unpaired electrons the reaction initiated this way is called free-radical polymerization. Typical initiators for free radical polymerization are compounds with a labile bond, e.g. peroxide —0—0— hydrogen peroxide is the most well-known example, but most widely used in the reactions of the type (3.1) are organic peroxides, e.g. di-ieri-butyl peroxide ... 

PMMA and related acrylic and methacrylic polymers have seen wide application in medicine becairse of their low cost, straightforward polymerization initiation by free radical or anionic mechanisms, ease of processing, and generally inert qirality. Application areas include orthopedics, dentistry, controlled-release systems, cosmetics, and ophthalmology. However, there are many other applications of methacrylate and acrylate polymers in medicine, some with low levels of use and others under development at this time. PMMA is the most widely used of this family of polymers. PHEMA will be addressed separately below. 

It is not always easy to deduce the mechanism of a polymerization. In general, no reliable conclusions can be drawn solely from the type of initiator used. Ziegler catalysts, for example, consist of a compound of a transition metal (e.g., TiCU) and a compound of an element from the first through third groups (e.g., AIR3) (for a more detailed discussion, see Chapter 19). They usually induce polyinsertions. The phenyl titanium triisopropoxide/aluminum triisopropoxide system, however, initiates a free radical polymerization of styrene. BF3, together with cocatalysts (see Chapter 18), generally initiates cationic polymerizations, but not in diazomethane, in which the polymerization is started free radically via boron alkyls. The mode of action of the initiators thus depends on the medium as well as on the monomer. Iodine in the form of iodine iodide, I I induces the cationic polymerization of vinyl ether, but in the form of certain complexes DI I (with D = benzene, dioxane, certain monomers), it leads to an anionic polymerization of 1-oxa-4,5-dithiacycloheptane. 

In these reactions, polymerizations initiated by free radicals or anions are to be preferred over those initiated by cations, since the latter tend to give frequent transfer reactions. For example, in the cyclization of natural rubber with the aid of concentrated acids of Lewis acids, only three trans-annular rings on the average are obtained [see reaction (25-20)]. 

In this type of polymerisation an initiating molecule is required so that it can attack a monomer molecule to start the polymerisation. This initiating molecule may be a radical, anion or cation. Chain growth polymerisation is initiated by free-radical, anion or cation proceeded by three steps initiation, propagation and termination. The chemical nature of the substituent group determines the mechanism. 

Generic structures of most of the relevant petroleum-based cosmetic monomers used today are shown in Table 1. All of them are based on some type of carbon-carbon unsaturated (olefinic) double bond. Even the oxyalkylene monomers are simply activated olefinic materials. Understanding the nature of the polymerization reaction is not essential to understanding polymer functionality. However, most cosmetically relevant polymers are produced by some type of addition polymerization reactions, which are carbon-carbon bondforming reactions occurring across the unsaturated double bond of the monomers. These reactions can be initiated by free radicals, anions, or cations and can be mn in bulk, in solution, as suspensions, or even as emulsions. 

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