Polymers, chain type double orientation

This brings us to double stereoselection and reinforcement of the mechanisms. If the site (a)symmetry were to control the orientation of the chain, and if, then, the orientation of the incoming propene is controlled by both the chain and the site, the highest stereoselection is obtained when the two influences reinforce one another. For 1,2-insertion this can be done most effectively for isotactic polymerization, since chain-end control naturally leads to isotactic polymer and this we can reinforce by site control with ligands of the bis(indenyl)ethane type. The chain-end influence of short chains is smaller than that of longer polymer chain and therefore short chain ends lead to lower selectivities. It may also be irrferred that making syndiotactic polymer via a 1,2-insertion mechanism on Ti or Zr complexes is indeed more difficult than making an isotactic polymer, because the two mechanisms now play a counterproductive role. 

Figure 3c. Since Head to Head linkages only makeup 1 to 2 percent of typical poly(l-hydroxyediylene), die polymer is lysed into 50 to 100 repeat unit long chains by digestion widi aqueous periodate. Tacticity and head or tail bonding are two examples of configurations, patterns of atom distributions produced by different bondings, that can exist in polymers. The word structure will be used to identify bond types, double versus single bonds, and atom types in a molecule or repeat unit while configuration will be used to describe differences in the order or orientation of tJiose bonds and atoms.

Polymerization of diene monomers such as 1,3-butadiene, when incorporated by 1,4 addition, will produce in-chain double bonds with cis (Z, 56) or trans (E, 57) structures with respect to the relative orientation of polymer s main chain. Identification of the configuration of these isomers may be complicated due to the possibility of 1,2-addition to form pendant vinyl groups (58), regioisomerism from H-T/H-H/ T-T combinations of type 58 units, and stereochemical isomerism in the polymer chain from type 58 units. 

The reactivity of the DSP crystal was thoroughly interpreted in terms of the topochemical concept proposed by Schmidt, in which potentially reactive double bonds are oriented parallel to each other and separated by approximately 3.5-4.2 A. The reaction proceeds with a minimum of atomic and molecular motion (2 ). The reactive double bonds in most of the topochemically polymeric crystals thus far found are related to the center of symmetry (centrosymmetric ct-type crystal) and dimerize to give highly crystalline polymers containing cyclobutanes with a 1,3-trans configuration in the main chain. 

However, when applying results of the type (57), it must be borne in mind that such equations apply to the amorphous constituent only, and must fail at advanced degrees of stretch where crystalliisation sets in since the influence of crystallisation on the double refraction of rubber-like polymers is considerable In addition to the intrinsic double refraction of the gel frame, the swollen gel may show structural double refraction and adsorption double refraction. The structural, or textural , birefringence is caused by the difference between the refractive indices of polymer substance and solvent. It was estimated by Wiener for rod-shaped particles and for platelets. Wiener s theory is not applicable to rods of molecular thickness, and the structual double refraction caused by oriented long-chain molecules has not yet been accessible to a reliable theoretical treatment. 

In a polymer made up of long, chainlike molecules, the side groups may be oriented in either an orderly or a random pattern with respect to the chain. There are two types of orderly arrangements (a) all the side groups may lie on the same side of die chain (b) the side groups may alternate with respect to the chain. These patterns are illustrated for polypropylene in Fig. 26.1. The structural difference between natural rubber (cis) and gutta-percha (trans), shown in Table 26.1, is a consequence of restricted rotation around double bonds. 

Series C compares LLDPE with different chemical structures, but transformed in the same processing conditions. Because of these changes in chemical structure, inducing for instance differences in the relaxation times, the quantitative levels of orientation are different from one polymer to another. Nevertheless, they all exhibit the same type of texture, with the same features as in series B for specimens cooled with a double-flux air ring (Fig. 15.28e) almost isotropic distribution of 6-axes, and c-axes preferentially in the film thickness. The amorphous chains, for their part, are isotropically located in the film plane (Fig. 15.28e). 

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