Polymerizations 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. 

The above results proved the potential viability of the adsorbed hydrophilic macromolecules as bonded phases in chromatography of biopolymers but it must be admitted that additional crosslinking of previously adsorbed macromolecules is usually needed in order to obtain stable composites. The cross-linked bonded polymeric phases, however, may suffer from the restricted flexibility of the chain segment and their steric repellency may be diminished. Moreover, the conformational adaptivity of cross-linked chains for binding with solutes is poorer than that of grafted or chemically bound macromolecules. 

Gem-dihydroperoxide (2), > /O-OH C no-oh W sol and decomp slowly with evoln of 02 and formation of expl polymeric peroxides can be reduced and hydrolyzed thermal decompn leads to pro- orpccivp Vinn/1 plptivacrp nf —0—0—, C—0— and then C—C bonds Polymeric peroxides formed as a decompn product are extremely expl... 

Some [MX]+ ions enter into reactions in which the ligand X and the reacting molecule become chemically bonded. Polymerization processes have been observed involving the [MC4H4]+ ions (147). The butadiene complex ions [MC4H4]+ of Co and Ni are unreactive to ethyne but the Fe, Ru, and Rh ions react to yield benzene and the bare metal ion. The [MC4H4]+ complex ions of Os+, Ir+, and Pt+ react with ethyne to form the MC4I I4 + ions that probably correspond to the benzyne complexes previously observed for platinum (126). 

Noble metal films, 15 251 Noble metal nanoparticles, 26 805 Noble metal recovery, in photocatalytic water decontamination, 19 87-89 N-0 bond polymerization inhibitors,... 

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... 

Proanthocyanidins (PAs, syn condensed tannins) are polymeric flavan-3-ols whose elementary units are linked by C-C and occasionally C-O-C bonds (polymerization degree between 3 and 11), Fig. (12) [19]. 

In addition (chain) polymerization, monomers containing an unsaturated (vinyl) bond polymerize in the presence of an initiator, which generates an active site at the end of the chain. Several chemical reactions take place simultaneously in the course of the polymerization. First, an initiation reaction via photo- or heat-decomposition of the initiator occurs to form the active species, which are either peroxides or azo compounds. The active species react with a monomer to generate the active site (i.e., initiation). 

Melamines and barbiturates or cyanurates are complementary with respect to their ability to form hydrogen bonds. Three intermolecular hydrogen bonds between adjacent molecules may lead to hydrogen-bonded polymeric or oligomeric structures in the crystalline state.278... 

Fig. 9.5 Singly bonded polymeric nitrogen (cubic modification).

Figure I. Effect of heating of soy milk before drying and effect of addition of N-ethylmaleimide (NEMI) to heated soy milk on the insolubilization of protein after drying. The curves are (a), dried without adding NEMI (b), dried after adding SEMI and (C), the values of (a) minus the values of (b). Curve (a) indicates total amount of insolubilized protein curve (b) indicates the amount of protein insolubilized by mechanisms other than by intermolecular disulfide bond formation and curve (c) indicates the amount of protein insolubilized through disulfide bond polymerization (3).

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. 

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