polyethylene vs polymethylene

polyethylene

noun
  • A polymer consisting of many ethylene monomers bonded together; used for kitchenware, containers etc. 

polymethylene

noun
  • Any of many organic compounds either having a polymeric structure of repeating -CH₂- groups 

How often have the words polyethylene and polymethylene occurred in a corpus of books? (source: Google Ngram Viewer )