Trans-Poly , melting

In contrast to natural rubber (cis-polyisoprene), gutta-percha (trans-poly-isoprene) is rather hard, but not brittle, and only a little elastic. It softens at about 30 °C, becomes plastic at 60 °C, and melts under decomposition at 100°C. 

The l,4-poly(butadienes) corresponding to the butene-2 compounds, however, differ in their melting points by almost 140°C (Table 1-2). 1,4-c/s-Poly(butadiene) is an elastomer l,4-trans-poly(butadiene) is thermoplastic. The other isomeric poly(butadienes) likewise show a marked difference in their properties. Aromaticized l,2-poly(butadiene), although not an isomer of poly(butadiene), shows semiconductor properties because of its conjugated double bonds. 

The two naturally occurring isomers of polyisoprene have quite different physical properties. The chemical structures of cis and trans poly dienes have been discussed in Section 2.5.4. When the chains have a cis configuration crystallization is relatively difficult. The degree of crystallinity in natural rubber is fairly low and the melting-point is just above ambient temperature ( 35°C). The chains in cw-polyisoprene can also coil relatively easily by rotation about single bonds and when the polymer is... 

Oxidative polymerization of trans-bis-deprotected 79 under Hay coupling conditions [54] yielded, after end-capping with phenylacetylene, the high-melting and readily soluble oligomers 80a-e with the poly (triacetylene) backbone [87,106] (Scheme 8). Poly(triacetylene)s [PTAs,-(C=C-CR=CR-C=C) -] are the third class of linearly conjugated polymers with a non-aromatic allcarbon backbone in the progression which starts with polyacetylene [PA,... 

A value of 102 erg/cm2 for the ys of the end surfaces of the low-melting form of poly-trans-1,4-isoprene was derived80 in the same manner. The lamella thickness... 

Figure 7.1 Effect of CHDM cis/trans ratio on the melting points of poly(1,4-cyclo-methylene terephthalate)s...

Berens.A.R., Folt,V.L. Resin particles as flow units in poly(vinyl chloride) melts. Trans. Soc. Rheol. 11,95-111 (1967). 

The microstructure and the properties depend on the cis/trans ratio of the ring bonding and on the stereochemistry between the rings. Poly(methylene-l,3-cyclopentane) obtained by cyclopolymerization of 1,5-hexadiene shows four different structures from which the tram isotactic structure is predominant, when using simple biscyclopentadienyl compounds. Higher substituted (pen-tamethyl) zirconocenes yield mainly as-connected polymers which are highly crystalline and have melting points up to 190 °C. 

Substituted polyacetylenes usually show a softening point in the range of ca. 200 to 400 °C (Table 27). It should be noted that these softening points are higher than those of usual vinyl polymers. Poly(l-chloro-2-phenylacetylene) does not melt but only decomposes (dehydrochlorinate) at high temperature. It is known that polyacetylene isomerizes at 145 °C from cis to trans and undergoes an exothermic reaction at 325 °C, but does not melt below 400 °C during its differential thermal analysis 9S). 

The solid-phase method of building up polyformals is applicable only to high-melting polymers. The required melting point is not known, but the poly-formal of trarw-l,4-cyclohexanediol melted at 206°—10° C. and that of the cis-/ trans- mixture of 2,2,4,4-tetramethyl-l,3-cyclobutanediol melted appreciably higher. The solution method did not appear to be applicable to building up the polyformals of these two diols, since inherent viscosities below 0.4 were obtained. The solution method may be most applicable to primary diols, such as cyclohexanedimethanol and decanediol, which gave polyformals with inherent viscosities of 0.9. 

A common method for polyester formation is via trans-esterification. The procedure described below uses a procedure that is modified from that described in Sorenson. In this case, aromatic di-acid 5 and di-ester 4 monomers are reacted together in the melt. During the polymerization, the molecular weight grows while volatile side-product is continuously removed from the reaction vessel by means of a vacuum pump. Scheme 2 describes the formation of a main-chain liquid crystalline poly(ester ether) containing... 

In solid, the molecules of trans isomers pack more closely and crystallize more readily than those of cis isomers. This is reflected in the fact that the melting point of fumaric acid is about 160°C higher than that of of maleic acid. Similarly, the differences in the properties of cis and trans isomers of polymers are also significant, as shown below with the example of poly(isoprene). 

The closely related trans-I,4-poly(2 ethylbutadiene) (gutta percha) has also two crystalline polymorphs. The stable, monoclinic a-crystal form (P2jC) grows from the melt above 318 K (T = 353 K) and has an entropy of fusion of 36.4 J/(K mol), indicative of full conformational order. The s nd polymorph, the orthorhombic P-crystal form (Pnam) grows at lower crystallization temperatures, but has only a somewhat lower entropy of fusion [29.7 J/(K mol)]. Conversion from p to a is not possiUe, or at least extremely slow. Both polymorphs have been asigned distinct chain conformations , i.e. neither seems to show large stale dynamic conformational disorder. 

In the 1970s, DuPont commercialized a silk-like Qiana fiber based on poly(bis[4-aminocy-clohexyljmethane dodecaneamide) or PACM,12 [87]. The base polymer is synthesized from bis(4-aminocyclohexyl)methane and dodecanedioic acid. The diamine exists in three isomeric forms trans-trans, cis-trans, and cis-cis. Its content of trans-trans isomer can be controlled by specific catalysts during its preparation. A normal diamine intermediate contained 51% trans-trans, 40% cis-trans, and 9% cis-cis isomers [88,89]. For Qiana fiber, the trans-trans content was about 70%. The polymer had a melt temperature of 290°C. 

---