Crystallinity trans-polybutadiene

If, however, the phenylene rings are para-oriented, the chains retain their axial symmetry and can crystallize more readily. Similarly, double bonds in trans configuration maintain the chain symmetry thus allowing for crystallite formation. This is highlighted by a comparison of the amorphous elastomeric ciy-polyisoprene (Tm = 28°C) with highly crystalline trans-polyisoprene (Tm — 74°C) which is a non-elastomeric rigid polymer, or c -l,4-polybutadiene (Tm = - 11°C) with ram-l,4-polybutadiene (Tm = 148°C). 

Styrene - Crystalline Polystyrene 85% Styrene - Styrene Butadiene Copolymer 45% Styrene Styrene Butadiene Copolymer 30% Styrene Styrene Butadiene Copolymer 23% Styrene - Styrene Butadiene Copolymer 5% Styrene - Styrene Butadiene Copolymer Cis Polybutadiene Cis-Trans Polybutadiene... 

Additional evidence for chain folding in solution-grown crystals comes from carbon-13 NMR studies of partially epoxidized 1,4-trans-polybutadiene crystals (49,50). This polymer was crystallized from dilute heptane solution and oxidized with m-chloroperbenzoic acid. This reaction is thought to epoxidize the amorphous portions present in the folds, while leaving the crystalline stem portions intact. 

Pure tra i-l,4-polyisoprenes as well as 1,4-polybutadienes can be synthesized by polymerization in inclusion compounds [266-269]. As typical hosts for this dienes, the inclusion compounds or clathrates of urea, thiourea, or perhydrotriphenylene [PHTP Eq. (36)] are used [270,271]. The host forms the frame of the crystal and the guest is placed in the cavities existing in the lattice. Polymerization is generally started by subjecting the inclusion compound to irradiation with a-, y-, or x-rays and proceeds by a radical mechanism [272,273]. Also, free radical initiators such as di-/cr/-butylperoxide could be used [274]. Inclusion in urea yields crystalline trans-, A polymers, whereas trans-lA-polyisoprene obtained in PHTP is amorphous. There is no trace of, A-cis units or of 1,2, 3,4, and cyclic units. The reason for the amorphous product is the presence of a substantial number of head-to-head and tail-to-tail junctions in addition to head-to-jail junctions [275, 276]. 

As an example of chemical structure and torsional angles, a sketch of a 1,4-poly butadiene chain in the planar all-trans conformation is shown in Figure 6. Rotational angles about bonds of the chain skeleton are measured relative to the planar conformation shown, for which (j)=0°. X-Ray diffraction studies on cis- and trans-polybutadiene indicate that CH2-CH2 bonds such as bond (i-l-2) connecting C,+i and Cj+2 in Figure 6 assume the trans conformation in the crystalline state. The rotational states accessible to these bonds in the amorphous state or in solution are concluded... 

EPM > CO > TP > CB. The highly crystalline TB had an etch rate about six times that of CB, ascribable to a morphology difference, while the partially crystalline TO had an etch rate somewhat higher than that of amorphous CO. Cis/trans content had little or no effect on the etch rate of the polyalkenamers. A mechanism involving crosslinking through vinyl units is proposed to explain the unexpected protection imparted to vinylene-rich polybutadienes by the presence of 1,2 double bonds. 

The data of Table II indicate that the etch rates for CB and its "homologues"—TP, CO (or TO), and EPM—tend to increase monotonically with a decrease in vinylene (-CH=CH-) unsaturation. The elastomeric EPM was chosen instead of crystalline polyethylene as a model for the fully saturated CB to avoid a morphology factor in etch rates, as was observed with crystalline TB. The difference in etch rates for the partially crystalline TO and the elastomeric CO (ratio of about 1.2 1.0) is attributable more to a morphology difference between these polyoctenamers than to the difference in their cis/trans content. Cis/trans content had likewise no perceptible effect on etch rates in the vinyl-containing polybutadienes (see Table I) if there was a small effect, it was certainly masked by the dominant effect of the vinyl groups. 

Natta, Porri, Carbonaro and Lugli (25) have prepared copolymers of 1,3-butadiene with 1,3-pentadiene in the whole range of compositions. The properties of the copolymers, in which all butadiene and pentadiene comonomer units are in the trans-1,4 configuration, clearly show the isomorphous replacement between the two types of units. The melting point/composition data show that the copolymer melting temperatures are a regular function of composition and are always comprised between those of trans-1,4-polybutadiene modification II and trans-1,4-polypentadiene. Also the X-ray diffraction spectra of the copolymers show that the trans-1,4-pentadiene units are isomorphous with the trans-1,4-butadiene units crystallized in the crystalline modification of the latter stable at high temperatures (form II). 

Another type of geometric arrangement arises with polymers that have a double bond between carbon atoms. Double bonds restrict the rotation of the carbon atoms about the backbone axis. These polymers are sometimes referred to as geometric isomers. The X-groups may be on the same side (cis-) or on opposite sides (trans-) of the chain as schematically shown for polybutadiene in Fig. 1.12. The arrangement in a cis-1,4-polybutadiene results in a very elastic rubbery material, whereas the structure of the trans-1,4-polybutadiene results in a leathery and tough material. Branching of the polymer chains also influences the final structure, crystallinity and properties of the polymeric material. 

