Polymers Section 120 of 123

Addition polymers

the why on every wrong step

Practice this lesson

13 min readfree

Take ethylene, CH2=CH2. Open its π bond, bond each carbon to a carbon of the next ethylene, and repeat that thousands of times: the result is polyethylene. A polymer made this way, by adding alkene molecules to one another, is an addition polymer. Nothing is lost along the way, so the repeat unit has exactly the same formula as the monomer. That match is the quickest way to recognize an addition polymer.

The step that repeats is one you already know: an addition to a C=C. What a polymer is showed that the growing chain end can be a radical, a cation or an anion. Of those three, the radical route makes the widest range of polymers, from polystyrene to PVC, and most of this page follows it. By tonnage, though, most polyethylene and polypropylene is made a fourth way, on metal catalysts, which the section on tacticity introduces.

The chain reaction, in three stages

Warm styrene with a little peroxide and it thickens into polystyrene. Three stages happen in the flask, the same three that any chain reaction has:

  • Initiation. The peroxide, RO–OR, has a weak O–O bond. Heat splits it homolytically: the bond breaks evenly, with one electron going to each oxygen, so two RO· radicals form. One of them adds to a monomer and starts a chain. AIBN, an initiator with an N=N between two carbons, does the same job, but it splits into two carbon radicals and a molecule of N2.
  • Propagation. The radical at the chain end adds to the next alkene. That makes a new radical at the end of a chain one unit longer, and the step repeats thousands of times.
  • Termination. Two radical chain ends meet, and both chains stop.

Termination can happen in two ways. In combination, the two radical carbons bond to each other, giving one chain as long as the two added together. In disproportionation, one radical takes a β-hydrogen from the other chain, meaning a hydrogen on the carbon next to that chain’s radical carbon. The chain that gains the hydrogen ends saturated, and the chain that loses it ends in a C=C, so there are two dead chains instead of one long one. Either way, two radicals are used up.

INITIATIONradicals: 0 → 2ROORheatRO+ROone electron to each oxygen: the weak O–O bond splits evenlyFIRST ADDITIONradicals: 1 → 1ROH₂CCHXROCH₂CHXthe radical ends up on the carbon that carries XPROPAGATIONradicals: 1 → 1~CH₂CHXH₂CCHX~CH₂CHXCH₂CHXone unit longer, still one radical at the end: this repeats thousands of timesCOMBINATIONradicals: 2 → 0~CH₂CHXXHCCH₂~~CH₂CHXXHCCH₂~the two radical carbons bond: one chain, as long as the two added togetherDISPROPORTIONATIONradicals: 2 → 0~CH₂CHXHCHXHC~~CH₂CH₂X+XHCCH~saturated enda C=C at the endthe β-hydrogen moves to the first chain; the two leftover electrons form the new π bond
Each stage drawn with fishhooks. Read the left-hand column: initiation makes radicals, the two addition steps keep one, and both kinds of termination remove two. Nothing carries a charge at any point: if a step you have drawn produces a cation or an anion, it was not a radical step.

Termination needs two radicals to find each other, and the radicals in the flask are very dilute. So each chain end adds monomer many thousands of times before it meets another radical, and that is why the chains are long.

This is the same initiation, propagation and termination pattern as Radical halogenation. Only the propagation step differs: there the radical takes an H from a C–H bond, and here it adds to a C=C bond.

Chain transfer: a fourth thing that can happen

Suppose the flask also holds a little thiol, RS–H, such as dodecanethiol, C12H25SH. The S–H bond is weak, so a growing chain radical can take the thiol’s hydrogen instead of adding to another monomer. That chain is now finished. The sulfur is left as a new radical, RS·, and it adds to a monomer and starts a fresh chain.

TRANSFERradicals: 1 → 1~CH₂CHXHSR~CH₂CH₂Xthis chain is finished+RSa new radicalRESTARTradicals: 1 → 1RSH₂CCHXRSCH₂CHXthe RS• radical adds to a monomer, and a new chain starts
Chain transfer to a thiol. Follow the hydrogen from sulfur to carbon.

This is chain transfer: one chain stops, and a new radical takes over. A chain end can take a hydrogen this way from the solvent, from a monomer, from an added thiol, or from its own backbone.

Chain transfer is how chain length, and so molecular weight, is controlled in practice. Added on purpose like this, the thiol is called a chain-transfer agent, and every chain comes out shorter.

