Syndiotactic propylene-butene copolymers

Copolymers of syndiotactic poly(propylene) behave in a similar manner.(64) Here the copolymers with ethylene, 1-pentene, 1-hexene and 4-methyl-1-pentene as co-units obey the same melting temperature-composition relation. On the other hand, the copolymer with 1-butene gives higher melting temperatures than the others. This result will also be discussed further in the next section. 

Both isotactic poly(propylene-co-butene) [81, 82] and syndiotactic poly(propylene-co-butene) [83] statistical copolymers exhibit isodimorphic behavior when crystallized. In isotactic poly(propylene-co-butene), the copolymer was found to crystallize over the entire range of butene content. A transition in crystal unit cell structure was observed at about 50 mol% butene, where below this composition, the crystalline phase resembled that for polypropylene and above this composition, the copolymer was essentially polybutene with propylene defect units [81]. The observed isodimorphic behavior can be explained by the similar helical chain structures exhibited by both polypropylene and polybutene in the crystal, which results in a relatively small free energy penalty for co-crystallization [42, 81-83],... 

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