Polymerization double bonds

Synthetic polymers can be classified as either chain-growth polymen or step-growth polymers. Chain-growth polymers are prepared by chain-reaction polymerization of vinyl monomers in the presence of a radical, an anion, or a cation initiator. Radical polymerization is sometimes used, but alkenes such as 2-methylpropene that have electron-donating substituents on the double bond polymerize easily by a cationic route through carbocation intermediates. Similarly, monomers such as methyl -cyanoacrylate that have electron-withdrawing substituents on the double bond polymerize by an anionic, conjugate addition pathway. 

Ionic ROP shows most of the characteristics described in Chap. 5. There is minimal discussion in this chapter of those characteristics that are similar to those for carbon-carbon and carbon-oxygen double-bond polymerizations. Ionic ROP shows analogous effects of solvent and counterion, propagation by different species (covalent, ion pair, free ion), and association phenomena. 

Polymers containing rings incorporated into the main chain (e.g., by double-bond polymerization of a cycloalkene) are also capable of exhibiting stereoisomerism. Such polymers possess two stereocenters—the two atoms at which the polymer chain enters and leaves each ring. Thus the polymerization of cyclopentene to polycyclopentene [IUPAC poly(cyclopen-tane-l,2-diyl)] is considered in the same manner as that of a 1,2-disubstituted ethylene. The... 

Cyclopentene yields mixtures of ROMP and double-bond polymerization with some Ti and V initiators. ROMP occurs exclusively with molybdenum and tungsten initiators, as well as Re, Nb, and Ta initiators. The relative amounts of cis and trans structures vary with the initiator and temperature [Dall Asta et al., 1962 Pampus and Lehnert, 1974]. Metallocene initiators polymerize cyclopentene through the double bond, but the polymer structure consists of cis 1,3-placement (Coates, 2000 Kaminsky, 2001 Kelly et al., 1997]. 

Cyclohexene does not polymerize by either route except when it is part of a bicyclic structure as in norbornene. Stereochemistry in the ROMP of norbomene is complicated since the polymer, LXVI in Sec. 7-8, has possibilities of isomerism at both the ring and the double bond. Most polymerizations by the typical ROMP initiators yield cis stereochemistry at the cyclopentane ring with varying amounts of cis and trans placements at the double bond [Ivin, 1987]. Metallocene initiators yield predominantly double-bond polymerization with 1,2-placement [Janiak and Lassahn, 2001]. 

Little is known about the R/S isomerism (i.e., erythro and threo ditactic structures are possible) at the stereocenters that result from double-bond polymerization. Cycloheptene and higher cycloalkenes undergo only ROMP double-bond polymerization does not occur because the larger rings can accommodate the double bond without being highly strained. 

The anionic polymerization of 1,3-dienes yields different polymer structures depending on whether the propagating center is free or coordinated to a counterion [Morton, 1983 Quirk, 2002 Senyek, 1987 Tate and Bethea, 1985 Van Beylen et al., 1988 Young et al., 1984] Table 8-9 shows typical data for 1,3-butadiene and isoprene polymerizations. Polymerization of 1,3-butadiene in polar solvents, proceeding via the free anion and/or solvent-separated ion pair, favors 1,2-polymerization over 1,4-polymerization. The anionic center at carbon 2 is not extensively delocalized onto carbon 4 since the double bond is not a strong electron acceptor. The same trend is seen for isoprene, except that 3,4-polymerization occurs instead of 1,2-polymerization. The 3,4-double bond is sterically more accessible and has a lower electron density relative to the 1,2-double bond. Polymerization in nonpolar solvents takes place with an increased tendency toward 1,4-polymerization. The effect is most pronounced with... 

An important quantity that can be deduced from the reaction profile is the rate of the cross-linking polymerization (Rp), i.e., the number of double bonds polymerized or of cross-links formed per second. Rp values were determined from the maximum slope of the kinetic curves (usually reached for conversion degrees between 20 and 40%). Table I summarizes the Rp values for the two photoresists tested under various conditions, namely conventional UV and continuous or pulsed laser irradiation at different light intensities. According to these kinetic data, Rp increases almost as fast as the light-intensity the ratio Io/Rp which is directly related to the product of the light-intensity and the required exposure time was found to vary only in the range 10-8 to... 