Since isomerically pure polymers were not available, three different kinds of BR, each relatively high in one of the three kinds of base units were used as standards [35]. The band near 1308 cm 1 was identified [38,39] with the cis isomer and used for analyses [43]. The 1308 cm 1 band is weak and relatively broad, with the appearance of an unresolved doublet (1306,1311 cm 1). The cis band at 730 cm 1 is more frequently used in spite of some difficulties. Relatively pure, crystalline stereoregular polymers have been prepared and structures were determined by X-ray diffraction for cis [44], trans [45] and syndiotactic vinyl [46] and isotactic vinyl [47]. Infrared spectra [48-50] have been published for the four stereoregular polybutadienes, with detailed analyses of the spectra and band assignments for cis [51], trans [51] and syndiotactic vinyl [51] polymers. For the spectrum of isotactic vinyl BR, bands at 1232, 1225, 1109, 943, 876, 807 and 695 cm"1... 

The dependence of the polymer microstructure on the ratio of catalyst components is related to the nature of these components. The structure of polybuta-diene obtained with an aluminum triisobutyl (AIBU3)-titanium tetrachloride catalyst system is a function of the Al/Ti molar ratio (Table II). Polybutadiene prepared at Al/Ti ratios of 0.5 to 8 in benzene or heptane and at 3° or 25° C. contain at least 90% 1,4- units. Polymerizations carried out at ratios of 1.0 and less at 25° C. in heptane and at ratios of 1.25 or less at 3° C. in heptane or benzene give crystalline polymers containing more than 96% trans-, A- structure (6). A similar temperature dependence of polymer structure has been reported in the polymerization of butadiene with a diethylcadmium-titanium tetrachloride catalyst system (3). Polybutadiene obtained with a triethylaluminum-titanium tetrachloride catalyst system at a 0.9 Al/Ti ratio at 30° C. in benzene is reported to contain 67% cis-1,4- units (19). 

Fig. 4.10. Thermal analysis and proton NMR second moment curves of trans-l,4-polybutadiene. The heat capacity for the solid (glass and low temperature crystal form I) have been computed by fitting at low temperature to an approximate skeletal vibration spectrum. The gradual decrease of the second moment of the proton resonance spectrum below T comes mainly from increasing mobility in the amorphous phase, Ref. The melting curve is of an approximately 90% crystalline sample. Ref.. Compare also to Fig. 1.3...

The factors mentioned above adequately explain the superior cold properties of the lithium-catalyzed polybutadiene. Compared to emulsion polybutadiene, the lithium-catalyzed polybutadiene has more of the cis-1,4 structure and less trans-1,4 and 1,2 structures. All of these changes are in the direction to increase the relative amounts of more desirable microstructures, at least from the standpoint of cold properties. In addition, there is also a decrease in the predominant structure, trans-1,4, compared to the emulsion polybutadiene. Therefore, it would be less crystalline or orient less. 

Other monocyclic monomers will imdergo ring-opening reactions cyclobutene and 1-methyl cyclobutene produce polybutadiene and cis-polyisoprene, respectively but this synthetic route cannot compete with estabhshed methods for preparing synthetic elastomers. Reactions are slower with the larger rings, as the strain decreases, but a useful elastomer with a high trans content and well-developed crystallinity (such as the product sold under the trade name Vestenamer) can be prepared from cis-cyclooctene (the trans compound is umeactive). 

As shown in Figure 1.7, the polymerization of monomers with two double bonds (e.g., butadiene and isoprene) leads to polymer chains with a residual double bond per monomer unit cis-trans isomerism is possible. Two important polymers that show this type of isomerism are 1,4-polybutadiene and 1,4-polyisoprene. The regularity of the trans configuration makes this type of isomer more crystalline, with a higher melting point compared to the cis configuration. 

Halogenation reactions of unsaturated polymers follow two simultaneous paths, ionic and free radical. Ionic mechanisms give soluble products from chlorination reactions of polybutadiene." The free-radical mechanisms, on the other hand, cause crosslinking, isomerization, and addition products. If the free-radical reactions are suppressed, soluble materials form. Natural rubber can be chlorinated in benzene with addition of as much as 30% by weight of chlorine without cycliza-tion. " Also, chlorination of polyalkenamers, both cis and trans, yields soluble polymers. X-rays show that the products are partly crystalline. The crystalline segments obtained from 1,4-trans-polyisoprene are diisotactic poly( 0 /rw-dichlorobutamer)s while those obtained from the 1,4-cis isomer are diisotactic polyOAfieo-l,2-dichlorobutamer)s. ... 

The polymer was amorphous at room temperature like cis-1, 4-polybutadiene. However, at —30°C the polymer was crystalline like the all cis polymer under the same conditions. The evidence indicated, therefore, that the mixed polymer contained long blocks of cis-1,4 units separated by shorter sequences of trans-l,A units, the latter being incapable of crystallization. 

At Al/Ti > 2.0 the catalytic behavior was quite different. The amorphous precipitate was catalytically active without mother liquor owing to the presence of trapped alkylaluminum compounds. Although the polymers prepared with the washed precipitates were about 60% iraws-1,4-polybutadiene, they nevertheless exhibited, at room temperature, crystallinity characteristic of sterically pure trans-1,4 poylmer. After ether extraction a very pure (> 90%) tmns-l,4-polybutadiene was left. The ether extract afforded very pure (94-95%) cis-l,4-polybutadiene. 

The latest more extensive work on the chlorination of PVC was part of an overall study of the chlorination of polyalkenamers. The chlorinated butadiene polymer (poly-butenamer) was found to be crystalline or at least had crystalline chain segments a unit cell for chlorination products of trans-1,4-polybutadiene was determined and for the crystalline material a structure of a diisotactic poly(erythro-l,2-dichlorobutadiene) was proposed. This proposal required the assumption that the addition of chlorine is stereospecific. It was also mentioned that the number of ordered units of about ten is sufficient to display crystallinity, sufficient to allow the determination of the structure by x-ray analysis. 

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