Polymer Classification by Reaction Type

Classification by End Use Chemical reactors are typically used for the synthesis of chemical intermediates for a variety of specialty (e.g., agricultural, pharmaceutical) or commodity (e.g., raw materials for polymers) applications. Polymerization reactors convert raw materials to polymers having a specific molecular weight and functionality. The difference between polymerization and chemical reactors is artificially based on the size of the molecule produced. Bioreactors utilize (often genetically manipulated) organisms to catalyze biotransformations either aerobically (in the presence of air) or anaerobically (without air present). Electrochemical reactors use electricity to drive desired reactions. Examples include synthesis of Na metal from NaCl and Al from bauxite ore. A variety of reactor types are employed for specialty materials synthesis applications (e.g., electronic, defense, and other). 

This is one of the oldest and most important classification criteria originally proposed by Carothers [3] in 1929 and the one that splits polymers into two major types addition and condensation polymers. The basis for the distinction is better understood by illustration with two examples belonging each one to one category polystyrene as an addition polymer and a polyester as a condensation polymer. They are produced by the reactions shown in Scheme 1.1. 

As disciissed in Chapter 1, under a scheme proposed by Carothers, polymers are classified as addition or condensation polymers depending on the type of polymerization reaction involved in their synthesis. This classification scheme, however, does not permit a complete difierentiation between the two classes of polymers. A more complete but still oversimplified scheme that is still based on the dilTerent polymerization processes places polymers into three classes condensation, addition, and ring-opening polymers. This scheme reflects the stractures of the starting monomers. Probably the most general classification scheme is based on the polymerization mechanism involved in polymer synthesis. Under this scheme, polymerization processes are classified as step-reaction (condensation) or chain-reaction (addition) polymerization. In this chapter, we will discuss the different types of polymers based on the different polymerization mechanisms. 

The formation of polymers from the monomers is known as polymerization reaction. When more than one basic unit forms the polymer, the process is also named copolymerization. The polymerization reactions can be classified into two main groups, addition polymerizations and condensation polymerizations (or polycondensations). For the addition polymerizations, the resulting polymer has the repeating unit with the same molecular formula as the monomer, and the molecular mass of the polymer is the sum of the molecular masses of all the monomer molecules. For the condensation polymerizations, the resulting polymer has the repeating unit with fewer atoms than that of the monomer or monomers, and the molecular mass of the polymer is less than the sum of molecular masses of the original monomer unit or units because small molecules are eliminated following this reaction. This classification is not adequate for the characterization of the polymer itself, because the same polymer can be formed by more than one type of reaction. For example, a polyamide can be formed by addition from a lactam or by condensation from an co-aminocarboxylic acid as shown below ... 

The reactions with formation of polymers also are classified based on another difference in their mechanism. This classification distinguishes step reactions and chain reactions. In step reactions the polymers are built from the monomer by random individual reactions to form dimers, trimers, tetramers, etc., each resulting molecule being able to participate in a subsequent reaction with a monomer or with an oligomer molecule. This type of reaction may start with molecules having two reactive functional groups in one molecule such as an o-aminocarboxylic acid. Another possibility consists of reactions between two different types of bifunctional molecules such as a diamine and a dicarboxylic acid as shown below ... 

In contrast with the usually slow progress of condensation polymerization the second major classification, addition, or vinyl-type polymerizations, usually proceed very rapidly, so rapidly that they are referred to as chain reaction polymerizations. This method of producing synthetic polymers uses the potential dual functionality present in a carbon-carbon double bond. The process is initiated by the use of radical or charged initiator species to form new sigma bonds from the carbon-carbon double bonds of the monomer, to link the monomer units (Eq. 20.6). 

A rational classification of polymers has been discussed for several decades. The debate began when Carothers proposed a classification according to the polymer reactivity and considered mainly two types of polymers polymers prepared through condensation reactions (condensation polymers) and those prepared by addition reactions (addition polymers). Chapter 1 of this handbook also discusses this topic, and although there are some overlaps in the topics covered here and in Chapter 1, the two chapters can be considered complementary. 

Natural polymers are broadly classified as polysaccharides, proteins, polynucleotide, and natural rubber. This classification is based on the type of monomer building blocks for the polymer and type of covalent bonds for formation of the polymer. Polysaccharides that are carbohydrate polymers are formed by condensation of monomeric units called monosaccharides, proteins or peptide polymers are formed from amino acids and polynucleotide are synthesized from nucleotide condensation reaction forming glycosidic, peptide, and phosphodiester bonds, respectively. All natural polymers are condensation polymers. 

The many ways to make polymers can be broken into two types of reactions based on the mechanisms of the polymerization, step and chain reactions (5). This reaction classification was termed condensation or addition reactions in the past but this archaic nomenclature is slowly dying away. The labels step and chain were developed for the two types of polymerizations by Flory and Mark (5, 6). All step reactions conduct the same stepwise reaction between all reactive entities in the reaction mixture. The... 

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