Formation of crosslinks

In distinction to other esters of acrylic acids containing double bonds in the alcohol radical and, therefore exhibiting a tendency to cyclopolymerization43 and formation of crosslinked polymers, 10 reacts with AN in DMF solution41 or in benzene/DMF42 only with the vinyl group of the acid part due to deactivation of the double bond in the 3-chloro-2-butenyl group by the chlorine atom. The copolymer of structure 11 is formed. 

Other subgroups of alkylating agents are the nitrosoureas (examples carmustine, BCNU lomustine, CCNXJ) and the triazenes (example dacarbazine, DTIC). Platinum derivatives (cisplatin, carboplatin, oxaliplatin) have an action that is analogous to that of alkylating agents (formation of crosslinks) and therefore are appended to this class, as well. 

Summarizing the fibrinolytic therapy, it should be emphasized that efficient treatment needs urgent application of plasminogen activator (within a few hours) to prevent the formation of crosslinks in the fibrin structure (Fig. 2) and to find the localization of thrombus to emerge plasmin on the surface of fibrin to prevent rapid inactivation of the enzyme by the inhibitor system of fibrinolysis (Fig. 3). 

Oxygen-free reactions of psoralens, when in close proximity to the target, proceed via the first excited states in which the 3,4-and the 4, 5 7r-bonds of the pyrone and furan moieties, respectively, can undergo C4-cyclization reactions with, e.g., unsaturated bonds of lipids, or the C5=C6 double bonds of thymine in DNA. In reactions with DNA the psoralen is believed to intercalate with DNA in the dark. Subsequent irradiation at 400 nm usually leads to furan-side 4, 5 -monoadduct formation, whereas irradiation at 350 nm increases the formation of crosslinks in which the furan and pyrone rings form C4 cycloadducts to thymines on opposite strands [95], Subsequent irradiation of the 4, 5 -monoadducts at 350 nm leads to formation of crosslinks and conversion into pyrone-side 3,4-monoadducts. Shorter wave-... 

The formation of crosslinks in silk fibroin increases the tenacity and resistance to deformation of the fibres, as reflected in the initial modulus and the yield point. This protective effect conferred by fixation of the bifunctional dye Cl Reactive Red 194 was not shown by the monofunctional Orange 16, which is unable to form crosslinks. The loss in tenacity of undyed silk that is observed on treatment at 90 °C and pH 7 for 2 hours is attributable to lowering of the degree of polymerisation (DP) by hydrolysis of peptide bonds. The crosslinking action of bifunctional dyes tends to compensate for this loss in DP and provides an intermolecular network that helps to maintain the physical integrity of the fibre structure [124] ... 

Heating of foods rich in proteins may lead to formation of crosslinking isopeptide bonds between the S-NH2 group of lysine and the p- and y-carboxyl groups of aspartic and glutamic acid residues or their amides. 

Scheme 8.11 Thermodynamic formation of crosslinked polymer 54 via radical crossover reaction of alkoxyamines in copolymers 52 and 53 [42],...

Sixou et al. (101) showed the circular dichroism of cholesteric CTA solutions in TFA depends on the CTA molecular weight. The intensity of the circular dichroic peak increases with molecular weight. Meeten and Navard (97) studied gel formation and liquid crystallinity in TFA-H2O solutions of CTA. When water was added to a liquid crystalline solution of CTA in TFA a gel phase formed presumably by the formation of crosslinks due to hydrogen bonding. They interpreted their results that liquid crystalline ordering involves both inter- and intramolecular forces. 

Since the fatty acid chains in each lipid were 18 carbons and 16 carbons, respectively, it is reasonable that they could form a mixed lipid phase. Furthermore the bis-dienoyl substitution of 15 favors the formation of crosslinked polymer networks. Ohno et al. showed that the dienoyl group associated with the sn-1 chain could be polymerized by lipid soluble initiators, e.g. AIBN, whereas the dienoyl in the sn-2 chain was unaffected by AIBN generated radicals. Conversely, radicals from a water-soluble initiator, e.g. azo-bis(2-amidinopropane) dihydrochloride (AAPD), caused the polymerization of the sn-2 chain dienoyl group, but not the sn-1 chain. These data provide clear evidence for the hypothesis of Lopez et al. that the same reactive group located in similar positions in the sn-1 and sn-2 chains of polymerizable 1,2-diacyl phospholipids are positionally inequivalent [23]. 

