Synthetic polymers homopolymers

Just as it is not necessary for polymer chains to be linear, it is also not necessary for all repeat units to be the same. We have already mentioned molecules like proteins where a wide variety of different repeat units are present. Among synthetic polymers, those in which a single kind of repeat unit are involved are called homopolymers, and those containing more than one kind of repeat unit are copolymers. Note that these definitions are based on the repeat unit, not the monomer. An ordinary polyester is not a copolymer, even though two different monomers, acids and alcohols, are its monomers. By contrast, copolymers result when different monomers bond together in the same way to produce a chain in which each kind of monomer retains its respective substituents in the polymer molecule. The unmodified term copolymer is generally used to designate the case where two different repeat units are involved. Where three kinds of repeat units are present, the system is called a terpolymer where there are more than three, the system is called a multicomponent copolymer. The copolymers we discuss in this book will be primarily two-component molecules. We shall discuss copolymers in Chap. 7, so the present remarks are simply for purposes of orientation. 

Vinylidene chloride copolymers were among the first synthetic polymers to be commercialized. Their most valuable property is low permeabiUty to a wide range of gases and vapors. From the beginning in 1939, the word Saran has been used for polymers with high vinylidene chloride content, and it is still a trademark of The Dow Chemical Company in some countries. Sometimes Saran and poly (vinylidene chloride) are used interchangeably in the Hterature. This can lead to confusion because, although Saran includes the homopolymer, only copolymers have commercial importance. The homopolymer, ie, poly (vinylidene chloride), is not commonly used because it is difficult to fabricate. 

Synthetic polymers have become extremely important as materials over the past 50 years and have replaced other materials because they possess high strength-to-weight ratios, easy processabiUty, and other desirable features. Used in appHcations previously dominated by metals, ceramics, and natural fibers, polymers make up much of the sales in the automotive, durables, and clothing markets. In these appHcations, polymers possess desired attributes, often at a much lower cost than the materials they replace. The emphasis in research has shifted from developing new synthetic macromolecules toward preparation of cost-effective multicomponent systems (ie, copolymers, polymer blends, and composites) rather than preparation of new and frequendy more expensive homopolymers. These multicomponent systems can be "tuned" to achieve the desired properties (within limits, of course) much easier than through the total synthesis of new macromolecules. 

A few reviews have dealt with the identification of synthetic polymers by Py-GC/MS [76]. In addition to compositional studies, applications of pyrolysis to synthetic polymers include sequence length characterization in copolymers [77] and tacticity measurements in stereoregular homopolymers [78]. 

Most naturally occurring polymers are largely homopolymers, but proteins and nucleic acids are copolymers composed of a number of different mers. While many synthetic polymers are homopolymers, the most widely used synthetic rubber, SBR, is a copolymer of styrene (S) and butadiene (B), with the R representing rubber. There are many other important copolymers. Here we will restrict ourselves to vinyl-derived copolymers. 

Synthetic water treatment polymers were first introduced in the 1950s. The first important synthetic polymers included hydrolyzed polyacrylamide and various high molecular weight (100,000+) homopolymers of polyacrylic acid and polymethacrylic acid, together with their sodium salts. 

As is well established, polymer/polymer blending is an important method to improve the original physical properties of one or both of the components, or to obtain new polymeric materials showing widely variable properties without parallel in homopolymers. There have been numerous blend studies for various polymer pairs from both the fundamental and practical viewpoints. A few reviews [7,42] have described a general scheme for preparation and characterization of the blends and micro composites of unmodified cellulose with synthetic polymers, mainly based on works performed until 1994. The present review will cover the articles published on this topic since the mid 1990s, with extensions to related works on cellulose derivatives and other natural polysaccharides. 

In many experiments, it appears that such drastic solvent extraction can remove the major part of the synthetic polymer, showing that the grafted polymer presents, in point of fact, a good "adhesive" bondability. However, in the case of surface modification, a heavy grafting is not necessary and the degre of permanence is a function of the insolubility of the homopolymer in the solvents used in the common course of subsequent treatments. So it can be inferred that this notion of grafting may be dependent on the efficiency of the solvent extraction. 

Fig. 9.2. Main types of (a) homopolymers and (b) heterochain synthetic polymers found in plastics (adapted from McNeill, 1991).

Among the structural factors that should be controlled in polymer syntheses (Fig. 1, Section I), perhaps the least exploited is the sequence of constitutional repeat units along a polymer main chain. We have already discussed the syntheses of block copolymers, where two or more homopolymer segments are connected, such as AAAAA-BBBBB- -, which is among the most primitive examples of sequence control in synthetic polymers. 

