Poly(4-methylpentene)
Isotactic polymer of 4-methylpentene
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Poly(4-methylpentene) is very unusual for a polyolefin made of just carbon and hydrogen. The monomer is made by dimerizing propylene, but we won't go into that here. What's most important is that it has a melting point of about 220-250 oC, which is incredibly high for a polyolefin, high enough that you can steam sterilize plastic materials used in an operating room. The range of temperatures is because it depends very much on sample history. One commercial supplier, Mitsui Chemicals, reported Tm's of 220-240 oC but those were for copolymers with an unspecified comonomer. In any event, these are high Tm values, and in fact, are higher than those of nylon 6 and isotactic polypropylene, meaning it should hold it's shape up to that temperature, right?.
Well, not so fast. You're confusing what happens to the crystalline portion of the sample with what happens to all of it altogether. And that means you have to look at the amorphous part also. "Amorphous part??" Yeah, well it turns out, almost no polymer is 100% crystalline no matter how much it might want to be. Things like rate of cooling or solvent used to crystallize from affect how fast the individual chains can align themselves with a crystal face. Because polymers are so big and entangle so much, some chains just never move fast enough to do this. That means there is always some amorphous content, polymer chains that may have some of their segments locked into crystalline domains but with other parts all mixed together in regions that aren't crystalline. Seems odd but this is the case for almost all polymers no matter how hard you try to make them crystallize.
And just to remind you, while the melting transition involves long polymer chain segments freeing themselves from the crystalline lattice (at around the crystalline melting point), the amorphous phase undergoes a different type of thermal transition. It's called the glass transition, and it's always much lower then Tm. The molecules go from a glassy state (hence the name) to a rubbery or liquid-like state as it passes through Tg. The molecular motion associated is very different than for Tm, consisting of only short-range reorientations that involve just a few repeat units. For this polymer, the Tg is only about 30 oC or so. Quite a drop from the Tm, no?
Take a look at the 3D model of this polymer at the top of the page. Play around with it, rotate it, maybe zoom in or out so that you can see how parts of the chain are arranged. This is an isotactic polymer because of the catalyst used to make it. That means all the pendent groups (three carbons plus a methyl attached to the penultimate carbon) are on the same side of the chain if it were stretched out into a linear zig-zag form. In fact, because the pendent groups get in each others way in that form, the chain actually twists along its axis to make a helix, and it's reported to adopt a 7-2 helix. That means seven repeat units for every two turns of the backbone around the central axis.
Look again at the model and see if you see that helix. Also notice that not all the pendent groups orient the same with respect to the backbone. Some are bent one way, others are bent a different way. In the crystal, they would all be bent the same, making for a very tightly packed domain around the chains. Simply put, everything (backbone and pendents) all fit tightly together in a long rod-like structure, and that's why the high Tm. And this despite the fact that the inter- and intramolecular forces are only the weak van der Waals forces, not the much stronger hydrogen bonding found in nylons. Now look at the space-filling model of this polymer below. Wow, crowded and looks messy but everything packs together tight!
Another interesting fact about this polymer in the solid state is its density. It's only about 0.83 g/cm3 which is the lowest of any polyolefin. Despite a crystallinity of about 30-40%, this polymer readily floats on water, so you could make a board out of it that wouldn't sink or melt in the hot sun of Death Valley. Why you'd want a plastic board in Death Valley and where you'd get water to float it in is beyond me, but hey, people do crazy things with unusual materials like this polymer.
So why the low density? Just one word: entropy! Yup, you may have heard of this word before (in fact, you really better have). It has to do with degrees of freedom and motions available. If only 30% of the solid polymer is crystalline, why, that leaves 70% that isn't. And that 70% is amorphous with lots of wiggle room for the backbone and the pendent groups with four carbons in them. Here's the paradox: you could say the amorphous phase is so loosely packed that it gives the chains lots of room to wiggle around. Or you could say that because the chains want lots of room to wiggle around, they don't pack well in the amorphous domains. Either way: entropy.
So what else is fascinating about this unique polymer? Well, two things actually. One, because of it's low density, there are lots of pathways for gases of various kind to get through the solid polymer. Using this property, gas separation membranes have been described in a patent, membranes that pass one size gas molecules fast but others not so fast. Useful in a world of increasing demand for pure gases.And two, PMP or poly(4-methylpentene) is transparent. Not a big deal, right? Polypropylene is transparent, too, and used for see-through frig containers to store left-overs. right? Well, the difference is this: PMP is transparent just as it is while polypropylene needs an additive to make it so. That additive is called a "clarifier" but actually, it's a nucleating agent that works by rapidly causing small crystallites to form as a thermally processed PP sample cools.
You might be asking yourself how deliberating causing crystallization would improve clarity. Crystallinity is the very reason that polymers are translucent, cloudy or downright opaque, no? Yes, and no. It's not the crystallinity per se, but the SIZE of the crystals in the polymer. If they're big enough to refract light, then yes, they reduce clarity. But the nucleating agents used in commercial PP make for lots of very small crystallites, so small that normal light doesn't "see" them so they don't refract. In other words, they're invisible. PMP doesn't need to have these expensive additives added so that makes it more useful in many ways- no impurities that can leach out, and no variation in the amount or distribution of the additive since there isn't any. Neat, huh?
So there you have it in a nutshell: a truly unique commercial polymer that you may not have heard of yet.
References, to name a few:
Misui Chemicals, http://www.mitsuichem.com/service/mobility/polymers/tpx/
Handbook of Thermoplastics, 2nd Ed; Olagoke Olabisi, Kolapo Adewale, editors; Ch 1: Polyolefins, p. 28.
USPatent 4,166,057, 1979.