Poly(para-methylstyrene) or PPMS
A Novel Styrene That Didn't Quite Make It
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Technology development, like life in general, occurs through either evolutionary progress or revolutionary jumps. Incremental improvements in properties are important but hardly earth (or market) shaking. It's the revolutionary developments in polymers that lead to new markets or replacement of existing materials with new ones.
A few decades ago, a new type of catalyst was developed at Mobil Chemical. Size-specific holes in zeolites were used to control the regioselectivity of aromatic Friedel-Crafts type reactions. One of the products obtained was p-methylethylbenzene which could be dehydrogenated to the p-methylstyrene (PMS for short). Conceptually, this was a fantastic development since normal conditions for aromatic alkylation lead to a messy mixture of products that can't be separated easily (the ortho-, meta- and para-styrenes all mixed together as in the figure below).
So what did Mobil do with this exciting new chemistry? Why, they tried to revolutionize the styrene industry. No small task, given that styrene is one of the most widely used monomers in commercial polymers. Think polystyrene cups (clear but easily broken in your hand if you're the nervous type) or foamed polystyrene used for cups, plates, packaging and the all-important blocks that insulate our buildings, cars and appliances.
Most companies try to institute change a little at a time, building a small pilot plant that supplies existing customers of similar products with samples to try out in their application. Not Mobil. Nope, they used an existing facility that wasn't doing much to scale up to- wait for it- a multi-million pounds per year production capability (if memory of a news release at the time serves me correct, and it may not). Wow! That's some "pilot plant."
The idea was that lots of little advantages would add up to a convincing case to switch from styrene to p-methylstyrene. Those advantages include the following:
- Monomer is cheaper to make based on cheaper starting material, toluene
- Monomer is safer since its vapor pressure is lower than that of styrene (35 vs 90 mmHg at 80 oC)
- PMS boiling point is higher by 25 oC (170 vs 145 oC)
- Monomer has similar toxicity as styrene
- Monomer is ca 1% less dense than styrene
- Monomer synthesis uses toluene starting material which is much less toxic than benzene used to make styrene
- Monomer polymerizes faster making conversion slightly easier
- Shrinkage on polymerization is 2% less than for styrene
- Copolymerization Q and e values are very similar for PMS and styrene
- Polymer has lower density than polystyrene meaning you use less material on a pound basis
- Polymer has higher Tg than PSty (113 vs 102 oC)
- Heat distortion temperatures is higher for PPMS (95 vs 89 oC)
- Polymer processes faster under thermal conditions (spiral fill test is 26% greater)
- Being less dense, roughly 4% less polymer by weight is needed for parts
- PPMS has better flame retardant properties than PSty
- Polymer has slightly lower physical properties such as strength and modulus BUT
- Polymer can be crosslinked (eg., with electron beams) and this gives significantly better thermal and physical properties including the ability to microwave bacon on it
Despite all of these small improvements over styrene and its polymer, no major customer of styrene would switch. In short, the market said "Why change what we have and know how to do for a little bit of improvement?" Overcoming an entrenched and well-understood material requires more than incremental improvements, it requires truly revolutionary advances. The advantages have to be astounding to convince conservative companies (most of them are, you know) to take a chance on something new.
Oh, but let's back up a bit! Turns out the monomer isn't quite dead yet, just mostly dead perhaps. Commercial PMS is available from a different source called Deltech (website at http://www.deltechcorp.com/). Their product has a slightly higher amount of impurity meta-methylstyrene but they say its use in fiber-reinforced unsaturated polyester leads to higher tensile strength than with styrene. So if you want to try it out, go get you some!
Long story short: even if you develop a better mousetrap, it doesn't mean anyone will buy it. In polymer technology, especially when trying to replace a commercially well-established material, you can't just dip your toe in the water; you have to make a huge splash, like with a cannonball dive, or maybe dropping an elephant in the pool. Not easy, maybe, but you would certainly make a splash!
1. "Para-methylstyrene," Kaeding, W. W.; Young, L. B.; Prapas, A. G., Chemtech, Sept., 1982 pp 556-562. This article is mostly unavailable but a copy was kindly provided by Michael E. Prater, Office of Society Services, Library Associate, American Chemical Society, 1155 16th Street, N.W., Washington, DC 20036.
2. Deltech link to PMS page: http://www.deltechcorp.com/deltech02/monomers10/pms.htm