Adhesive Emulsions. Thermoplastic, synthetic polymers can be prepared as emulsions for use as adhesives. For example, while EVAc hot-melt adhesives described in the previous section contain less than 40% VAc, when the content of VAc in the copolymer is increased to 60%, and the copolymer is prepared in the form of aqueous emulsions, a very useful and versatile adhesive polymer is obtained. Although the VAc homopolymer, poly(vinyl acetate), is a brittle solid, with a Tg = 28 °C, the ethylene units present in the EVAc copolymer act as an internal plasticizer, and lower the Tg to below room temperature. The plasticization results from the reduction of interchain interaction of the VAc polymer chains by the ethylene units interspersed among the strongly interacting VAc units. This reduction of the Tg has important consequences because the formation of a flexible adhesive film from the emulsion depends upon the Tg of the polymer. 

Polymer molecules may be linear or branched, and separate linear or branched chains may be joined by crosslinks. Extensive crosslinking leads to a three-dimensional and often insoluble polymer network. Polymers in which all the monomeric units are identical are referred to as homopolymers those formed from more than one monomer type are called copolymers. Various arrangements of the monomers A and B in the copolymer molecules (Fig. 8.1) can be produced with consequent effects on the physical properties of the resulting polymer. Synthetic polymers may have their main chains substituted in different ways, depending on the conditions of the reaction, such that atactic (random), isotactic or syndiotactic forms are produced, as diagrammatically represented in Fig. 8.1. 

Synthetic polymers in general can be classified (1) by thermal behavior, i.e., thermoplastic and thermosetting (2) by chemical nature, i.e., amino, alkyd, acrylic, vinyl, phenolic, cellulosic, epoxy, urethane, siloxane, etc. and (3) by molecular structure, i.e., atactic, stereospecific, linear, cross-linked, block, graft, ladder, etc. Copolymers are products made by combining two or more polymers in one reaction (styrene-butadiene). See cross-linking block polymer epitaxy homopolymer plastics. 

Evolved gas analysis, particularly in the form of TGA-DTA-MS, has obvious synthetic polymer applications. It has been applied to study the thermal behavior of homopolymers, copolymers, polymeric blends, composites, residual polymers, solvents, additives, and toxic degradation polymers. In the latter context, hydrogen chloride evolution from heated polyvinylchloride materials is readily quantified by TGA-DTA-MS and such data are of major significance in... 

Synthetic peptide-based polymers are not new materials homopolymers of polypeptides have been available for many decades and have only seen hmited use as structural materials [5,6]. However, new methods in chemical synthesis have made possible the preparation of increasingly complex polypeptide sequences of controlled molecular weight that display properties far superior to ill-defined homopolypeptides [7]. Furthermore, hybrid copolymers, that combine polypeptide and conventional synthetic polymers, have been prepared and combine the functionality and structure of peptides with the processabihty and economy of polymers [8,9]. These polymers are well suited for applications where polymer assembly and functional domains need to be at length scales ranging from nanometers to microns. These block copolymers are homogeneous on a macroscopic scale, but dissimilarity between the block segments typically results in microphase heterogeneity yield-... 

The polymers we have discussed so far are formed from only one type of monomer and are called homopolymers. Often, two or more different monomers are used to form a polymer. The resulting product is called a copolymer. Increasing the number of different monomers used to form the copolymer dramatically increases the number of different copolymers that can be formed. Even if only two kinds of monomers are used, copolymers with very different properties can be prepared by varying the amounts of each monomer. Both chain-growth polymers and step-growth polymers can be copolymers. Many of the synthetic polymers used today are copolymers. Table 28.6 shows some common copolymers and the monomers from which they are synthesized. 

Two older reviews summarize work in this field [124,125]. The following derivatives have been employed as porphyrins Fe(II,III) protoporphyrin-EX (heme, hemin), Fe(II,III) or Co(II) protoporphyrin-IX-diester, chlorophyllins with different metal ions in the core, Fe(II) tetraphenylporphyrin, Mg(II) or Fe(II,III) octaethylporphyrin, Fe(II,III) tetrakis[o-(alkylamido)phenyl]-porphyrin. Polymers with N-donor groups are based on proteins such as poly(L-lysine), poly(L-histidine), poly(Y-benzyl-L-glutamate) or synthetic polymers such as homopolymers and copolymers with vinylpyridine, iV-vinylimidazole or ethyleneimine. 

In this book the term polymer is used to mean a particular class of macromolecules consisting, at least to a first approximation, of a set of regularly repeated chemical units of the same type, or possibly of a very limited number of different types (usually only two), joined end to end, or sometimes in more complicated ways, to form a chain molecule. If there is only one type of chemical unit the corresponding polymer is a homopolymer, if there is more than one type it is a copolymer. This section deals briefly with some of the main types of chemical structural repeat units present in the more widely used synthetic polymers and with the polymerisation methods used to produce them. Further details of the structures of individual polymers will be given in later sections of the book. 

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