Unsaturated polymers double bonds

The presence of the unsaturated substituent along this polyester backbone gives this polymer crosslinking possibilities through a secondary reaction of the double bond. These polymers are used in paints, varnishes, and lacquers, where the ultimate cross-linked product results from the oxidation of the double bond as the coating cures. A cross-linked polyester could also result from reaction (5.J) without the unsaturated carboxylic acid, but the latter would produce a gel in which the entire reaction mass solidified and is not as well suited to coatings applications as the polymer that crosslinks upon drying. ... 

Photoinitiation. Since photolysis of polysdanes generates sdyl radicals, which can add to carbon—carbon double bonds, these polymers have been used for the free-radical polymerization of unsaturated organic monomers (135,136). Though about one-tenth as efficient as other organic photoinitiators, polysdanes are nevertheless quite insensitive to oxygen effects, which somewhat compensates for their lower efficiency. 

Until now the discussion has centered on the addition polymers obtained fiom unsaturated monomers by reaction of the C=C or C=0 double bond. However, polymers obtained by other methods (ring-opening polymerization, polycondensation, etc.) offer interesting stereochemical phenomena also. As a rule, in these classes of macromolecular compounds the monomer units are clearly defined, the direction of the chain is often distinguishable and the stereo-isomeric elements present in the chain already preexist in the monomer. There are, however, numerous exceptions and further clarification is called for. 

Norbornene and norbornene methyl ester have been polymerized forming a polymer that lacks backbone carbon-carbon double-bond unsaturation. The polymerization catalyst mixture consisted of palladium acetate, a phosphine such as tricyclohex-ylphosphine, a Lewis acid such as dimethyl zinc, and hexafluoroisopropanol. Polynorbornenes prepared in this manner typically had M s> 200,000 daltons with polydispersies less than 2, while poly(norbornene methyl ester) had M s of roughly 100,000 daltons. 

Unsaturated polyesters contain a relatively high number of double bonds per polymer molecule as well as reactive end groups which remain unreacted after the cross-linking reaction. These unreacted groups have a negative effect on the network properties. These unfavorable structural defects are largely eliminated in what is known as vinyl ester resins ... 

Owing to their double bonds, unsaturated HRs can be grafted with polymers or with various chemical functions. Thus, unsaturated HRs are dissolved in vinyl monomers then they can be polymerized in solution or emulsion. In the same way, polystystyrene or poly(methyl methacrylate) can be grafted on liquid polybutadiene [35]. 

Because no molecule is spHt out, the molecular weight of the repeating unit is identical to that of the monomer. Vinyl monomers, H2C=CHR (Table 2) undergo addition polymerization to form many important and familiar polymers. Diene (two double bonds) monomers also undergo addition polymerization. Normally, one double bond remains, leaving an unsaturated polymer, with one double bond per repeating unit. These double bonds provide sites for subsequent reaction, eg, vulcanization. 

Allyl alcohol, CH2=CH—CH2OH (2-propen-l-ol) [107-18-6] is the simplest unsaturated alcohol. One hydrogen atom can easily be abstracted from the aHyhc methylene (—CH2—) to form a radical. Since the radical is stabilized by resonance with the C=C double bond, it is very difficult to get high molecular weight polymers by radical polymerization. In spite of the fact that aHyl alcohol has been produced commercially for some years (1), it has not found use as a monomer in large volumes as have other vinyl monomers. 

The use of TAG as a curing agent continues to grow for polyolefins and olefin copolymer plastics and mbbers. Examples include polyethylene (109), chlorosulfonated polyethylene (110), polypropylene (111), ethylene—vinyl acetate (112), ethylene—propylene copolymer (113), acrylonitrile copolymers (114), and methylstyrene polymers (115). In ethylene—propylene copolymer mbber compositions. TAG has been used for injection molding of fenders (116). Unsaturated elastomers, such as EPDM, cross link with TAG by hydrogen abstraction and addition to double bonds in the presence of peroxyketal catalysts (117) (see Elastol rs, synthetic). 

