Homopolymers and heteropolymers

HR RH HR RHHR RHHR RH Syndiotactic - - - - 4 V HH H H H HH HH H14 HH HH HR R H HR HR R H HR R H R H Atactic - - - HH H H H H H Hit HH HH HH shown in Fig. 1.4. A more general case is described by a probability for a monomer to add to the growing chain with its R group on the same side as Fig. 1.4 Tacticities of vinyl polymers, illustrated with all backbone carbons in the plane of the page and with H and R groups pointing either into or out from the page. 

Homopolymers are made from the same monomer, but may differ by their microstructure, degree of polymerization or architecture. Examples of different microstructure of homopolymers such as tacticity, structural or sequence isomerisms were described in Section 1.2. Throughout this book we demonstrate that the degree of polymerization N tor the number of backbone bonds n) of macromolecules is a major  

Simple gas Low-viscosity liquid High-viscosity liquid Simple soft solid Tough plastic solid 

Gaseous fuels Liquid fuels and solvents Oils and greases Candles and coatings Bottles and toys 

Another important feature controlling the properties of polymeric systems is polymer architecture. Types of polymer architectures include linear, ring, star-branched, H-branched, comb, ladder, dendrimer, or randomly branched as sketched in Fig. 1.5. Random branching that leads to structures like Fig. 1.5(h) has particular industrial importance, for example in bottles and film for packaging. A high degree of crosslinking can lead to a macroscopic molecule, called a polymer network, sketched in Fig. 1.6. Randomly branched polymers and th formation of network polymers will be discussed in Chapter 6. The properties of networks that make them useful as soft solids (erasers, tires) will be discussed in Chapter 7. 

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One of the unique features of the chemistry of carbon is its ability to form long chains of atoms. This property is the basis of an important area of industrial chemistry concerned with the manufacture of polymeric materials with a variety of properties see plastics). The molecules in these materials are essentially long chains of atoms of various lengths. In some polymers, cross-linkage occurs between the chains. Synthetic polymers are formed by chemical reactions in which individual molecules (monomers) join together to form larger units see POLYMERIZATION). Two types of polymer, homopolymers and heteropolymers, can be distinguished. 

Polymers with which we will deal throughout this chapter are water soluble. They can be either ionic or nonionic. Some of them are synthetic, others are of biological origin (proteins, for instance). Both homopolymers and heteropolymers exist. Some polymers own amphiphilic monomers that induce surface-active properties to the whole polymeric structure. Water plays a very important role in determining the polymer properties in solution. The properties are also greatly modified by the addition of salts or by a pH modification. Frequently encountered nonionic polymers in polymer-surfactant interactions and their subsequent adsorption behavior at solid surfaces are poly(ethylene oxide) (PEO), poly(vinyl pyrrolidone) (PVP), polyacrylamide, and poly(vinyl alcohol). 

A report in the literature describes the preparation of diacylium cations from tetrahaloterephthalic acids and their reactivity with free tetrahaloterephthalic acids to produce homopolymers and heteropolymers with an anhydride backbone. The perhalo polyanhydrides are surprisingly stable to hydrolysis and are stable at relatively high temperatures. 

In Fig. 8.7, the mean radii of gyration as a function of the temperature for the sequences from Table 8.1 are shown in comparison with the homopolymer. For all temperatures in the interval plotted, the homopolymer obviously takes more compact conformations than the heteropolymers, since its mean radius of gyration is always smaller. This different behavior is an indication for a rearrangement of the monomers that is particular for heteropolymers the formation of the hydrophobic core surrounded by the hydrophilic monomers. Since the homopolymer trivially also takes in the ground state a hydrophobic core conformation (since it only consists of hydrophobic monomers), which is obviously more compact than the complete conformations of the heteropolymers, we conclude that hydrophobic monomers weaken the compactness of low-temperature conformations. Thus, homopolymers and heteropolymers show a different phase behavior in the dense phase. Homopolymers fold into globular conformations which are hydrophobic cores with maximum number of hydrophobic contacts. Heteropolymers also form very compact hydrophobic cores which are, of course, smaller than that of the homopolymer due... 

Examples considered in this paper deal with binary heteropolymers prepared by the method of chemical modification of homopolymers and by the free-radical copolymerization of a mixture of two monomers, Mi and M2. The products of each of these processes is a mixture of an enormous... 

