Step-growth reaction

All polymers described in this chapter bear the furan ring in the chain s backbone, and are the result of step growth reactions which involve the elimination of a condensation product. 

We can create thermoplastic polymers by chain growth or step growth reactions. In either case the polymer chains consist of a string of monomer residues, each of which is attached to two other monomer residues. The polyethylene molecule shown in Fig. 1.1 is an example of a thermoplastic polymer made via chain growth polymerization, as shown in Fig. 1.7,... 

Step growth polymerization can also yield highly crosslinked polymer systems via a prepolymer process. In this process, we create a prepolymer through a step growth reaction mechanism on two of the sites of a trifunctional monomer. The third site, which is chemically different, can then react with another monomer that is added to the liquid prepolymer to create the crosslinked species. We often use heat to initiate the second reaction. We can use this method to directly create finished items by injecting a mixture of the liquid prepolymer and additional monomer into a mold where they polymerize to create the desired, final shape. Cultured marble countertops and some automotive body panels are created in this manner. 

Step-growth reactions, 24 16 Step-scan ftir photoacoustic analysis, 19 564... 

The formation of synthetic polymers is a process which occurs via chemical connection of many hundreds up to many thousands of monomer molecules. As a result, macromolecular chains are formed. They are, in general, linear, but can be branched, hyperbranched, or crosslinked as well. However, depending on the number of different monomers and how they are connected, homo- or one of the various kinds of copolymers can result. The chemical process of chain formation may be subdivided roughly into two classes, depending on whether it proceeds as a chain-growth or as a step-growth reaction. 

In step growth reactions, on the other hand, neither are specific activated centers present to force the connection of the monomers, nor does the process occur as a cascade reaction. Instead, the monomers are tied together in discreet, independent steps via conventional organic reactions such as ester-, ether-, amide-, or urethane formation. Depending on whether small molecules are set free in the connection step, one distinguishes between polycondensations (2.9) and polyadditions (2.10) ... 

Sengupta showed that the reaction of bis-arenediazonium salt 91 with vinyl(triethoxy)silane 92 afforded poly(phenylene-vinylene) 93. Although the reaction apparently proceeds through the Heck reaction mechanism, which is described in Section 11.19.4, a part of the step-growth reaction is indeed a transformation of the carbon-silicon bond of 92 to the carbon-carbon bond (Equation (44)). 

Step growth reactions involving the homo- and heteropolycondensation of various bifunctional unsaturated monomers in the presence of transition metal-based coordination catalysts have appeared to be a very useful synthetic tool for the preparation of low and high molecular weight polymers with an unsaturation in the polymer backbone. These reactions lead to unsaturated hydrocarbon and non-hydrocarbon polymers where polymeric chains are formed by carbon-carbon and carbon-heteroatom coupling respectively. 

The most successful polymerizations carried out by using a Diels-Alder step-growth reaction are those which generate a highly reactive A-B monomer in situ by the reaction of a bismaleimide with cyclopenta-dienone (12), 2-pyrone (6, 13), or thiophene dioxide (5) derivatives. The intermediate 1 1 adduct loses carbon monoxide, carbon dioxide, or sulfur dioxide, respectively, all to generate the same type of reactive AB monomer, which is converted rapidly to polymer. High molecular weight polymers are obtained (Reaction 4). 

The polymerization reaction is typical of step-growth reactions in that it follows second-order kinetics for the reaction of tetracyclone ends with acetylene ends (Figures 1 and 2). 

Although the utilization of the Diels-Alder synthesis as a step-growth reaction for polymerization requires, in most cases, rather unusual monomers, an occasional lengthy and trying synthesis, torturous purifications, and, in certain cases, difficult reaction conditions, it does afford high... 

A typical step-growth reaction is one in which a single epoxy ring reacts with the active hydrogen of the curing agent. The general reaction is ... 

Time to vitrification data for the other two cases are not available. For a hypothetical linear step growth reaction of Aj -t- B2, with reasonable values of Ma, Mb,... 

The earliest polymers of practical use were prepared by step-growth reactions, most notable among them Bakelite, a phenol-formaldehyde copolymer first marketed in 1910 [4]. Its name was long almost synonymous with synthetic plastics and resins, has become generic, and is no longer restricted to phenol-formaldehyde copolymers. Most but not all step-growth polymerizations are condensations. 

Growth of polymer molecules is caused by a kinetic chain of reactions. (The name chain-growth reflects the existence of a chain reaction. Unfortunately, macromolecules are often also called chains because they are composed of linked identical entities. There is no necessary connection between the two usages. Some polymer chains are made by chain-growth polymerizations, and some are made by step-growth reactions.)... 

Number Average Degree of Polymerization in Step-Growth Reactions... 

To obtain a high polymer in equilibrium step-growth reactions, p must be close to unity (Section 5.4.2). Then, with p very close to 1,... 

The need to drive the polymerizations to completion is common to all step-growth reactions that are carried out under conditions in which polymerization-depolymerization equilibria are significant (Section 5.4.2). This is accomplished in general by removal of a volatile product such as water or an alcohol. The rale of polymerization is often limited by the rate of transfer of such condensation products into the vapor state. A complete kinetic description of the process must then involve both the chemical reaction rate and the rate of mass transfer. The latter depends on the details of reactor design and stirring and therefore so does the rate of polymer production [1]. 

Heal removal and mixing are problems in bulk chain-growth polymerizations for the reasons outlined in the previous section. Thus, homogeneous bulk step-growth reactions are driven to high conversions to achieve high molecular weights... 

Poly (ethylene terephihalate) and nylon-6,6 manufacture are homogeneous bulk step-growth reactions. The molecular weight of the polymer produced is limited by the high viscosity of the reaction mixture at very high conversions. Post polymerization techniques such as that described in connection with reaction (5-39) can be used to increase the polymer molecular weight for some applications. 

It is important to define the terms used in describing functionality and to clearly distinguish between the actual and potential functionality and to show the relationship between stoichiometry and functionality. Functionality can be defined as the number of other molecules that a compound can react with. This definition of functionality also means that within step-growth polymerizations the actual functionality is dependent on stoichiometry. The phenol-formaldehyde reaction is a typical step-growth reaction in... 

Polyesters, polyetherimide, melamine-formaldehyde, polyurethanes, polyurethane-ureas and polyamides (nylons) are examples of condensation polymers prepared by REX. " Because a small molecule is produced in condensation reactions, vent ports are employed. Between the ports are melt sealing screw sections, to prevent back mixing of volatilizing melt. Sealing screw sections are constructed by a right handed-left handed sequence of screw elements. Another chemistry feature of step-growth reactions is their sensitivity to errors in stoichiometric feed proportions. This poses problems with solid feed materials, which are normally converted to liquid form for more... 

Random copolymers have grown to be the most versatile, economical, and easily synthesized types of copolymers. A wide variety of free-radical, ionic-addition, and ring-opening polymerization techniques, as well as many step-growth reactions, are employed. 

Epoxy resins are complex network polyethers usually formed in a two-staged process. The first stage involves a base-catalyzed step-growth reaction of an excess epoxide, typically epichlorohydrin with a dihydroxy compound such as bisphenol A This results in the formation of a low-molecular-weight prepolymer terminated on either side by an epoxide group. 

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