Ozonc-rcsjstant elastomers which have no unsaturation are an exceUent choice when their physical properties suit the appHcation, for example, polyacrylates, polysulfides, siHcones, polyesters, and chlorosulfonated polyethylene (38). Such polymers are also used where high ozone concentrations are encountered. Elastomers with pendant, but not backbone, unsaturation are likewise ozone-resistant. Elastomers of this type are the ethylene—propylene—diene (EPDM) mbbers, which possess a weathering resistance that is not dependent on environmentally sensitive stabilizers. Other elastomers, such as butyl mbber (HR) with low double-bond content, are fairly resistant to ozone. As unsaturation increases, ozone resistance decreases. Chloroprene mbber (CR) is also quite ozone-resistant. 

The terminal double bond is active with respect to polymerisation, whereas the internal unsaturation remains in the resulting terpolymer as a pendent location for sulfur vulcanisation. The polymer is poly(ethylene- (9-prop5iene- (9-l,4-hexadiene) [25038-37-3]. 

An alternative route to cross-linking is to start with a linear polymer and then cross-link the molecules by tying the molecule through some reactive group. For example, it is possible to cross-link unsaturated polyesters by an addition polymerisation across the double bond as shown schematically in Figure 2.12. 

Like NR, SBR is an unsaturated hydrocarbon polymer. Hence unvulcanised compounds will dissolve in most hydrocarbon solvents and other liquids of similar solubility parameter, whilst vulcanised stocks will swell extensively. Both materials will also undergo many olefinic-type reactions such as oxidation, ozone attack, halogenation, hydrohalogenation and so on, although the activity and detailed reactions differ because of the presence of the adjacent methyl group to the double bond in the natural rubber molecule. Both rubbers may be reinforced by carbon black and neither can be classed as heat-resisting rubbers. 

Their physieal properties are essentially those of the alkanes. It is the unsaturated linkages that dominate the ehemistry and the main reaetion is one of addition (e.g. hydrogen, halogen, and hydrogen halides) aeross the double bond to produee saturated eompounds. This reaetivity is utilized in the manufaeture of long-ehain polymers, e.g. polyethylene and polypropylene. 

Butyl rubber (BR) and polyisobutylene (PIB) are widely used in adhesives as primary elastomeric binders and as tackifiers and modifiers. The main difference between these polymers is that butyl is a copolymer of isobutylene with a minor amount of isoprene (which introduces unsaturation due to carbon-carbon double bonds), while polyisobutylene is a homopolymer. 

Degradation of rubbers and resins can also be produced by ozone attack. Ozone directly reacts with, and cleaves, the carbon-carbon double bonds of rubbers and resins. Thus only polymers with backbone unsaturation will be cracked by ozone. Unlike oxidation, ozone attack cannot be accelerated by increasing the... 

Ozone adds readily to unsaturated organie eompounds and ean eause unwanted eross-linking in rubbers and other polymers with residual unsaturation, thereby leading to brittleness and fraeture. Addition to alkenes yields ozonides whieh ean be reduetively eleaved by Zn/H20 (or I /MeOH, ete.) to yield aldehydes or ketones. This smooth reaetion, diseovered by C. D. Harries in 1903, has long been used to determine the position of double bonds in organie moleeules, e.g. ... 

Braun [22] showed from ozonolysis that for fractions of bulk PVC the number of internal double bonds and the rate of thermal degradation, although dependent on each other, were independent of the molecular weight. This clearly demonstrated the role of internal unsaturation on the stability of the polymer. After careful chlorination of the double bonds, an increase in thermal stability was observed and the number of double bonds as determined by oxidation with potassium permanganate were reduced. It was also shown that one polyene sequence was formed from each isolated double bond. 

The classic chemical technique for measuring the degree of unsaturation in diene polymers is iodometry (iodine value) [102]. Kubo et al. [103] extensively measured the iodine value to determine the amount of residual double bonds present in the HNBR. However, this method exhibited significantly poorer precision as compared with IR and NMR spectroscopies [99-101]. Acid... 

Synthetic polymers are classified by their method of synthesis as either chain-growth or step-growth. The categories ate somewhat imprecise but nevertheless provide a useful distinction. Chain-growth polymers are produced by chain-reaction polymerization in which an initiator adds to a carbon-carbon double bond of an unsaturated substrate (a vinyl monomer) to yield a reactive inter-... 

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