Compare the three fates by what each does to the number of radicals, because that is what exam questions turn on. Propagation makes one chain longer and keeps one radical. Chain transfer stops one chain and starts another, and still keeps one radical. Termination stops two chains and destroys two radicals. Only termination lowers the radical concentration, which is why a transfer agent shortens the chains without slowing the reaction down.

Head-to-tail, and why

What a polymer is showed why a radical adds to the CH2 end of vinyl chloride, CH2=CHCl: that leaves the more stable radical, on the carbon that carries the chlorine. Sterics agree, because the CH2 carbon is also the less crowded one to attack. That page also named the two ends: the substituted carbon is the head and the CH2 the tail. So every new monomer joins by its tail to the head at the chain end, and the X groups of any CH2=CHX land on alternate carbons of the chain.

Worked example — draw three repeat units of PVC and show that it is head-to-tail

Step 1. The monomer is CH2=CHCl. Its two ends are different: one CH2, unsubstituted, and one CHCl, substituted.

Step 2. A chain radical R· attacks the CH2 carbon, giving R–CH2–ĊHCl. The radical sits on the carbon that carries the chlorine, the better of the two radicals available.

Step 3. That radical attacks the CH2 end of the next monomer: R–CH2–CHCl–CH2–ĊHCl.

Step 4. Once more: R–CH2–CHCl–CH2–CHCl–CH2–ĊHCl.

Step 5 — read the backbone. Number the chain carbons from the R end. C1 is CH2, C2 carries Cl, C3 is CH2, C4 carries Cl, C5 is CH2 and C6 carries Cl. Each chlorine has one CH2 between it and the next, so the two are 1,3 to each other, and no two sit on neighboring carbons. That is what head-to-tail means.

The contrast. The selectivity is high but not perfect, so now and then a monomer adds the wrong way round. Two chlorines then sit on neighboring carbons: a head-to-head join. The chain end is now a CH2 radical, and the next monomer adds normally, so two CH2 groups end up side by side: a tail-to-tail join. Exams draw a chain and ask you to find that defect, so look for any pair of neighboring carbons that both carry the substituent. A chain with a few such defects is still described as head-to-tail.

head-to-tail: Cl on every other carbon1Cl23Cl45Cl67Cl8Cl on 2, 4, 6, 8: never on neighborsone monomer reversed1Cl2Cl345Cl67Cl8head-to-head: Cl on 2 and 3tail-to-tail: no Cl on 4 or 5
The carbons are numbered from the left. The highlighted bonds are the two wrong-way joins.

The polymers worth recognizing

MonomerPolymerWhere you meet it
Ethylene, CH2=CH2PolyethyleneBags, bottles, pipe
Propylene, CH2=CHCH3PolypropyleneRope, containers, carpet
Vinyl chloride, CH2=CHClPVCPipe, flooring, insulation
Styrene, CH2=CHPhPolystyrenePackaging, foam cups
Tetrafluoroethylene, CF2=CF2PTFE (Teflon)Non-stick coatings
Methyl methacrylate, CH2=C(CH3)CO2CH3PMMA (Plexiglas)Transparent sheet
Acrylonitrile, CH2=CHCNPANAcrylic fiber, carbon-fiber precursor
Isoprene, CH2=C(CH3)CH=CH2PolyisopreneNatural rubber

Every one of these is drawn the same way. Take the monomer, break the π bond, and write the repeat unit in brackets with a bond out of each side: –[CH2–CHX]–n for a monomer with one substituent. The figure draws each monomer in the table except isoprene, which the next section draws, beside its repeat unit.

ethylene → polyethyleneH₂CCH₂CH₂CH₂npropylene → polypropyleneH₂CCHCH₃CH₂CHCH₃nvinyl chloride → PVCH₂CCHClCH₂CHClnstyrene → polystyreneH₂CCHCH₂CHntetrafluoroethylene → PTFEF₂CCF₂CF₂CF₂nacrylonitrile → PANH₂CCHCNCH₂CHCNnmethyl methacrylate → PMMAH₂CCCH₃COOCH₃CH₂CCH₃COOCH₃nisoprene → polyisopreneisoprene is a diene, so it adds differently:its drawings are in the next section
Each monomer beside its repeat unit. Check that every substituent carries over unchanged.

Dienes, and the polymer that keeps its double bond

The last row of the table is isoprene, 2-methylbuta-1,3-diene, which is a conjugated diene. Number its carbons C1 to C4 along the diene, so that the methyl sits on C2.

