27677
The elucidation of the reaction mechanisms leading to either the desired production of ethene and / or propene or undesired deactivation has challenged researchers for many decades .
 Chemical: ethene; Chemical: propene
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: Chemical 
Text: {0} 
Answer:
 Chemical: ethene; Chemical: propene