The grease starts out as long chain triglycerides. But there’s an easy reaction with methanol or methoxide anion (essentially wood alcohol or its base form), referred to as a transesterification, that converts the grease to long chain methyl esters and glycerine. And the long chain fatty esters are essentially the molecular analog of the compounds found in diesel, but with an oxidized group on the end that doesn’t really matter that much.
Sometimes figuring out what to do with the leftover glycerine is a bit of a problem.
Thank you for the explanation, the word “polymer “ just didn’t sound right to me, when we’re talking about fat.