Isotactic polypropylene chain orientation

For the growth of isotactic polypropylene chains and higher polyolefin chains at the chiral coordination sites of ansa-metallocene catalysts, the following explanation is now firmly established [11,12] Formation of the new C-C bond requires that the a-olefin substituent and the C(a)-C(P) bond of the metal-bound polymeryl chain are oriented anti to each other along the incipient C-C bond, while the C(a)-C(P) chain segment must reside in an open quadrant of the chiral metallocene coordination site. The latter is thus considered to control the enantiofacial orientation of the a-olefin in the insertion transition state TS (Fig. 4) by way of the C(a)-C(P) chain-segment lever . 

Snetivy D and Vancso G J 1994 Atomic force microscopy of polymer crystals 7. Chain packing, disorder and imaging of methyl groups in oriented isotactic polypropylene Po/yme/ 35 461... 

We find an additional crystalline structure in rapidly quenched isotactic polypropylene products. In these materials, the polymer chains do not have the necessary time to orient,... 

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). 

Figure 4 shows that the orientation of the two chains follow similar paths. The similarity suggests that the local environment is very much the same for both blend constituents. In contrast, a phase-separated binary mixture of polymer segments will exhibit a different orientation for each component. For example, high-density polyethylene and isotactic polypropylene form multiphase systems when blended together (16). When the incompatible blends are deformed, the component which constitutes the continuous phase always orients to a higher degree than... 

The chemical structure of a polymer determines whether it will be crystalline or amorphous in the solid state. Both tacticity (i.e., syndio-tactic or isotactic) and geometric isomerism (i.e., trans configuration) favor crystallinity. In general, tactic polymers with their more stereoregular chain structure are more likely to be crystalline than their atactic counterparts. For example, isotactic polypropylene is crystalline, whereas commercial-grade atactic polypropylene is amorphous. Also, cis-pol3nsoprene is amorphous, whereas the more easily packed rans-poly-isoprene is crystalline. In addition to symmetrical chain structures that allow close packing of polymer molecules into crystalline lamellae, specific interactions between chains that favor molecular orientation, favor crystallinity. For example, crystallinity in nylon is enhanced because of... 

U.V. Makedonov, A.Z. Margolin, H.la. Rapoport, LS. Shibriaeva About the reasons of the change of effective constants of the rate of reactions of continuation and chains breaks in the period of oxidation induction of isotropic and oriented isotactic polypropylene////zg/zzzzo/eczz/z7r compounds (1986), 28 (A), No 7, 1380-1386 (in Russian). 

FIGURE 7.17 Polymers of propene. The main chain is shown in a zigzag conformation. Every other carbon bears a methyl substituent and is a stereogenic center, (a) All the methyl groups are on the same side of the carbon chain in isotactic polypropylene, (b) Methyl groups alternate from one side to the other in syndiotactic polypropylene, (c) The spatial orientation of the methyl groups is random in atactic polypropylene. 

Makedonov Yu.V., Margolin A.L., Rapoport N.Ya. and al. (1986)." On the causes of changes in the effective rate constants continue and chain termination in the induction period of oxidation of isotropic and oriented isotactic polypropylene". Vysoconwlek. Soedin. V.28 A. No 7. PP.1380-1385. (In Russian)... 

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...

Isotactic polypropylene (iPP), when quenched from the melt in cold water, gives rise to a metastable, solid mesophase, which transforms into the stable a form by annealing at elevated temperatures [25-27]. The X-ray powder diffraction pattern of this disordered form of iPP presents only broad halos at 20 = 14.8 and 21° (Fig. 33A). Oriented fibers of iPP in this mesomorphic form can be obtained by stretching the corresponding unoriented films [25]. The X-ray fiber diffraction patterns present broad peaks on well-defined layer lines as shown in Fig. 33B the chain periodicity corresponds to 6.5 A, as in the crystalline a form. 

In one study. X-ray diffraction, Fourier transform infrared (FTIR) spectroscopy and refractive index measurements were utilized to characterize the state of molecular orientation in one-way and two-way (bi-axially) drawn isotactic polypropylene (iPP) films [6]. It was shown that the use of all three techniques leads to much greater confidence in the orientation averages deduced than can be obtained by using any two of the techniques. It was observed that, with one-way drawing, the chain axes of both crystalline and amorphous regions orient towards the direction of drawing. The crystalline chains are more highly oriented than the amorphous chains and tend to orient towards the plane of the... 

Uniaxially oriented isotactic polypropylene (iPP) was imaged using afm, showing microfibrils and macromolecules. Fibrils with an average diameter of 150 nm were observed. Individual polymer chains with 1.17 nm chain-chain distance were seen. The authors propose that the (110) crystal plane was being resolved with this work (90). Other workers, who were able to clearly resolve right-and left-handed helices (Fig. 13) with pendant methyl groups visible, accomplished atomic scale resolution of iPP (91). 

Figure 3-9. Infrared absorption spectrum of isotactic polypropylene in the various physical states, (a) Melt (—) and normal solid at room temperature (—) New bands appear associated to regularity bands originating from k = 0 phonons of the chain as ID crystal (b) stretch-oriented film in polarized light showing which of the bands are of A( j (------i or E(X i—) species (from [74]).

Most polypropylene used in various textile products is produced from isotactic polymers. Isotacticity of polypropylene has significant influence on tape or fiber property. Only iso- and syndiotactic polymer have sufficient molecular order to create polycrystalline structure and hence acceptable physical and tensile properties for oriented tapes. Isotactic polymer chain is characterized by the arrangement of the functional group on one side of the plane. 

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