Orientation of the repeating unit

Polymers that are built from the repetition of identical "repeat units" are called "homopolymers" (from ancient Greek "d/iog — same).2 Linear homopolymer chains are obtained upon linking chemically identical units exclusively at both ends. However, repeat units are not always symmetrical in the chain direction. Depending on the orientation of the repeat unit, different microstructural... 

In Sgroups, repeating units are enclosed in square brackets and a subscript n is placed to the right of the closing bracket. The subscript blk indicates a block copolymer and mon a monomer in a source-based representation. Superscripts indicate the orientation of the repeating units (hh = head-to-head, ht = head-to-tail, eu = either unknown) in a structure-based description. Crossing bonds (bonds... 

Here it is immaterial whether II-1 or II-2 is chosen as the simplest stereorepeating unit. Infinitely long poly(propylene) molecules from II-l, of course, differ from those from II-2 only in the orientation of the repeating units. Thus, chains consisting of II-l and II-2 are not enantiomeric to each other, which is in contrast to their configurational monomeric units. 

Some of the more important factors influencing the orientation of the repeating units are ... 

The stability of the free radicals is possibly the greatest influence on the orientation of the repeating unit in a polymer chain. 

All polymer molecules have unique features of one sort or another at the level of individual repeat units. Occasional head-to-head or tail-to-tail orientations, random branching, and the distinctiveness of chain ends are all examples of such details. In this chapter we shall focus attention on two other situations which introduce variation in structure into polymers at the level of the repeat unit the presence of two different monomers or the regulation of configuration of successive repeat units. In the former case copolymers are produced, and in the latter polymers with differences in tacticity. Although the products are quite different materials, their microstructure can be discussed in very similar terms. Hence it is convenient to discuss the two topics in the same chapter. 

The structure and molecular orientation of the monolayers have been studied taking in account the different conformations of the repeating unit in the collapse area. Molecular Dynamic Simulation (MDS) has been also reported. The MDS un-... 

The well known rule for classifying stereoregular polymers states that in isotactically regular polymers the substituent groups on the asymmetric carbons of the repeating units are oriented similarly either above or below... 

The spectra of mixed polysaccharides containing 2,3-anhydro-mannose units show in the 700-950 cm-1 region a new band at 855 cm-1 characteristic of Cj-H(e) bonds. The change of the spatial orientation of the Q-H bond from axial to equatorial has evidently occurred as a result of the conversion of the repeating unit from conformation C1 to a half-chair conformation ... 

Achieving orientation of all repeating units along the flow axis is unlikely because of the excluded volume effect. 

The distrubution of substituent groups from the backbone ofa polymer is tacticity and it is displayed in Figure 2 of Appendix II. Polymers in which every repeat unit has the same left- or right-handed orientation with respect to the backbone of the chain are called isotactic. Polymers in which the optical orientation of each repeat unit with respect to the backbone of the chain is the opposite of the preceding repeat unit are called syndiotactic. Polymers in which there is no order or pattern to the distribution of optically active sites with respect the the backbone are called atactic. 

The length of the repeat unit of PpBAT is about 1.5 times that of PpPTA, but the X-ray diffraction patterns are very similar, yielding the same unit cell dimensions (a 0.797, b 0.519, c 1.29 nm). Due to the polarity of the 4,4 -diaminobenzanilide molecule, its amide group can take two orientations in the chains leading to disorder in the crystal structure, which is the cause of the apparent repeat unit of 1.29 nm. The disorder also affects the lateral packing. Not more than 5 out of 6 possible hydrogen bonds per two monomeric units can be formed between the chains, which has been confirmed by IR spectroscopy [114]. 

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