Dicyclopentadiene polymerized sulfur

Figure 2. Change in stress-strain behavior with time, with dicyclopentadiene polymerized sulfur...

Although both linear polyethene and isotactic polypropene are crystalline polymers, ethene-propene copolymers prepared with the aid of Ziegler catalysts are excellent elastomers. Apparently, a more or less random introduction of methyl groups along a polyethene chain reduces the crystallinity sufficiently drastically to lead to an amorphous polymer. The ethene-propene copolymer is an inexpensive elastomer, but having no double bonds, is not capable of vulcanization. Polymerization of ethene and propene in the presence of a small amount of dicyclopentadiene or 1,4-hexadiene gives an unsaturated heteropolymer, which can be vulcanized with sulfur in the usual way. 

The rationale in using these particular dienes is that only the strained double bond of dicyclopentadiene and the terminal double bond of 1,4-hexadiene undergo polymerization with Ziegler catalysts. Consequently the polymer chains contain one double bond for each molecule of dicyclopentadiene or 1,4-hexadiene that is incorporated. These double bonds later can be converted to cross-links by vulcanization with sulfur (Sections 13-4 and 29-3). 

A method, using differential scanning calorimetry, has been developed to estimate quantitatively orthorhombic and monoclinic sulfur in sulfur materials. Sulfur cooled from the melt at 120°C immediately gives monoclinic sulfur which reverts to orthorhombic sulfur within 20 hr. Limonene, myrcene, alloocimene, dicyclopentadiene, cyclododeca-1,5,9-triene, cycloocta-1,3-diene, styrene, and the polymeric polysulfides, Thiokol LP-31, -32, and -33 each react with excess sulfur at 140° C to give a mixture of poly sulfides and unreacted sulfur. In some cases substantial amounts of this unreacted sulfur may be held indefinitely in a metastable condition as monoclinic sulfur or S8 liquid. Limonene, myrcene, and dicyclopentadiene are particularly effective in retarding sulfur crystallization. 

Incompatibilities and Reactivities Strong oxidizers, fuming nitric acid, sulfuric acid [Note Polymerizes to dicyclopentadiene upon standing ]... 

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