Fig. 14 Accumulated weight fraction distribution development with and without terminal double bond polymerization...

Nonlinear polymer formation in emulsion polymerization is a challenging topic. Reaction mechanisms that form long-chain branching in free-radical polymerizations include chain transfer to the polymer and terminal double bond polymerization. Polymerization reactions that involve multifunctional monomers such as vinyl/divinyl copolymerization reactions are discussed separately in Sect. 4.2.2. For simplicity, in this section we assume that both the radicals and the polymer molecules that formed are distributed homogeneously inside the polymer particle. 

Cationic polymerization is applied almost exclusively to monomers with olefinic double bonds. Susceptible are double bonds whose carbon atoms carry electron-donating substituents such as alkyl groups. Thus, isobutene with two methyl groups adjacent to the double bond polymerizes readily, propene with only one is sluggish, and ethene with none is inert a-methyl styrene is more reactive than styrene vinyl ethers are reactive, but vinyl chloride is not. The most important commercial product is butyl rubber, produced by copolymerization of isobutene with small amounts of isoprene, initiated by A1C13, BF3, or TiCl4 [82]. 

Reduction of the double bond Polymerization Nitroacctylcncs Polynitro aliphatic compounds Nitration of hydrt>carbons Substitution of halogen Electrolytic methtnls Addition reaction Michael addition Diels-Aider addition Oxidative dimerization a. u>-Dinitroalkanes gcm-Dinitroalkanes I rinitromethane (nitroform) derivatives Properties of nitroform Manufacture of nitroform... 

The development takes into account transfer to monomer, transfer to polymer, and terminal double bond polymerization. For the vinyl acetate system where transfer to monomer is high, the generation of radicals by transfer to monomer is much greater than the generation of radicals by initiation, so that essentially all radicals present have terminal double bonds hence, effectively all dead polymer molecules also have a terminal double bond. Thus, for vinyl acetate polymerization, the terminal double bond polymerization can be significant, and has been built into the development. The equations for the moments of the molecular weight distribution and the average number of branches per polymer molecule are as follows ... 

It thus appears that the principal structural features found In poly(1,4-dimethylenecyclohexane) can be explained by conventional carbonium ion chemistry. There is no indication that cyclopolymerization occurs in these polymerizations and there is much evidence to indicate that the double bonds present in this diene react independently. Some of them are involved in polymerization reactions, but a large proportion isomerize to relatively stable endocyclic double bonds. Under polymerization conditions where isomerization is favorable, soluble, unsaturated polymers having complex structures are obtained. When isomerization reactions are not favorable (low temperatures, use of Ziegler-Natta catalysts), the double bonds polymerize independently and crosslinked products are obtained. 

Miscellaneous Routes. Polyamides have been prepared by other reactions, including addition of amines to activated double bonds, polymerization of isocyanates, reaction of formaldehyde with dinitriles, reaction of dicarboxylic acids with dllsocyanates, reaction of carbon suboxide with diamines, and reaction of diazlactones with diamines. These reactions are reviewed in Reference 4. 

Tip 8 Terminal double bond polymerization. Transfer to monomer and termination by disproportionation lead to dead polymer molecules with a TDB. This TDB may react with a polymer radical, thus forming a radical center somewhere along the chain of the combined molecules. This radical center, on propagation with monomer, will evenmally form a trifunctionally branched chain. 

The bismaleimides can be reacted with a variety of bifimctional compounds to form polymers by rearrangement reactions. These include amines, mercaptans, and aldoximes (Figure 4.22). If the reaction is carried out with a deficiency of the bifunctional compound, the polymer will have terminal double bonds to serve as a cure site for the formation of a cross-linked polymer via a double bond polymerization mechanism during molding. The cross-linked in this case occurs without the formation of any volatile by-products. 

Free-radical polymerization, like coordination polymerization discussed in Chapter 2, involves the sequential addition of vinyl monomer(s) to an active center. For FRP the active centers are free radicals. The increase in chain length is very rapid an individual chain is initiated, grows to high MW and is terminated in a few seconds or less. After termination, the high-MW polymer chain does not react further (barring side reactions such as chain transfer to polymer or terminal/internal double bond polymerization) and is considered dead . Dead chains have a residence time of minutes or hours in the reactor, such that the final polymer product is an intimate mixture of chains formed under time and/or spatially varying conditions. 

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