Up and down chain

Statistical substitution between chains having the same helix handedness, but different orientations of the side groups ("up" and "down chains) was proved or at least suggested as probable for a variety of isotactic vinyl polymers. In particular we will mention here the cases of isotactic polypropylene (53), isotactic poly-w-butene-1 (54), isotactic polyaldehydes (55) and isotactic-1,2-poly-4-methylpentadiene (56). 

Further refinement of the crystal structure consisting of double helices is difficult, because the x-ray photograph is not well-defined, and the possibility of a disordered structure must be considered, e.g., right and left-hand helices, and up and down chains. Although there are some unexplained feature of the double helical model, such as the mode of rapid double helix formation during crystallization, the author and his coworkers believe the result to be essentially correct. 

Fig. 17.—Three Possible Arrangements of Chitin Chains. (The chain sense is indicated by arrows. (A) is for chitin 1 (B) and (C) show two possible ways of deriving chitin 2 from chitin 1 by a solid-state transformation. (B) shows up and down chains sliding into each other (C) shows chains folding upon themselves. ")...

Fig. 41. Model of the conversion of parallel- cked arrays of microfibrils of up and down chains of cellulose I to antiparallel-packed fibrils of cellulose 11 during mercerization (redrawn fiom Ref. 108). (See Color Plate 16.)...

Fig. 2 (A) Chain conformation of isotactic polypropylene in the crystalline state. Symbols R and L identify right- and left-handed helices, respectively, in 3/1 conformations. Subscripts up and dw ( dw standing for down ) identify chains with opposite orientation of C - C bonds connecting tertiary carbon atoms to the methyl groups along the z-axis (B) Limit-ordered model structure (a2 modification, space group P2 /c) [113] (C) Limit-disordered model structure (otl modification, space group Cl/c) [114]. In the a2 modification up and down chains follow each other according to a well-defined pattern. The al modification presents a complete disorder correspon ng to a statistical substitution of up and down isomorphic helices...

The infrared spectrum of 2D2 in the 670-600 cm range shows only a band near 620 cm below T (Figure 3-56). The band is not split, as mentioned above, because the precise phase coupling is lost by the random distribution of up and down chains. At the solid-solid transition a very weak additional band appears near 655 cm (Figure 3-56) indicating the formation of gauche conformations at the deuterated chain ends, thus reiterating what has been already learned from Cl9. From a quantitative measure of the absorption intensities of the these modes it transpires that approximately 25% of chain ends are conformationally distorted. 

It was also reported that the a form in iPP can be recrystallized and/or annealed from a less ordered otj form with a random distribution of up and down chain packing of methyl groups to a more ordered ot2... 

Another example of crystal lattice affected by disorder due to isomorphic substitution of different chains is provided by the a form of iPP (Fig. 2.15) [29,101,102]. As shown in Figure 2.25a, in the crystal the threefold helical chains of iPP of a given chirality may be up or down depending on the orientation of the bond vector b that connects the backbone methine carbon atoms to the lateral methyl carbon atoms, with respect to the c-axis. Isomorphous up and down (anticlined) [16] chains have a similar external steric hindrance because the methyl groups may assume the same position (Fig. 2.25b). Therefore, disorder in the positioning of up and down chains may be present in the crystals of the a form because if isomorphic and anticlined helices of iPP substitute each other in the same site of the lattice, the same steric interactions with neighboring chains are involved. 

In the limit ordered model proposed for the a form of iPP (Fig. 2.25c) up and down chains follow each other according to a well-defined pattern [101,102]. The limit disordered model (Fig. 2.25d) corresponds to a statistical substitution of up and down isomorphic helices in each site of the lattice [29]. The real crystalline modifications of the a form of iPP are intermediate between the limit ordered and limit disordered models of Figure 2.25, the degree of disorder in the positioning of up and down chains being dependent on the thermal and mechanical history of the sample [150]. 

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