5.13 Polymers of Alkene Derivatives
All the many varied and miscellaneous kinds of plastic that humans make have one thing in common. They're bad for the environment. Okay, two things in common: they're also all polymers. Just as isomers are made of the same parts, polymers are made of many parts, subunits that were once separate molecules that got glommed together into a long strand. These subunits, logically, are called monomers.
There are different types of polymerization. One type happens easily in alkenes and their derivatives. Ethylene, a molecule made of two CH2 subunits, can join itself end to end to end into a long string of CH2CH2 subunits. The resulting polymer has only single bonds instead of the original monomer's double bonds. Polypropylene is similar to polyethylene; the CH2=CHCH3 monomers hook up to form -CH2CH(CH3)- subunits.
Plastics that can be melted down and then molded or deposited into a new shape are called thermoplastics. Such plastics can in theory be recycled, however there are significant logistical problems that prevent true recycling of most plastics, such as the necessity to remove impurities. Polyethylene and polypropylene are thermoplastics.
Another monomer that is used to make plastic polymers is styrene or vinylbenzene, C6H5CH=CH2. It's a liquid that, in the presence of a catalyst, readily polymerizes to polystyrene, a brittle plastic that gets used a lot for drinking cups for some reason. It's also the material that gets formed into the infernal squicky sensory assault known as styrofoam. (Not only is starch based foam packing material better for the planet, it's also more pleasant for anyone who has to touch it or hear it!)
Styrene and a short chain of polystyrene.
Notice that it is strictly the vinyl group that participates in the polymerization, and not the aromatic ring due to the latter's extreme stability.
We've already seen butadiene and acrylonitrile, and we know that they're precursors to plastics. One type of strong thermoplastic is poly- acrylonitrile butadiene styrene, or ABS. It is the stuff of water pipes (one type) and fit-together brick toys, and has frustrated many a 3D printing enthusiast. Being a copolymer, the monomers of its three precursor materials are lined up together in random order. The acrylic component adds strength, the styrene adds stiffness, and the butadiene prevents brittleness.
Acrylonitrile, butadiene, styrene, and a short chain of ABS showing the random ordering.
Oh, speaking of vinyl, the plastic commonly known as vinyl is actually polyvinyl chloride or PVC. Vinyl chloride's structure is CH2=CHCl, so PVC consists of repeating -CH2CH(Cl)- units. Vinyl polymers also find use as school glue, made from a mixture of polyvinyl alcohol (PVA) -(-CH2CH(OH)-)n- and polyvinyl acetate -(-CH2CH(OCOCH3)-)n-. PVA is soluble in water, but the acetate is not; mixing them allows the PVA to carry the PV acetate into solution with it. Besides being soluble, PVA is also a thermoplastic.
PVC, PVA, and polyvinyl acetate.
Tetrafluoroethylene CF2=CF2 can be polymerized to polytetrafluoroethylene also known as PTFE or Teflon. It is the slipperiest substance known, and at room temperature it is immune to attack from all but the nastiest of chemicals.
PTFE.
Super glue is made from cyanoacrylate esters. Methyl cyanoacrylate is one of the most common, with the structure CH2C(CN)CO2CH3. Esters with longer alcohol chains, such as octyl cyanoacrylate CH2C(CN)CO2CH2CH2CH2CH2CH2CH2CH2CH3, do not form quite as strong a bond but the resulting bond is more flexible. Water catalyzes the polymerism of cyanoacrylates to polycyanoacrylates.
Methyl cyanoacrylate polymer.
Polymers of alkenes also occur in nature. Polybutadiene is synthetic, but polyisoprene is the stuff of natural rubber. Specifically, rubber is cis-polyisoprene, while trans-polyisoprene is a stiff polymer called gutta-percha that was once used to make golf balls.
cis-polyisoprene and trans-polyisoprene.