A growing radical adds to C1, which puts the new radical on C2. That radical is allylic: it sits next to the C3=C4 double bond, so the unpaired electron is shared between C2 and C4. This is the same sharing between two ends that 1,2- vs 1,4-addition showed for HBr and a diene.

The next monomer adds at C4. That leaves a C2=C3 double bond in the middle of the repeat unit. Most units join this way, by 1,4-addition.

1. the chain radical adds to C1RCH₃H₂CCCHCH₂C1C2C3C4CH₃RCH₂CCHCH₂C1C2C3C42. the radical is allylic: C2 and C4 share itCH₃RCH₂CCHCH₂C1C2C3C4CH₃RCH₂CCHCH₂C1C2C3C43. the next monomer adds at C4, so C2=C3 stays in the chainCH₃CH₂CCHCH₂C1C2C3C4nthe repeat unit of polyisopreneone C=C in every unit, from C2 to C3
Isoprene numbered C1 to C4. Follow the radical from C2 to C4, and see where the double bond ends up.

So every repeat unit of 1,4-polyisoprene carries one C=C, and each of those double bonds can be cis or trans. Natural rubber is the all-cis form: the chain enters and leaves each double bond on the same side. Gutta-percha, which is hard and inelastic, is the all-trans form. (Rubber trees build natural rubber with enzymes rather than from isoprene itself, but the structure is the same.)

cis: natural rubbertrans: gutta-perchaCH₃HC1C2C3C4chainchainCH₃HC1C2C3C4chainchainchain bonds on the same side of C2=C3chain bonds on opposite sides
Same repeat unit, same formula. Only the side on which the chain leaves C3 differs; the highlighted bonds are the chain.

Those double bonds are what vulcanization needs. What a polymer is showed that sulfur bridges form at C=C bonds and the carbons beside them. Structure and properties shows what that does to the rubber. A saturated chain such as polyethylene has nothing equivalent to offer.

Two monomers in one chain: copolymers

Styrene and butadiene can be polymerized together, so that one chain holds both kinds of unit. The product is SBR, the rubber in car tires. It is a copolymer, a chain built from two monomers, like the butyl rubber in What a polymer is. A polymer made from a single monomer is a homopolymer.

In a random copolymer, the two units are mixed along the chain in no set order. SBR is one, and so is SAN, made from styrene and acrylonitrile.

In a block copolymer, met in What a polymer is, each chain is a long run of one monomer followed by a long run of the other. In the solid, the A blocks gather together, and so do the B blocks, so the material separates into small domains of each kind. Each domain keeps the properties of its own homopolymer, and the material shows both sets at once.

randomABBABAAABABBAABABBBABAABblockAAAAAAAAAAAABBBBBBBBBBBBblock chains in a solidAAAAAAABBBBBBBAAAAAAABBBBBBBAAAAAAABBBBBBBAAAAAAABBBBBBBA domainB domain
Two ways to order two monomers along a chain, and how block chains sit in a solid.

ABS, made from acrylonitrile, butadiene and styrene, is built on the same domain idea. Tiny rubbery polybutadiene particles, with SAN chains grafted onto them, are spread through a rigid SAN matrix. The rubbery particles absorb the energy of an impact, which is why ABS is tough.

Tacticity: the same polymer, three materials

Look at polypropylene. Every second carbon in its chain carries a methyl group. Draw the backbone as a flat zig-zag, and each methyl points either toward you or away from you. The pattern of those directions along the chain is the polymer’s tacticity, and three patterns have names:

  • Isotactic: every methyl on the same side. The chains line up and pack closely, so the material is strong.
  • Syndiotactic: the methyls alternate sides in a regular pattern. These chains also pack well.
  • Atactic: no pattern. The chains cannot pack, so atactic polypropylene is disordered and soft.

What a polymer is used the same packing argument for the two polyethylenes: atactic chains cannot lie close together, just as branched ones cannot.

SAME CONNECTIVITY, SAME FORMULA, THREE MATERIALSCH₃CH₃CH₃CH₃isotacticevery methyl on the same side:the chains line up; melts near 165 °CCH₃CH₃CH₃CH₃syndiotactica regular alternation is a pattern too,so these chains line up as wellCH₃CH₃CH₃CH₃atacticno pattern: no two stretches of chainmatch, so nothing lines up; a sticky goo
Three polypropylenes, drawn with wedges and dashes. Only the side each methyl points to changes.A drawing habit: lay the backbone down flat as a zig-zag first, then decide carbon by carbon whether each methyl comes forward (wedge) or goes back (dash). Drawn any other way, the three look identical, and no formula, molecular weight or spectrum of the monomer tells them apart.

