Molecular weight changes a lot the melting point for instance, but does it change the heat of "vaporization" (including some cracking)
per mass unit? I expect the influence is small.
From existing papers, polystyrene is the common plastic in waste after polyolefins. It gives much liquid styrene upon heating to moderate temperature, so the required heat per monomer mole for this particular reaction would only be the inverse of polymerization, which is certainly known.
Though, styrene is a bad fuel, and I suppose it will contain big amounts of carcinogenic aromatic compounds. If much hydrogen is available - for instance by full pyrolysis of a part of the polystyrene or polyolefins - you might
hydrogenate the liquid to get ethylcyclohexane and the like, which would be acceptable fuels.
Papers consider instead to
recycle the styrene into plastics. Obscure to me: how expensive it is to get decently pure monomers from the pyrolysis mushy, and how this cost compares with cleaner paths.
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If this can fit in your study, I wish you check the pyrolysis of waste
tyres. The tetramer, pentamer and trimer of isoprene would make excellent fuels after hydrogenation (phytane, farnesane) - for rockets, Diesel engines, jet engines, and as transformer oil, lubricants... If the oligomer result from pyrolysis, fine. If not, take isoprene as an intermediate. Superior transformer oil and rocket fuel have more value than a bad heating fuel, making economic recycling more credible.
Isoprene itself is cheap, but if obtaining the proper oligomers (head-to-tail) directly by pyrolysis, it avoids a delicate operation, and the price ratio between waste tyres and good transformer oil should afford moderate yields. Maybe hydrogenation can be conducted during the pyrolysis and reduce the proportion of by-products like limonene.
The competitor to waste tyres in that particular transformation would be hevea and gutta-percha.
http://www.chemicalforums.com/index.php?topic=56069.msg202845#msg202845(I've put many mistakes there, some are noted within the thread, more should remain, caution).