Natural rubber tacticity

The possibility of tacticity is not, of course, restricted to vinyl polymers. Other types of polymer which contain asymmetric carbon atoms and which ha/e been obtained in tactic forms include poly(propylene oxide) and natural rubber hydrochloride ... 

If all the units along the chain are trans or if all the units along the chain are ois the polymer is called tactic in the first case trans-tactic, in the second case cis-tactic. It may be noted that, in the case of 1,4 polyisoprene, these two possibilities correspond to guttapercha and natural rubber respectively tactic polymers of isoprene are naturally occurring polymers. 

Of these three PP isomers (called that because they all have the same formula, just different stereoconfigurations), isotactic makes the best plastic. Atactic polypropylene is soft, elastic, and rubbery but not as good as rubber, natural, or synthetic. It is usually separated from the isotactic propylene and discarded as waste, which adds considerable cost to the remaining isotactic. The iso tactic form has a high degree of crystallinity with the chains packed... 

---