SolutionsPolymerization
1. 1,2- and 1,4-polymerization
Write the repeating unit of polybutadiene obtained by 1,2-addition and by 1,4-addition.
- 1,2-Addition: –[CH2–CH(CH=CH2)]n–, with pendant vinyl groups.
- 1,4-Addition: –[CH2–CH=CH–CH2]n–, with a double bond in the main chain.
2. Natural rubber
What is the structure of natural rubber? How does it differ from gutta-percha?
Both are 1,4-polyisoprenes, –[CH2–C(CH3)=CH–CH2]n–. In natural rubber all the double bonds are Z (cis) and the polymer is elastic; in gutta-percha they are E (trans) and the polymer is hard and hardly elastic.
3. Vulcanization
Why is vulcanized rubber stronger and more elastic than natural rubber?
Sulfur forms polysulfide bridges between the chains (at the allylic positions). This cross-linking stops the chains from sliding past each other and makes them recover their shape after stretching.