By far the most interesting application to date is the ability to crosslink polymers in the solid state, and much research has been devoted to studying the reactions involved. The initial physical process of energy absorption and the final chemical change—formation of crosslinks—can be readily determined. However, there is still considerable doubt as to the intermediate reactions, and this problem offers an appropriate start of this review. 

As shown in Table II, in the presence of polymer, the enclosed nitrous oxide is completely consumed during irradiation. In the place of nitrous oxide, nitrogen and water are formed. The yield of nitrogen or water corresponds stoichiometrically to the loss of nitrous oxide. A large G value, about 2000, is given for the disappearance of nitrous oxide. Estimation of the G value is based on the assumption that the available energy for the consumption is only that absorbed directly by the gas dissolved in the polymer solid. The G values for the formation of water and nitrogen should be equal to 2000. Moreover, the summation of the amount of the excess formation of crosslinks and unsaturation becomes stoichiometrically almost equal to the loss of nitrous oxide, as shown in Table III. The equation of material balance of nitrous oxide, therefore, should be written as follows ... 

These schemes satisfy the fact that the summation of G values for the excess formation of crosslinks and unsaturation becomes almost equal... 

Cationic polymerization of furan (53) and alkylated furans is often complicated by the formation of crosslinked, insoluble materials (77MI11102). Early reports postulated 1,2-polyaddition as a primary reaction followed by crosslinking through the resultant dihydrofuran moieties. More recent results (80MI11106) shed considerable doubt on the 1,2-addition sequence, at least beyond the trimer stage. The ultimately formed poly(furans) are believed to be composed mainly of units with structural features similar to those of tetramers (54 Scheme 14). 

The concentration of modifier will tend to diminish during the reaction, and the more reactive the modifier, the more serious is the effect likely to be. An extension of the Flory treatment of crosslinking to take into account the presence of modifier demonstrates the point. When branches arise by transfer to polymer and crosslinks by the subsequent coupling of branched radicals, it is easy enough to show that the rate of formation of crosslinks, X, as a function of conversion is ... 

Figure 5. Rate of formation of crosslinks dX/da in relation to fractional conversion a according to equation (6) no modifier, O modifier at initial concentration 0.1 phm, X, K = 0.5 A, K=1.0 Q K=2.0...

Polymerization catalysts such as sulfur ( ),water (4,, 6), oxygenated organics (7-10) and silica from the surface of glass (11,12) have been used to promote the polymerization. Many of these catalysts promote the formation of crosslinked chloropolymer at conversions above 50 percent, thus rendering them unsuitable for reaction with nucleophiles. 

Statistically defined structures may also arise from the formation of crosslinks in a melt the resulting gels are described within a percolation framework which predicts the existence of definite meshes [7, 8]. Contact-lenses, jellies or even jellyfish are common examples of gels. Latex beads with specific functionalities attached, such as antigens, are used in biodiagnostics. 

Considering calibrated gels, the effect of linkage of chemical units on NMR is easily disclosed when chain ends are frozen by the formation of crosslinks. The linear molecular weight dependence of %c 1 may serve as a reference for calibrating other gels made from the same polymer species. 

Side reactions including cis-to-trans isomerisation and sulfidic cyclisation are observed along with the formation of crosslinks in the BR cured with sulfur alone. In the sulfur-donor vulcanisation of BR, cis-to-trans isomerisation is the predominant feature of the vulcanisation reaction sequence and seems to obey first-order kinetics with respect to the concentration of accelerator. 

Contrary to the high-pressure polycondensation, when the polycondensation of the salt monomers was conducted in a molten state under atmospheric or reduced pressure for the preparation of the polyimides having Tm below 300°C, this often led to the formation of crosslinked aliphatic polyimides that were insoluble even in concentrated sulfuric acid. Therefore, the high-pressure polycondensation process provides a simple and effective method for the synthesis of the linear polyimides with well-defined structures that caused high crystallinity, compared with the other synthetic methods. 

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