The Collaborative International Dictionary of English v.0.48:

Pentamethylene \Pen`ta*meth"yl*ene\, n. [Penta- + methylene.]
   (Chem.)
   A hypothetical hydrocarbon, C5H10, metameric with the
   amylenes, and the nucleus of a large number of derivatives;
   -- so named because regarded as composed of five methylene
   residues. Cf. Trimethylene, and Tetramethylene.
   [1913 Webster]