Monomeric units, arrangement

The next major commodity plastic worth discussing is polypropylene. Polypropylene is a thermoplastic, crystalline resin. Its production technology is based on Ziegler s discovery in 1953 of metal alkyl-transition metal halide olefin polymerization catalysts. These are heterogeneous coordination systems that produce resin by stereo specific polymerization of propylene. Stereoregular polymers characteristically have monomeric units arranged in orderly periodic steric configuration. 

Microstructure of copolymers typically refers to the proportion and the arrangement of the monomer units in the polymeric backbone. In their structure the copolymers may contain the monomeric units arranged randomly, they may alternate regularly, may form large blocks of one type of monomer, or may appear as side chain blocks connected to a polymer main chain (see Section 1.1). This distribution also depends on the relative amounts of each monomer present in the copolymer [7]. Analytical pyrolysis, particularly Py-GC-MS, has been used successfully for the analysis of microstructure of copolymers (see e.g. [8]). Pyrolysis generates small fragments that represent sections of the polymer and can make distinctions between random and block copolymers fairly straightfonward. 

Several polymers form helices which are simple in the sense that A< is a fraction of 360°, like 180°, 120° or 90°. These are called m/l -helices, m giving the number of monomeric units arranged along one 360°-turn. Examples are polypropylene, polystyrene or poly-1-butene, which all form 3/1-helices. The all-trans-confoimdition of polyethylene and the all-gauche-conioimation of poly(oxymethylene) correspond to 2/1-helices. 

Generally polymers involve bonding of the most substituted carbon of one monomeric unit to the least substituted carbon atom of the adjacent unit in a head-to-tail arrangement. Substituents appear on alternate carbon atoms. Tacticity refers to the configuration of substituents relative to the backbone axis. In an isotactic arrangement, substituents are on the same plane of the backbone axis that is, the configuration at each chiral center is identical. 

Statistical characteristics of the chemical structure of a heteropolymer, which pertain to the second type, describe the pattern of arrangement of units along macromolecules. The best known among such characteristics are fractions P Uk of directed sequences Uk incorporating k monomeric units. The simplest of them are the dyads U2, the complete set of which for a binary copolymer is composed of four pairs MiMi,MiM2,M2Mi,M2M2. Their calculation turns out to be rather useful for two reasons. 

The copolymer formed is composed of both the monomeric units A and B arranged at random. The A B ratio and the degree of randomness is found to depend on the quantity of the individual monomers taken, their amenability for copolymerisation and the polymerisation mechanism used. 

Note 2 A copolymer comprising linear molecules of uniform relative molecular mass and uniform elemental composition but different sequential arrangements of the various types of monomeric units, is not uniform (e.g., a copolymer comprising molecules with a random arrangement as well as a block arrangement of monomeric units). 

Similarly to homopolymers, source-based nomenclature has been applied to copolymers [4]. The principal problem is to define the kind of arrangement in which various types of monomeric units are related to each other. Seven types of separate arrangements have been defined, which are shown in Table 1, where A, B and C represent the names of monomers. The monomer names are linked either through an italicized qualifier or connective (infix), such as -CO- , to form the name of the copolymer, as in poly(styrene-co-acrylonitrile). The order of citation of the monomers is arbitrary. 

Copolymers have gained considerable importance both in scientific research and in industrial applications. A consistent and clearly defined system for naming these polymers would, therefore, be of great utility. The nomenclature proposals presented here are intended to serve this purpose by setting forth a system for designating the types of monomeric-unit sequence arrangements in copolymer molecules. 

The nomenclature system presented here is designed for copolymers. By definition, eopolymers are polymers that are derived from more than one species of monomer [4], Various classes of eopolymers are discussed, whieh are based on the charaeteristie sequenee arrangements of the monomeric units within the copolymer moleeules. Generally, the names of monomers are used to speeify monomeric units the latter ean be named using the trivial, semi-systematie or systematie form. The classes of copolymers are as follows ... 

Copolymers with an unspecified arrangement of monomeric units Rule 1.1... 

An unspecified sequence arrangement of monomeric units is represented by (A-co-B)... 

Statistical copolymers are copolymers in which the sequential distribution of the monomeric units obeys known statistical laws e.g. the monomeric-unit sequence distribution may follow Markovian statistics of zeroth (Bemoullian), first, second or a higher order. Kinetically, the elementary processes leading to the formation of a statistical sequence of monomeric units do not necessarily proceed with equal a priori probability. These processes can lead to various types of sequence distribution comprising those in whieh the arrangement of monomeric units tends towards alternation, tends towards... 

An alternating copolymer is a copolymer comprising two species of monomeric units distributed in alternating sequence. The arrangement... 

If copolymer structures comprise several types of periodic sites, only some of which are always occupied by particular species of monomeric units (A, B...), and sites of the other types are occupied by two or more types of monomeric unit (U, V...) in irregular arrangement, the names of the monomers in the latter sites are embraced by parentheses and are separated by semicolon(s). 

A block polymer is a polymer comprising molecules in which there is a linear arrangement of blocks, a block being defined as a portion of a polymer molecule in which the monomeric units have at least one constitutional or configurational feature absent from the adjacent portions [4]. In a block copolymer, the distinguishing feature is constitutional, i.e. each of the blocks comprises units derived from a characteristic species of monomer. 

Examples. In the following examples, the subscripts k, m,. ..represent different multiplicity of the monomeric units for different blocks. They may be indeterminate or specific (see Rule 5.3). In each case, the first line gives a representation of the block sequence arrangement, the second the corresponding name and the third an illustration of a specific case. 

The type of arrangement of the monomeric units in polymeric dienes can be determined qualitatively and quantitatively by IR spectroscopy. For this purpose thin films are prepared by dropping an approximately 2% solution in carbon disulfide (spectroscopically pure) on to suitable rock salt plates and allowing the solvent to evaporate at room temperature.The plates are placed in the spectrometer beam and the IR spectrum is recorded.The different types of chemical linkages are associated with characteristic IR bands as summarized in Table 3.9. 

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