Free radical polymerization, alkyl vinyl

The rates of radical-forming thermal decomposition of four families of free radical initiators can be predicted from a sum of transition state and reactant state effects. The four families of initiators are trarw-symmetric bisalkyl diazenes,trans-phenyl, alkyl diazenes, peresters and hydrocarbons (carbon-carbon bond homolysis). Transition state effects are calculated by the HMD pi- delocalization energies of the alkyl radicals formed in the reactions. Reactant state effects are estimated from standard steric parameters. For each family of initiators, linear energy relationships have been created for calculating the rates at which members of the family decompose at given temperatures. These numerical relationships should be useful for predicting rates of decomposition for potential new initiators for the free radical polymerization of vinyl monomers under extraordinary conditions. 

Substituted olefins that are capable of forming secondary or tertiary carbo-nium ion intermediates polymerize well by cationic initiation, but are polymerized with difficulty or not at all free radically. In general, vinyl or /-alkenes that contain electron donating groups (alkyl, ether, etc) polymerize well via a carbo-cationic mechanism. 

Use of triphenylmethyl and cycloheptatrienyl cations as initiators for cationic polymerization provides a convenient method for estimating the absolute reactivity of free ions and ion pairs as propagating intermediates. Mechanisms for the polymerization of vinyl alkyl ethers, N-vinylcarbazole, and tetrahydrofuran, initiated by these reagents, are discussed in detail. Free ions are shown to be much more reactive than ion pairs in most cases, but for hydride abstraction from THF, triphenylmethyl cation is less reactive than its ion pair with hexachlorantimonate ion. Propagation rate coefficients (kP/) for free ion polymerization of isobutyl vinyl ether and N-vinylcarbazole have been determined in CH2Cl2, and for the latter monomer the value of kp is 10s times greater than that for the corresponding free radical polymerization. 

For free-radical polymerization, classical results have been obtained concerning the tacticity of hydroxytelechelic poly(methyl methacrylate)109) and copolymers, 46) initiated by H202/UV. Most of the units are in a syndiotactic (64 %) or heterotactic (30 %) configuration. For poly(vinyl acetate) obtained in the presence of H202 at 120 °C 98), the polymer contains less syndiotactic (22%) and somewhat more heterotactic (38%) units with 80% of head-to-tail linkage mode. For the copolymerization of alkyl methacrylate by the H202/UV system113) quite different results, explained by the nature of the medium, especially by the solubility effect (see Table 1.1), have been obtained. 

The vinyl silane reacts more slowly than the methacrylate in free-radical polymerizations. In addition, vinyl silanes are known to hydrolyze relatively quickly compared to alkyl-modified silanes [4], These two facts make it reasonable to assume that the cause of the oligomerization seen... 

Extensive work studying the interaction of poly(N-ethyl-4-vinylpyridinium bromide) (PEVP) and related (partially alkylated) cationic copolymers with anionic lipid membranes has been performed by Yaroslavov, Menger, and Kabanov. In all cases, 4-vinyl pyridine was free radically polymerized and postfunctionalized with alkyl bromides (Figure 17). The initial... 

On the other hand, butyllithium-aluminum alkyl initiated polymerizations of vinyl chloride are unaffected by free-radical inhibitors. Also, the molecular weights of the resultant polymers are unaffected by additions of CCI4 that acts as a chain-transferring agent in free-radical polymerizations. This suggests an ionic mechanism of chain growth. Furthermore, the reactivity ratios in copolymerization reactions by this catalytic system differ from those in typical free-radical polymerizations An anionic mechanism was also postulated for polymerization of vinyl chloride with t-butylmag-nesium in tetrahydrofuran. ... 

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. 

Alternating copolymers ( interpolymers ) of maleic anhydride and alkyl vinyl ethers are prepared by free radical polymerization in solvent, non-solvent or bulk media Products prepared from ethyl-, butyl-, hexyl-, and octyl-, and decyl- vinyl ethers are described in Table I. The decyl-copolymer was prepared by Dr A. W. Schultz. A methyl-copolymer ( Gantrez AN General Aniline and Film Corp.), not described in Table I, was included in some studies. Molecular weight estimates were... 