Typically, polymer-grade l-LA with high chemical purity and optical purity (i.e., over 98-99% l-LA and less than 1-2% d-LA) is used for commercial PLA production. When l-LA is dehydrated at high temperature into L-lactide, some l-LA may be converted into d-LA. d-LA mixed with l-LA contributes to meso-lactide (the cyclic dimer of one d-LA and one l-LA) and heteropolymer PLA (with both d-LA and l-LA units). Heteropolymer PLA exhibits slower crystallization kinetics and lower melting points than homopolymer PLA (of pure l-LA units or pure d-LA units). 

Therefore, from these and previous inferences, it may deduced that, at the molecular level, ED/W binary mixtures are very heterogeneous as a consequence of a separation of microphases. This probably implies that the aggregation of different type of clusters, such as tetrahedral or Wj oligomers, small ED homopolymers, and ED/W heteropolymers, should lead to interstitial solvation of one species into the other. Obviously this polyaggregation of clustered species provides the greatest effect corresponding to the hypothetical formation of a 2ED 3W solvent/cosolvent complex, whose existence seems to be still far away from being proven. 

Another classification of polymers is based on the number of different types of monomers and their distribution along the polymer chain. Thus, polymers are classified as homopolymers, alternating copolymers, random copolymers, block copolymers, and heteropolymers. They are schematically represented in Figure 12.2. Homopolymers are made up of one type of monomeric unit. Well-known examples of synthetic homopolymers are poly(styrene), poly(vinyl alcohol), poly(vinyl chloride), poly (ethylene), poly (ethylene oxide), and so on. Various natural polysaccharides, such as amylose, cellulose, dextran, chitin, and others, belong to this class as well. Copolymers contain two (or a few more) types of monomers that may be... 

Polymerization involves the reaction of the monomer building blocks into polymers. The polymerization reaction types involve addition reactions and condensation reactions. Addition reactions typically involve the use of ethene to form polyethene and polyethylene. Condensation reactions typically involve different reaction products reacting to form a heteropolymer and a small molecular by-product. The reaction of 1,6-diaminoethane and hexanedioic acid to form nylon and water is a classic example. Polymers made from one monomer type are termed homopolymers, and those formed with two different monomers are referred to as copolymers. 

The synthesis of exocellular polysaccharides by lactic acid bacteria is a very widespread character. L. mesenteroides and Streptococcus mutans produce glucose homopolymers such as dextran and glucan fructose homopolymers (levans) and heteropolymers are also synthesized. Dextran of L. mesenteroides is the best known, as much for its different structures and its biosynthesis as for its various applications. 

Interconversions based on a subunit model of the A, B, and S forms of isoenzymes of the j8-o-2-acetamido-2-deoxyhexosidase in human tissues have been accomplished by means of preparative polyacrylamide gel electrophoresis." It was concluded that the A, B, and S forms are composed of a heteropolymer containing a- and j8-chains, a jS-chain homopolymer, and an a-chain homopolymer, respectively. Separation of the isoenzymes of jS-D-2-acetamido-2-deoxyhexosidase in human tissues by electrophoresis on cellulose acetate membranes has been used in the diagnosis of GM -gangliosidosis." The relative abundances and properties of the three isoenzymes were determined, and the relevance of the results to the disease was discussed. 

The crystallization of horse spleen apoferritin was in fact a fortuitous coincidence, because, as we mentioned earlier, attempts to crystallize horse-liver ferritin were not successful, whereas the iron-rich ferritin from horse spleen could be crystallized (Laufberger, 1937). This was certainly related to the relatively high content of H subunits (average composition L12H12) in horse liver (something that was only discovered 50 years later). It has generally proved very difficult if not impossible to crystallize heteropolymers, and the best results in crystallographic terms have been obtained with recombinant homopolymers. As will be discussed later in this chapter,... 

Its derivation implies a succession of two formal procedures. First, it is necessary to color the homopolymer globule units marking every z -th unit by color of/ with the probability waj(rl) which coincides with the ratio of concentration of units Ma, at point rl to the overall concentration of all units at this point. As a result of such a coloring, the joint distribution for configurations and conformations of proteinlike heteropolymers is obtained. Integration of this distribution over coordinates of all units results in the desired molecular-structure distribution (Eq. 23). 

Fig. 5 Comparison of phase diagrams calculated for the melt of a proteinlike heteropolymer (b) with the phase diagram of a Markovian copolymer according to criterion II (a) and criterion I (c). Proteinlike heteropolymer consisting of / = 103 units is obtained for polymeranalogous reaction in a homopolymer globule at the value of the Thiele modulus h equal to 35... 

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