Condensation polymers linking units

As an example of a polyester fiber, consider the condensation polymer Dacron (also sold as a film—Mylar). The monomeric repeating unit of Dacron is shown in Fig. 7.8.2, with several linked monomers indicated in Fig. 7.8.3. While the... 

Multiple amide groups link the resultant polymer. For this reason, this condensation polymer is a polyamide however, DuPont did not like this name, so we now know this polymer to be an example of a nylon. If the units joined by the... 

A condensation polymer is one in which the repeating unit lacks certain atoms which were present in the monomer(s) from which the polymer was formed or to which it can be degraded by chemical means. Condensation polymers are formed from bi- or polyfunctional monomers by reactions which involve elimination of some smaller molecule. Polyesters (e.g., 1-5) and polyamides like 1-6 are examples of such thermoplastic polymers. Phenol-formaldehyde resins (Fig. 5-1) are thermosetting condensation polymers. All these polymers are directly synthesized by condensation reactions. Other condensation polymers like cellulose (1-11) or starches can be hydrolyzed to glucose units. Their chemical structure indicates that their repealing units consist of linked glucose entities which lack the elements of water. They are also considered to be condensation polymers although they have not been synthesized yet in the laboratory. 

Polymers are extremely large molecules that are made of repeat units called monomers. Addition polymers are formed from unsaturated monomers that commonly link through free-radical reactions. Most condensation polymers are formed by linking two types of monomer through a dehydration-condensation reaction. 

Redistribution during thermal processing is a common self-reaction in condensation polymers such as PA, PEST, and PC that often contain nucleophilic amine, hydroxy, or phenolic end-groups, along with electrophilic groups such as amide, ester, or carbonate linking the individual monomer units. 

In 1969, Craven reported the first example of the DA reaction with the furan/ maleimide system [136]. He synthesized polymers which consisted of chains of the saturated condensation polymer backbone bearing the furan group reacted with maleimides. Since then several patents and papers have been published, all concerning the fabrication of a thermally reversible polymer network bearing DA-reactive furan and maleimide units, either as pendant groups (for reversible cross-linking) [137-147], or as part of the polymer backbone (for reversible polymerization) [148-154]. 

In all the preceding polymerization methods we have seen how to utilize the double bond in an unsaturated organic compoimd to link many molecules together into a polymeric chain. Also, in all of these processes the polymer was produced starting from a single monomer. In contrast in this section we will look at polymers that are prepared from the reaction of two difunctional monomers with each other. In all the polymerization reactions that we have seen so far there was no side-product formation. For example, ethylene was converted into polyethylene acrylonitrile was converted into polyacrylonitrile and so on. During this conversion the entire stmc-tural unit of the monomer was incorporated into the polymer without any side-product formation. However, in the preparation of condensation polymers a small molecule (such as water or methanol) is eliminated as the side-product. Another important difference is that condensation polymerization is usually a step-growth polymerization. This means that the polymerization proceeds in a series of steps. To make this point clear let us recall the polymerization of ethylene by the free-radical method. In the free-radical process the polymerization of various chains are initiated by the... 

The term silanol is used for any OH group attached to silicon and will be understood to include what has otherwise been referred to as siloxanol" and silicol groups. The term polymerization, strictly speaking, means a linking together of monomer units to form a polymer of the same composition, but, in the silica system the monomer. Si(OH)4, condenses to form polymer that ultimately has the composition (Si02)n- However, the term polymerization has been so widely used for the formation of condensation polymers that it will also be so used here. 

Curve I is for a linear to branched-chain polymer with methylene or dimethylene ether linkages. In curves II, III, and IV the linking unit is methylene, and the cross-linking is greatest in IV. Curve V is for a linear polymer series with hexahydrotriazine linkages. The condensation of hexa-methylolmelamine has been interpreted in a similar fashion [51]. 

Unsaturated polyester resins are low MW condensation polymers which are transformed to a cross-linked network via radical initiated polymerization. The double bonds in the backbone copolymerize with unsaturated monomer (reactive diluent) present in the system. During polymerization, relatively short low MW polyester chains are cross-linked by short bridges consisting of, on average, around two to three styrene units, forming a densely cross-linked polymer network (see Figure 2.17). 

Condensation polymers containing sucrose or glucose units in the main chain were also reported. Cross-linked polyurethanes prepared from polyethylene glycol (PEG), diphenylmethane diisocyanate (MDl) and a sugar (glucose, fructose, or sucrose), were reported by Zetterlund et The storage modulus, the Tg, and the stress at break increased with an increase of sucrose content and the strain at break decreased with an increase of sucrose content. 

The second step is a condensation reaction that involves the linking together of monomer units with the Hberation of water to form a dimer, a polymer chain, or a vast network. This is usually referred to as methylene bridge formation, polymerization, resinification, or simply cure, and is illustrated in the following equation ... 

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