Polymerization step-growth-type

Bulk polymerizations, such as addition (free-radical- and ionic-based) and step-growth types. Grafting polymerization by small molecules. Interchain copolymer formation, based on chain cleavage, graft copolymerization, and end-group block copolymerization. 

Transition metal catalyzed coupling reactions of aromatic halides, such as Heck or Suzuki coupling, are used extensively for the synthesis of low-molecular-weight organic compounds. With bifunchonal substrates, polymers can be obtained. By contrast to all other polymerization reactions considered in this chapter, such reactions represent step-growth-type polymerizations. 

Three broad types of polymerization processes are generally recognized chain-addition polymerization, step-growth polymerization, and coordination polymerization. 

Suggest a mechanism for the polymerization, using curved arrows to symbolize the flow of electrons, and indicate whether it is of the chain-reaction or step-growth type. 

Several polymer types and classes are known to exhibit photoconductivity. Consequently no preferred method of synthesis exists. The known photoconductive polymers are prepared by almost all common methods like free-radical, cationic, anionic, coordination, and ring-opening polymerization, step-growth polymerization, and polymeranalo-gous reactions. The only common requirement for all photoconductive materials is that they have to be of extreme purity. It is well known [40-42] that even traces of impurities act as traps and have drastic influence on both quantum yield and carrier mobility. 

Our primary purpose in this section is to point out some of the similarities and differences between step-growth and chain-growth polymerizations. In so doing we shall also have the opportunity to indicate some of the different types of chain-growth polymerization systems. 

Step-growth polymerizations can be schematically represented by one of the individual reaction steps VA + B V —> Vab V with the realization that the species so connected can be any molecules containing A and B groups. Chain-growth polymerization, by contrast, requires at least three distinctly different kinds of reactions to describe the mechanism. These three types of reactions will be discussed in the following sections in considerable detail. For now our purpose is to introduce some vocabulary rather than develop any of these beyond mere definitions. The principal steps in the chain growth mechanism are the following ... 

Synthesis and Properties. Polyquinolines are formed by the step-growth polymerization of o-aminophenyl (aryl) ketone monomers and ketone monomers with alpha hydrogens (mosdy acetophenone derivatives). Both AA—BB and AB-type polyquinolines are known as well as a number of copolymers. Polyquinolines have often been prepared by the Friedlander reaction (88), which involves either an acid- or a base-catalyzed condensation of an (9-amino aromatic aldehyde or ketone with a ketomethylene compound, producing quinoline. Surveys of monomers and their syntheses and properties have beenpubhshed (89—91). 

Monomers of die type Aa B. are used in step-growth polymerization to produce a variety of polymer architectures, including stars, dendrimers, and hyperbranched polymers.26 28 The unique architecture imparts properties distinctly different from linear polymers of similar compositions. These materials are finding applications in areas such as resin modification, micelles and encapsulation, liquid crystals, pharmaceuticals, catalysis, electroluminescent devices, and analytical chemistry. 

Dendrimers produced by divergent or convergent methods are nearly perfectly branched with great structural precision. However, the multistep synthesis of dendrimers can be expensive and time consuming. The treelike structure of dendrimers can be approached through a one-step synthetic methodology.31 The step-growth polymerization of ABx-type monomers, particularly AB2, results in a randomly branched macromolecule referred to as hyperbranch polymers. 

Hyperbranched polyesters are prepared by the step-growth polymerization of A13,-type monomers where A and B are —OH and —COOH groups or derivatives such as CH3COO-, HO-CH2CH2-O-, (CH3)3SiO—, -COOCH3, or -COC1... 

Hyperbranched polymers are characterized by their degree of branching (DB). Hie DB of polymers obtained by the step-growth polymerization of AB2-type monomers is defined by Eq. (2.1) in which dendritic units have two reacted B-groups, linear units have one reacted B-group, and terminal units have two unreacted B-groups191 ... 

The functional groups that typically participate in this type of polymerization are carboxyl, amine, and alcohol groups. Examples of step growth polymers include polyesters and nylons, which are often spun into fibers used to manufacture carpeting and fabrics, and polycarbonates, which are converted into compact discs, jewel cases, and the large bottles used in water coolers. 

The type of copolymer formed during step growth polymerization depends on the reactivity of the functional groups and the time of introduction of the comonomer. A random copolymer forms when equal concentrations of equally reactive monomers polymerize. The composition of the copolymer, then, will be the same as the composition of the reactants prior to polymerization. When the reactivities of the monomers-differ, the more highly reactive monomer reacts first, creating a block consisting predominandy of one monomer in the chain the lower reactivity monomer is added later. This assumes that there is no chain transfer and no monofunctional monomer present. If either of these conditions were to exist,... 

We encounter homogeneous catalysts in both step-growth and chain-growth polymerization processes. We saw several examples of these types of reactions in Chapter 2. For example, the acid catalyzed polymerization of polyesters occurs via a homogeneous process as do some metallocene catalyzed polymerization of polyolefins. 

The dimer formed still has reactive groups. Hence, the polymerization will continue to take place in a stepwise fashion. Each step in this type of polymerization is identical and, therefore, the rate and mechanism of each step remains same as in the initial step. Polyestrification reactions are examples of step growth polymerization... 

The first strategies to random hyperbranched polymers involved exclusively step-growth polymerizations. This limited the potential applications for these architectures to areas where only condensation-type polymers are acceptable. Frechet et al. [21] presented the first example of a hyperbranched vinyl polymerization in 1995, ] initiating the birth of a second generation of hyperbranched... 

The polymerization of AB -functional vinyl monomers is fundamentally different from the step-growth polymerization of AB2-monomers. Condensation of AB2-monomers results immediately in the formation of hyperbranched polymers since the reactivity of the end-groups are the same, regardless of what type of repeat unit (linear or dendritic) that is formed. 

The first type, termed sequential IPN s, involves the preparation of a crosslinked polymer I, a subsequent swelling of monomer II components and polymerization of the monomer II in situ. The second type of synthesis yields materials known as simultaneous interpenetrating networks (SIN s), involves the mixing of all components in an early stage, followed by the formation of both networks via independent reactions proceeding in the same container (10,11). One network can be formed by a chain growth mechanism and the other by a step growth mechanism. 

At the beginning of this chapter we described step growth polymerization chiefly to contrast it with chain polymerization. We now consider this subject in more detail and discuss various types of step growth polymers. 

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