Structural Arrangement of Monomer Units

STRUCTURAL ARRANGEMENT OF MONOMER UNITS 3-2a Possible Modes of Propagation... 

This type of arrangement (III) is usually referred to as a head-to-tail (H-T) or 1,3-placement of monomer units. An inversion of this mode of addition by the polymer chain propagating alternately via Eqs. 3-9 and 3-8 would lead to a polymer structure with a 1,2-placement of substituents at one or more places in the final polymer chain. 1,2-Placement is usually... 

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. 

When ternary clathrates form, the structural problem concerning the arrangement of guests in the channel remains undefined. From this point of view useful information can be derived just from copolymerization, which acts as an unconventional probe for structural analysis. If we admit that interchange between included monomers is slow, the sequence of monomer units in the copolymer corresponds to that of the guests in the channel before polymerization. The polymer chain behaves as a recording tape or a permanent copy of an otherwise elusive intermolecular arrangement (23). 

The conformation of a polymer in its crystal will generally be that with the lowest energy consistent with regular placement of structural units in the unit cell. Helical conformations occur frequently in polymer crystals. Helices are characterized by a number fj where / is the number of monomer units per j number of complete turns of the helix. Thus, polyethylene could be characterized as a li helix in its unit cell with an -trans conformation. The arrangement of the molecules in the polyethylene crystal structure is illustrated in Fig. 2.8. 

The stractme of an amorphous polymer chain comprises various structural levels as it has been cleai-ly presented by Jancar [4] (i) constitution (atomic/molecular structm-e of monomer units), (ii) configuration (spatial arrangement of primary bonds independent of bond rotation), and (iii) conformation (spatial arrangement, which can be altered by bond rotation). The majority of amorphous polymer chains used as matrices contain the carbon backbone (ethyleiie/prop-ylene rubber, EPR poly(vinyl chloride), PVC polystyrene, PS poly(methyl methacrylate), PMMA poly(vinyl acetate), PVAc etc.). The approximate characteristics of the vinyl-type backbone are as follows C-C bond length b = 0.15 nm, C-C bond angle 9= 109°, C-C bond energy = 350 kj/mol. 

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 structural changes that accompanied the [2 + 2] photodimerization of the metastable a -polymorph of ort/zo-ethoxy-tranx-cinnamic acid have been studied [93]. In this study, the photochemical reaction was carried out at 293 K, and observed in situ by single-crystal X-ray diffraction. In the structure of the title compound, the three molecules in the asymmetric unit are arranged to form two potential reaction sites, but only one of these was found to be photoreactive. Since only two out of three molecules in the asymmetric unit take place in the photodimerization reaction, the crystal of the final product contains an ordered arrangement of the photodimer and the unreacted monomer. 

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... 

Tanaka, Chatani, and Tadokoro improved this model by refining the crystal structure of polyisobutene (182). The resulting structure is a 2/1 helix in which the structural unit contains four nonequivalent monomer units. In the crystal cell there are always eight monomer units arranged in three turns but the 8/3 helical symmetry is no longer retained. This example represents one of the most notable exceptions to the equivalence principle. Displacement from the exact helical conformation is small, however, and all the pairs of torsion angles fall inside the same energy well. 

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). 

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