Only a monomer with two different groups on one of its carbons gives a tacticity. Polyethylene and PTFE carry two identical atoms on every carbon, H and H or F and F, so they have none.

Each methyl-bearing carbon carries four different groups, but two of them are the chain running left and the chain running right, which differ only far away, at the chain ends. An R or S label would depend on those distant ends, so tacticity describes how each carbon relates to its neighbors instead.

Radical polymerization gives mostly atactic polymer. The radical at the chain end is planar, so the next monomer can arrive on either face, and the side each new substituent ends up on is close to random. For example, polystyrene made with radicals is atactic.

Propylene itself cannot be polymerized usefully with radicals. A chain radical takes an allylic hydrogen from propylene, one of the hydrogens on its CH3, next to the C=C. The allylic radical that forms is too stable to add to more monomer, so the chains stop after a few units. Both atactic and isotactic polypropylene are made on metal catalysts instead.

Isotactic polypropylene, the useful kind, needs a Ziegler–Natta catalyst or a related metal catalyst, which holds the growing chain on a metal center and controls which face of each propylene binds. Atactic polypropylene is a sticky goo, while isotactic polypropylene is rope and car bumpers. The repeat unit is the same.

A classic Ziegler–Natta catalyst is TiCl4 with a trialkylaluminum such as Al(C2H5)3. The aluminum compound hands an ethyl group to the titanium. That leaves a Ti–C bond with an empty coordination site beside it, meaning a place on the metal where another molecule can bond.

Propylene binds in that empty site through its π bond. It then inserts into the Ti–C bond: the chain moves from the titanium onto one alkene carbon, and the titanium bonds to the other. The chain is now one monomer longer, and the empty site is back, in the place the chain used to occupy.

1. a Ti–C bond, and an empty siteTiCH₂chainempty site2. propylene binds in the empty siteTiCH₂chainH₂CCHCH₃ab3. insertion: the chain moves onto the alkenechainTiCH₂CHCH₂CH₃emptyagainnew C–Cab
Other groups on the titanium are left out. Ti ends up on the CH₂ (a); the chain ends up on the CH (b), through the highlighted bond.

This bind-then-insert pattern is the same migratory insertion as the Heck reaction in Palladium cross-coupling. The insertion happens at one defined, crowded site on the metal. The shape of that site lets propylene bind to the titanium by only one of its two faces, so every methyl ends up on the same side of the chain, and the polymer is isotactic. It also prevents branching. The chain end is a metal–carbon bond, not a radical, so it cannot curl back and take a hydrogen from its own chain.

Branching, and the two polyethylenes

What a polymer is showed that LDPE’s chains carry short branches and HDPE’s do not, and why that changes how they pack. Backbiting is where those branches come from.

It is chain transfer to the growing chain’s own backbone. Call the radical carbon at the chain end C1 and count back along the chain. The chain end curls round so that C1 comes close to a hydrogen on C5, four carbons back. C1 to C5 and that hydrogen form a six-membered ring, and the hydrogen moves across it from C5 to C1.

1. the chain end curls back on itselfCH₂CH₂CH₂CH₂CHH~CH₂C1C2C3C4C5six atoms: C1 to C5, and the H on C52. the radical is now on C5~CH₂CHCH₂CH₂CH₂CH₃C5C4C3C2C1
Follow the numbered carbons from the ring into the branch.

The radical is now on C5, in the middle of the chain, and the chain grows on from there. C1 to C4 are left behind as a four-carbon butyl branch.

Radical polymerization of ethylene at high pressure backbites constantly, which is why LDPE is branched. With a Ziegler–Natta catalyst, the chain end is a metal–carbon bond rather than a radical, so it cannot backbite, and the chains of HDPE stay linear. The branches keep LDPE’s chains apart, so it melts at a lower temperature than HDPE.

What carries forward

Radical polymerization runs by initiation, propagation and termination, with chain transfer setting the chain length. The more stable radical decides head-to-tail linking. Tacticity and branching then change the material without changing the repeat unit. When a question gives you an alkene and asks for the polymer, break the π bond and bracket the repeat unit. When it gives you two materials with the same formula, look for the difference in architecture.