Other monomers that copolymerize with alkyl vinyl ethers are vinyl ketones [47], acrolein diacetate [48], acrylamide [49], alkoxy 1,3-butadienes [50], butadiene [51], chloroprene [52], chlorotrifluoroethylene [53], tri-and tetrafluoroethylene [54], cyclopentadiene [55], dimethylaminoethyl acrylate [56], fluoroacrylates [57], fluoroacrylamides [58], A-vinyl car-bazole [59,60], triallyl cyanurate [59,60], vinyl chloroacetate [61,62], N-vinyl lactams [63], A-vinyl succinimide [63], vinylidene cyanide [64, 65], and others. Copolymerization is especially suitable for monomers having electron-withdrawing groups. Solution, emulsion, and suspension techniques can be used. However, in aqueous systems the pH should be buffered at about pH 8 or above to prevent hydrolysis of the vinyl ether to acetaldehyde. Charge-transfer complexes have been suggested to form between vinyl ethers and maleic anhydride, and these participate in the copolymerization [66]. Examples of the free-radical polymerization of selected vinyl ethers are shown in Table IV. 

Normally, free radical polymerization results in essentially atactic polymers although steric and electrostatic effects generally favour one configuration or the other and truly random polymers are probably rare. In some cases, free radical polymerization results in polymers which are largely syndiotactic examples of monomers which behave in this way are chlorotri-fluoroethylene, methyl methacrylate and vinyl acetate. More commonly, however, highly tactic polymers are prepared by means of ionic polymerizations. Most of the common vinyl polymers have been obtained in isotactic and syndiotactic forms by the use of ionic initiators, particularly lithium alkyls and Ziegler-Natta catalysts. 

The reactions of alkyl hydroperoxides with ferrous ion (eq. 11) generate alkoxy radicals. These free-radical initiator systems are used industrially for the emulsion polymerization and copolymerization of vinyl monomers, eg, butadiene—styrene. The use of hydroperoxides in the presence of transition-metal ions to synthesize a large variety of products has been reviewed (48,51). 

The main industrial use of alkyl peroxyesters is in the initiation of free-radical chain reactions, primarily for vinyl monomer polymerizations. Decomposition of unsymmetrical diperoxyesters, in which the two peroxyester functions decompose at different rates, results in the formation of polymers of enhanced molecular weights, presumably due to chain extension by sequential initiation (204). 

Cationic Polymerization. For decades cationic polymerization has been used commercially to polymerize isobutylene and alkyl vinyl ethers, which do not respond to free-radical or anionic addition (see Elastomers, synthetic-BUTYLRUBBEr). More recently, development has led to the point where living cationic chains can be made, with many of the advantages described above for anionic polymerization (27,28). 

The refined grade s fastest growing use is as a commercial extraction solvent and reaction medium. Other uses are as a solvent for radical-free copolymerization of maleic anhydride and an alkyl vinyl ether, and as a solvent for the polymerization of butadiene and isoprene usiag lithium alkyls as catalyst. Other laboratory appHcations include use as a solvent for Grignard reagents, and also for phase-transfer catalysts. 

The free radical initiators are more suitable for the monomers having electron-withdrawing substituents directed to the ethylene nucleus. The monomers having electron-supplying groups can be polymerized better with the ionic initiators. The water solubility of the monomer is another important consideration. Highly water-soluble (relatively polar) monomers are not suitable for the emulsion polymerization process since most of the monomer polymerizes within the continuous medium, The detailed emulsion polymerization procedures for various monomers, including styrene [59-64], butadiene [61,63,64], vinyl acetate [62,64], vinyl chloride [62,64,65], alkyl acrylates [61-63,65], alkyl methacrylates [62,64], chloroprene [63], and isoprene [61,63] are available in the literature. 

Photoinduced free radical graft copolymerization onto a polymer surface can be accomplished by several different techniques. The simplest method is to expose the polymer surface (P-RH) to UV light in the presence of a vinyl monomer (M). Alkyl radicals formed, e.g. due to main chain scission or other reactions at the polymer surface can then initiate graft polymerization by addition of monomer (Scheme 1). Homopolymer is also initiated (HRM-). 

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