Chapter 31: Synthetic Polymers
One reaction, repeated until the molecule is enormous
Before anything else, the two words this section turns on. The monomer is the small molecule you start from. The repeat unit is the fragment that recurs along the finished chain, and it is what is left of the monomer after the bond that reacted has been consumed - so ethylene is CH₂=CH₂ but the repeat unit of polyethylene is -CH₂CH₂-, with no double bond and an open valence at each end where a neighbor attaches. Drawing the monomer inside the brackets instead of the repeat unit is the single most common error on this material. A polymer is written as that repeat unit in brackets with a…
Radical chain growth, step by step
The outline this section builds on is three stages. Initiation: a weak bond in an initiator breaks homolytically on warming - AIBN sheds nitrogen gas and gives two carbon radicals - and one of those radicals adds to a monomer. Propagation: the chain-end radical adds another monomer, forming a bond and regenerating a radical at the far end of the unit just added, so the step can immediately repeat, typically a thousand or more times in under a second. Termination: two radicals meet and pair their electrons, and both chains stop. The regiochemistry is set by radical stability: the growing…
Charged chain ends, and polymers that refuse to stop growing
The rule this section rests on is that a charged chain end is fussy about its neighbors in a way a radical is not. Cationic chain growth needs electron-donating substituents - alkyl groups, or an OR group whose lone pair can be donated onto the cationic center - so isobutylene and the vinyl ethers work and ethylene does not. Anionic chain growth needs electron-withdrawing substituents that can delocalize negative charge - a nitrile, an ester carbonyl, a nitro group - so acrylonitrile and methyl methacrylate work. Styrene works by all three mechanisms, because a benzene ring stabilizes a…
Same chain, three stereochemistries, three materials
First the three cases this section compares. Polymerize a monosubstituted alkene and every second backbone carbon carries a substituent, a hydrogen and two lengths of chain, so its configuration can vary; the pattern those centers form is called tacticity. An isotactic chain has the same configuration at every center, so on an extended backbone all the substituents project from the same side. A syndiotactic chain alternates strictly, so they alternate sides. An atactic chain has no pattern at all. Both regular arrangements let neighboring chains register with one another and crystallize; the…
Two monomers, four architectures
The four arrangements this section compares, first. Use two monomers and the product is a copolymer, and the design variable is the sequence in which they appear. In a random (statistical) copolymer the two appear in no particular order. In an alternating copolymer they strictly alternate. In a block copolymer a long run of one is joined to a long run of the other. In a graft copolymer a backbone of one carries side chains of the other. Which you get is decided by the relative rates at which each chain end adds each monomer - except for blocks, which cannot arise from a competition at all and…
Step growth: everything reacts with everything
The defining features first. In step-growth polymerization every molecule in the pot carries a reactive group at both ends, so any two can join at any time - monomer with monomer, dimer with trimer, or two long chains with each other. There is no initiator and no privileged growing end, which is the whole contrast with chain growth. A diacid plus a diamine gives a polyamide (nylon 6,6) and expels water; a diacid plus a diol gives a polyester (poly(ethylene terephthalate)) and expels water; a diol plus a carbonate source gives a polycarbonate; a diol plus a diisocyanate gives a polyurethane…
Why 95% finished means two different things
The contrast this section quantifies, stated first. In chain growth an individual chain is born, reaches full length in a fraction of a second, and dies; running the reaction longer therefore produces more chains rather than longer ones, and conversion controls yield rather than chain length. In step growth every molecule present is reactive at both ends, so the whole population lengthens together and chain length is locked to conversion. The relationship, due to Carothers, is that the number-average degree of polymerization equals one divided by one minus the conversion: DPn = 1/(1 − p).…
Two averages, and what the gap between them tells you
The three quantities first. A polymer sample has a distribution of chain lengths rather than a single molecular weight, so it needs an average, and there are two sensible ones. The number-average, Mn, counts every chain equally: add the masses of all the chains and divide by how many there are. The weight-average, Mw, lets each chain count in proportion to its own mass, so the heavy chains count for more. Because of that weighting Mw is always at least as large as Mn, and they are equal only if every chain has the same length. Their ratio, Ð = Mw/Mn, is the dispersity, and it measures how…
Cutting double bonds in half and rejoining them
What metathesis does, before the consequences. Olefin metathesis cuts two carbon-carbon double bonds in half and rejoins the halves the other way round, so the number of double bonds is the same before and after. It runs through a metal carbene - a metal doubly bonded to carbon - which combines with an alkene to give a four-membered ring containing the metal, a metallacyclobutane; that ring then breaks along either of two pairs of bonds, back the way it came or the other way, and the second option releases a new alkene and leaves a new carbene on the metal. No carbocation, radical or…
Why one plastic is a bottle and another is a tire
The framework first, because everything else hangs off it. Two thermal transitions matter and they are not the same thing: the melting temperature, Tm, is where crystalline regions are destroyed and exists only if the polymer has crystals, while the glass transition, Tg, is the range over which the disordered regions gain enough mobility for chain segments to move, and every polymer has one. A fully amorphous polymer has a glass transition and no melting point at all. Cross-linking gives the second axis: a thermoplastic has separate chains that slide past one another on heating, so it melts…
Part of Organic Chemistry, a free and open textbook licensed CC BY-SA 4.0.