Condensation reactions crossed

Nucleophilic reactions, condensation reactions cross-linking, ester linking... 

Haward et al.t have reported some research in which a copolymer of styrene and hydroxyethylmethacrylate was cross-linked by hexamethylene diisocyanate. Draw the structural formula for a portion of this cross-linked polymer and indicate what part of the molecule is the result of a condensation reaction and what part results from addition polymerization. These authors indicate that the crosslinking reaction is carried out in sufficiently dilute solutions of copolymer that the crosslinking is primarily intramolecular rather than intermolecular. Explain the distinction between these two terms and why concentration affects the relative amounts of each. 

Polymerization. Thermal polymerization or curing of an ink film at elevated temperatures can foUow many different chemical paths. Condensation and cross-linking reactions may be accompHshed with or without the use of catalysts. However, this method of drying generally has not been widely used for printing inks, except those used for metal and glass decoration, and some clear coatings. 

The segments derived from the condensation reaction of the butanediol and the diisocyanate agglomerate into separate phases, which are hard and crystalline. The elastomeric chains are thus cross-linked to form a network similar in many ways to that given by the simple... 

As these condensation reactions can occur at the two ortho and the para positions in phenol, w-cresol and 3,5-xylenol, cross-linked structures will be formed. It has been pointed out by Megson that because of steric hindrance the amount of cross-linking that can take place is much less than would involve the three reactive groups of all the phenolic molecules. It is now generally considered that the amount of cross-linking that actually takes place is less than was at one time believed to be the case. 

The results in Table IV suggest that the condensation reactions cannot be described adequately by the ion-induced dipole model. In this regard the results agree with conventional studies which have frequently found a higher power inverse dependence of the cross-section on the field strength E for condensation reactions than for hydrogen transfer reactions. 

A low ion pair yield of products resulting from hydride transfer reactions is also noted when the additive molecules are unsaturated. Table I indicates, however, that hydride transfer reactions between alkyl ions and olefins do occur to some extent. The reduced yield can be accounted for by the occurrence of two additional reactions between alkyl ions and unsaturated hydrocarbon molecules—namely, proton transfer and condensation reactions, both of which will be discussed later. The total reaction rate of an ion with an olefin is much higher than reaction with a saturated molecule of comparable size. For example, the propyl ion reacts with cyclopentene and cyclohexene at rates which are, respectively, 3.05 and 3.07 times greater than the rate of hydride transfer with cyclobutane. This observation can probably be accounted for by a higher collision cross-section and /or a transmission coefficient for reaction which is close to unity. 

The raw materials are usually mutually contaminated to about 5 %. Thus, DSC can be ruled out. The compounds are made by aldol condensations and cross Canizzaro reactions between acetaldehyde and formaldehyde. An elegant method of distinguishing between the two raw materials takes advantage of the 10 fold solubility difference between the two in methanol and water. 

The cross-condensation reaction of benzyl benzoate (46) and 44 was carried out under solvent-free conditions. Treatment of a 1 1 mixture of 46 and 44a with Bu OK at 120 °C for 1 h gave the cross-condensed product 47a in 42% yield (Scheme 7). Similar reaction of 46 with 44b gave 47b in 45% yield. Because heating of46,44, and Bu OK in toluene under reflux for 16 h did not give any product, it is clear that the solvent-free reaction is again effective for the cross-condensation. In these cases, self-condensation of 44 itself did not occur probably because of the high reactivity of 46 [9]. 

Polycondensation pol5mers, like polyesters or polyamides, are obtained by condensation reactions of monomers, which entail elimination of small molecules (e.g. water or a hydrogen halide), usually under acid/ base catalysis conditions. Polyolefins and polyacrylates are typical polyaddition products, which can be obtained by radical, ionic and transition metal catalyzed polymerization. The process usually requires an initiator (a radical precursor, a salt, electromagnetic radiation) or a catalyst (a transition metal). Cross-linked polyaddition pol5mers have been almost exclusively used so far as catalytic supports, in academic research, with few exceptions (for examples of metal catalysts on polyamides see Ref. [95-98]). 

Scheme 8-33 [Cp2Zr(H)CI] 1-catalyzed cross-condensation reactions...

Scheme 6 Typical condensation reactions leading to a cross-linked polymer.

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