Cross-linking products

In some cases of multifunctional monomers the possibility exists for branches on branches, which ultimately result in cross-linked products and effectively infinite molecular weights well before p reaches unity. 

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. ... 

Sketch the structure of the average endene and centrene molecules, as well as the structure of one of the cross-linked products. 

A cross-linked product with unsaturation at the chain ends does, indeed, have a higher modulus. This could be of commercial importance and indicates that industrial products might be formed by a nonequilibrium process precisely for this sort of reason. 

Uses. Furfuryl alcohol is widely used as a monomer in manufacturing furfuryl alcohol resins, and as a reactive solvent in a variety of synthetic resins and appHcations. Resins derived from furfuryl alcohol are the most important appHcation for furfuryl alcohol in both utihty and volume. The final cross-linked products display outstanding chemical, thermal, and mechanical properties. They are also heat-stable and remarkably resistant to acids, alkaUes, and solvents. Many commercial resins of various compositions and properties have been prepared by polymerization of furfuryl alcohol and other co-reactants such as furfural, formaldehyde, glyoxal, resorcinol, phenoHc compounds and urea. In 1992, domestic furfuryl alcohol consumption was estimated at 47 million pounds (38). 

Many friction material formulations contain 5—15 wt % of friction particles, the granulated cross-linked products of the reaction of CNSL, a phenol substituted at the meta position with a unsaturated side chain, and formaldehyde. Friction particles range in size from 50 to 500 p.m. They reduce frictional wear and increase pedal softness (86). 

Polyurethane foams are formed by reaction with glycerol with poly(propylene oxide), sometimes capped with poly(ethylene oxide) groups with a reaction product of trimethylolpropane and propylene oxide or with other appropriate polyols. A typical reaction sequence is shown below, in which HO—R—OH represents the diol. If a triol is used, a cross-linked product is obtained. 

There is, quite clearly, scope or a very wide range of epoxy resins. The nonepoxy part of the molecule may be aliphatic, cycloaliphatic or highly aromatic hydrocarbon or it may be non-hydrocarbon and possibly polar. It may contain unsaturation. Similar remarks also apply to the chain extension/cross-linking agents, so that cross-linked products of great diversity may be obtained. In practice, however, the commercial scene is dominated by the reaction products of bis-phenol A and epichlorohydrin, which have some 80-90% of the market shtu"e. 

Ozone cracking is a physicochemical phenomenon. Ozone attack on olefinic double bonds causes chain scission and the formation of decomposition products. The first step in the reaction is the formation of a relatively unstable primary ozonide, which cleaves to an aldehyde or ketone and a carbonyl. Subsequent recombination of the aldehyde and the carbonyl groups produces a second ozonide [58]. Cross-linking products may also be formed, especially with rubbers containing disubstituted carbon-carbon double bonds (e.g. butyl rubber, styrene-butadiene rubber), due to the attack of the carbonyl groups (produced by cleavage of primary ozonides) on the rubber carbon-carbon double bonds. 

One of the most important furan resins from an industrial standpoint is undoubtedly that obtained from 2-furfuryl alcohol. The final cross-linked product displays outstanding chemical, thermal and mechanical properties8. ... 

Uvarova and Gorshkova131 reported the thermal polymerization of 2-furfuryl vinyl ether and of a series of vinyl ethers of alkyl-1-(2-furyl) eaibinols. Treatment of these compounds at 150 °C for 24 hours produced brown cross-linked products with 2-furfuryl vinyl ether, and soluble, low molecular-weight polymers with the others. It seems likely that in all these systems the ring participated in the growth of the chains, but the lack of experimental details and of structural determination makes it difficult to assess the results. 

In contrast with the above situation, the polymerization of 2-furfurylidene methyl ketone, di-2-fiirfurylidene ketone and their homologues has been the subject of a large volume of (mainly technical) publications because of the useful applications of the final cross-linked products. As pointed out in the introduction, this review does not deal with the technological aspects of furan resins and in this section only the mechanistic aspects of the first phase of these polymerizations will be discussed. 

