Conformation of Polymer Chains in Crystals and Conformational Polymorphism

Double bonds present along the chains of polymers of 1,3-dienes with 1,4 additions of the monomeric units are stereoisomeric centers that may assume cis or trans configurations. Polidienes may also contain up to two stereoisomeric tetrahedral centers. Stereoregular polydienes can be cis or trans tactic, isotactic or syndiotactic, and diisotactic or disyndiotactic if two stereoisomeric tetrahedral centers are present. In the latter case, erythro and threo structures are defined depending on the relative configurations of two carbon atoms [1]. 

The requirement concerning the regularity in the configuration (stereoregularity) is more stringent than that 

2 CONFORMATION OF POLYMER CHAINS IN CRYSTALS AND CONFORMATIONAL POLYMORPHISM 

The conformation assumed by polymer chains in the crystalline state depends on the configuration of the stereoisomeric centers present along the chains, and is defined by two basic principles [1,2,23-25]. 

The equivalence principle. The conformation of a polymer chain in the crystalline state is defined by a succession of equivalent structural units, which occupy geometrically (not necessarily crystallo-graphically) equivalent positions with respect to the chain axis. The chain axis is parallel to a crystallographic axis of the crystal. 

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Conformation of Polymer Chains in Crystals and Conformational Polymorphism, 33... 

CONFORMATION OF POLYMER CHAINS IN CRYSTALS AND CONFORMATIONAL POLYMORPHISM 39 TABLE 2.2 Continued)... 

Crystallization from the melt or solution produces the most common polymorph of PVDF, the a phase (also known as form II). The chain conformation, shown schematically in Fig. 5.1(a) is trans gauche trans gauche (tg" tg ). Each chain conformation possesses a net dipole moment, originating from the CF2 bond, with a component perpendicular to the polymer chain. In the case of the tg tg" conformation the dipole components normal to the chain axes are antiparallel and thus cancel each other. Therefore the a phase is a non-polar form. 

Data concerning the chain conformations of isotactic polymers are reported in Table 2.1. In all the observed cases the torsion angles do not deviate more than 20° from the staggered (60° and 180°) values and the number of monomeric units per turn MIN ranges between 3 and 4. Chains of 3-substituted polyolefins, like poly(3-methyl-l-butene), assume a 4/1 helical conformation (T G )4,45,46 while 4-substituted polyolefins, like poly(4-methyl-1-pentene), have less distorted helices with 7/2 symmetry (T G )3.5-39 When the substituent on the side group is far from the chain atoms, as in poly(5-methyl-1-hexene), the polymer crystallizes again with a threefold helical conformation (Table 2.1). Models of the chain conformations found for the polymorphic forms of various isotactic polymers are reported in Figure 2.11. 

Whereas most of the early work on crystallization, etc., were concerned with predominantly isotactic chains, the recent developments in synthetic methodologies have enabled the preparation of highly syndiotactic polymers [13,14]. Since the high stereoregularity of these syndiotactic polymers facilitates their crystallization, several papers have been published on the x-ray crystal structure and polymorphism of syndiotactic polystyrene [15-18]. The chain conformation in the crystalline state has also been analyzed using NMR [19]. Similarly, the crystal structure of syndiotactic polypropylene has also been studied by a number of authors [20-22]. 

Finally, a few comments about the uniqueness of polymer crystal structures and phase space localization are warranted. Almost all crystallizable polymers exhibit polymorphism, the ability to form different crystal structures as a result of changes in thermodynamic conditions (e.g., temperature or pressure) or process history (e.g., crystallization conditions) [12]. Two or more polymorphs of a given polymer result when their crystal structures are nearly iso-energetic, such that small changes in thermodynamic conditions or kinetic factors cause one or another, or both, to form. Polymorphism may arise as a result of competitive conformations of the chain, as in the case of syndiotactic polystyrene, or as a result of competitive packing modes of molecules with similar conformations, as in the case of isotactic polypropylene. In some instances, the conformational change may be quite subtle isotactic polybutene, for example, exhibits... 

The ordered structures of some polymers are governed by the influence of specific diluents. This involves a specific type of polymorphism, the more general aspects of which will be discussed in the chapter concerning thermodynamic quantities. Syndiotactic poly(styrene) is a polymer that is rich in compound formation with solvent mediated polymorphic behavior.( 126-130) The polymer can crystallize in four major crystalline modifications that involve two different chain conformations. In the a and p modifications the chains adopt an all trans planar zigzag conformation. These two modifications are formed by crystallization from the melt and, under special conditions, from solution. In contrast the y and 5 modifications are characterized by a helical conformation. The 5 polymorph can only be prepared in the presence of solvent. Its exact crystal structure depends on the nature of the solvent. Compound formation between the 5 form of the polymer and the solvent has been demonstrated. Complete elimination of the solvent results in the pure, helical y form. 

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