Monomer units, different arrangements

The term block copolymer refers to the copolymers in which the two different monomer units are arranged in linear blocks. The simplest example of a block copolymer is an AB diblock copolymer (Fig. 2.2) [1]. 

The presence of more than one type of monomer adds extra complexity to FRP kinetics. The monomers in the system form different radical structures, with the relative rates of chain growth dependent on the structure of both monomer and radical. It is these propagation mechanisms that control polymer composition (the relative amounts of each monomer unit incorporated into the copolymer) and sequence distribution (the way in which these monomer units are arranged along the chain backbone), while the effect of radical structure on termination and transfer rates controls copolymer molecular weight. Reactivity of... 

Heteropolymers or random copolymers, in which at least two different monomer units are arranged in a more-or-less random manner along the chain. 

Diblock copolymers consist of contiguous sequences of two different covalently bound monomer units, arranged in an -A-A-A-B-B-B-B- structure. In an appropriate solvent, the diblock copolymers spontaneously self-assemble into micelles with cores which are essentially pure in one component and a diameter... 

The relative reactivities (in free-radical copolymerizations) of TBTM and MMA are 0.79 and 1.00 respectively (15). With equal concentrations of monomer, an excess of MMA in the polymer would be expected. In the following discussion A will represent the MAA or TBTM unit and B will represent the MMA unit. For A-centered triads four different arrangements are possible AAA, AAB, BAA, and BAB. Analogous sequences apply to the B-centered triads. For a random compositon, Bernoullian statistics should apply (14). With P (the proportion of TBTM) equal to 0.5, the probabilities of each of the A-centered triads is P 2 or 0.25. The AAB and BAA triads are indistinguishable and appear as a single resonance. 

It is noteworthy that the value of g is different from zero and relatively large. This result suggests that the electric and magnetic contributions to the nonlinearity are essentially of the same order of magnitude. This large value of the magnetic contributions is probably due to the near centrosym-metric arrangement of the monomer units in the helical polymer structure... 

The synthesis of biopolymers in vivo leads to macromolecules with a defined sequence of units. This effect is very important for living organisms and is different in comparison with random copolymerization in which sequences of units are distributed according to stochastic rules. On the other hand, the predicted sequence of units can be achieved by a set of successive reactions of respective monomer molecule addition. In template copolymerization, the interaction between comonomers and the template could pre-arrange monomer units defining sequence distribution in the macromolecular product. 

Natta and co-workers had produced stereospecific polymers. For example, olefins like propylene have been polymerised in such a way as to yield long linear head to tail chains consisting of sequences of monomer units having the same steric structures. These polymers are called isotactic polymers and they crystallise easily, whereas those monomeric units of different steric arrangement phased at random do not crystallise well. These polymers are called atactic. Polymers of regular, alternating structure are called syndyotactic polymers. 

A synthetic copolymer provides additional degrees of freedom in the arrangement of the repeating units. For example, the spectrum of a copolymer of vinylidine chloride and isobutylene, shown in Fig. 13.5, indicates that various tetrad sequences (sequences of four monomer units) display significantly different spectra. Copolymers composed of more than two monomer types, including biopolymers, have much more complex spectra, as we discuss later. 

The fundamental difference between the non-covalent and the covalent approach is that the latter involves template molecules which are covalently bound to monomer units prior to their addition to the reaction mixture. Subsequent co-polymerisation in the presence of a cross-linking reagent results in the incorporation of the template molecule within a polymer matrix. An important requirement is that the bond between the template and polymer is readily reversed as, to remove the template molecules, the covalent bonds between template and polymer must be cleaved. This cleavage step is very important and must be performed under conditions that will not profoundly alter the functionality or spatial arrangement at the imprinted site. The polymer is finally washed to leave the vacant imprinted sites. 

A major but defining difference between polymers and biopolymers can be found in their structures. Polymers, including biopolymers, are made of repetitive units called monomers. Biopolymers often have a well-defined structure, though this is not a defining characteristic (e.g. lignocellulose). The exact chemical composition and the sequence in which these units are arranged is called the primary structure in the case of proteins. Many biopolymers spontaneously fold into characteristic compact shapes which determine their biological functions and depend in a complicated way on their primary structures. In contrast, most synthetic polymers have much simpler and more random (or stochastic) structures [7, 8]. 

The relative arrangement of groups and atoms in successive monomer units in a polymer chain not only affects the crystallinity but also induces completely different properties in polymers. One example of this effect is found in polypropylene steroisomers. (The three stereoisomers of polypropylene can be obtained by replacing R by CH3 in Figure 1.13.) The structural difference results in profound variations in the properties of polypropylene isomers. As is evident from Table 1.4, the three polypropylene isomers appear to be three altogether different materials. 

Differences here include branching, network formation, and polymers derived from isomeric monomers, for example, polyfethylraie oxide), 1, poly(vinyl alcohol), 11, and polyacetaldehyde. 111, in which the chemical composition of the monomer units is the same, but the atomic arrangement is different in each case. This makes a considerable difference to the physical properties of the polymers, e.g., the glass transition temperature Tg of structure I is 206 K, for 11 = 358 K, and for 111 = 243 K. 

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