C]methylated glycophorin work in this laboratory indicates that it results from head-to-head dimerization of glycophorin molecules. This phenomenon could be studied by isolation of the cross-linked product (intact, or enzymically digested). 

There are examples in which base radicals undergo reaction with adjacent base residues. The 5-(2 -deoxyuridinyl)methyl radical (63, Scheme 8.30) can forge an intrastrand cross-link with adjacent purine residues. Cross-link formation is favored with a guanine residue on the 5 -side of the pyrimidine radical and occurs under low-oxygen conditions. A mechanism was not proposed for this process, but presumably the reaction involves addition of the nucleobase alkyl radical to the C8-position of the adjacent purine residue. Molecular oxygen likely inhibits crosslink formation by trapping the radical 63, as shown in Scheme 8.24. The radical intermediate 89 must undergo oxidation to yield the final cross-linked product 90,... 

Note Cross-linked products are shown with Z representing any of the functional groups in the first column. 

Chlorosulphonated polyethylene is obtained by reacting low density polythylene with chlorine in presence of sulphur dioxide using carbon tetrachloride as solvent. The product contains 30 per cent chlorine and 1.5 per cent sulphur. This is a sticky rubbery material and is soluble in chlorinated hydrocarbon solvents. It can be vulcanised by heating with metal oxides like litharge or magnesium oxide in presence of water. The cross-linked product is found to be resistant to chemical attack and is used in gaskets, hoses, etc. 

To get polyurethane foams the polymer is formed along with gas evolution. When these two processes take place simultaneously the gas bubbles are trapped in polymer matrix yielding a cellular product. Slightly cross-linked products are flexible while highly cross-linked products are rigid. Both flexible and rigid foams are of commercial importance. 

Thus the approach described can be apolied to the description of the kinetics of intramolecular cross-linking and the conformational properties of cross-linked products. 

Reaction with monofunctional metal containing reactants leads to the formation of linear products (form I.) which are soluble in a number of polar solvents, whereas reaction with difunctional reactants leads to formation of cross-linked products (form II.) which are insoluble in all attempted solvents. 

An unsaturated polyester resin consists of a linear polyester whose chain contains double bonds and an unsaturated monomer such as styrene that copolymerizes with the polyester to provide a cross-linked product. The most common unsaturated polyester is made by step growth polymerization of propylene glycol with phthalic and maleic anhydrides. Subsequent treatment with styrene and a peroxide catalyst leads to a solid, infusible thermoset. 

Alkyds were synthesized by Kienle in the 1920s from trifunctional alcohols and dicar-boxylic acids. Unsaturated oils, called drying oils, were transesterified with the phthalic anhydride in the reaction so that an unsaturated polymer was obtained which could later be reacted producing a cross-linked product. 

Cross-linked products are formed when the functionality of either reactant is greater than 2. Linear products are formed when the functionality of both reactants is 2. 

While termination leads to the irreversible disappearance of an active center, chain transfer results in the growth of a second chain while the first one is terminated. Here, the active center is transferred to another molecule (solvent, initiator, monomer,...) where it is able to initiate further chain growth. The resulting dead polymer, on the other hand, can continue its growth only when activated in a subsequent transfer step. Because this re-activation in general does not occur at the terminal monomer unit but somewhere in the chain, branched or cross-linked products will result ... 

The formation of the cross-linked products demonstrated an apparent correlation to H bonding sequences. What is the mechanism behind the observed sequence specificity One of two possible mechanisms, one kinetic and the other thermodynamic, may be involved in aqueous media. The kinetic mechanism involves intermolecular... 

Based on these results, the possibility of a kinetic mechanism for the sequence-specific cross-linking of 19 and 20, and similarly, 21 and 22, was mled out, which left the thermodynamic mechanism that requires the selective stabilization of the sequence-matched, cross-linked products. The observed sequence specificity is likely to be due to the stabilization of the products by intramolecular H bonds... 

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