Polymer Nomenclature and Repeat Units

Introduction

Polymer names can be confusing because the same material may have a systematic name, a common name, an abbreviation and a trade name. Poly(tetrafluoroethene), PTFE and Teflon all refer to one polymer. IUPAC recognises two systematic approaches: source-based names, which state what the polymer was made from, and structure-based names, which describe the repeat unit. Knowing both, and being able to draw repeat units accurately, lets you move confidently between monomers, structures and names.

Core explanation

Source-based names. Write "poly" followed by the monomer name. If the monomer name has more than one word, contains a locant or a substituent, put it in brackets: poly(propene), poly(chloroethene), poly(methyl methacrylate). For single-word monomers brackets are optional (polystyrene or poly(styrene)), although using them consistently is clear. Traditional monomer names are widely used, so polyethylene and poly(ethene) are both acceptable.

Structure-based names. These name the constitutional repeat unit (CRU) as a divalent group inside "poly(…)". Polyethylene becomes poly(methylene), because its smallest repeating unit is –CH₂–. Poly(propene) becomes poly(1-methylethylene) — more correct, but used mainly in formal contexts. Structure-based names are valuable when the same structure could arise from different monomers.

Drawing repeat units for addition polymers. For an alkene monomer R¹R²C=CR³R⁴:

1. Draw the two carbon atoms of the double bond with a single bond between them. 2. Attach the substituents as they were on the monomer. 3. Add an open bond on each side and enclose in square brackets with subscript n.

For propene, CH₂=CH–CH₃, the repeat unit is –[CH₂–CH(CH₃)]ₙ–. The methyl group is a side group ; it is never part of the backbone.

Drawing repeat units for condensation polymers. Identify the linkage (ester –COO–, amide –CONH–) and remember the lost small molecule. For a diol HO–R–OH plus a diacid HOOC–R′–COOH, the repeat unit is –[O–R–O–CO–R′–CO]ₙ–: each diol loses an H from each OH and each diacid loses an OH from each COOH, and two water molecules are lost per repeat unit formed. For a single monomer containing both groups, such as a hydroxy acid HO–R–COOH, the repeat unit is –[O–R–CO]ₙ–.

Working backwards. From an addition-polymer repeat unit, restore the C=C between the two backbone carbons. From a polyester or polyamide, break each linkage and add back H to O or N and OH to the acyl carbon.

Common and trade names. Many condensation polymers have names that code their monomers. In nylon-6,6 the first number gives the carbon atoms in the diamine and the second the carbon atoms in the diacid; nylon-6 has one number because it comes from a single six-carbon monomer. PET stands for poly(ethylene terephthalate), whose systematic source-based name is poly(ethane-1,2-diyl benzene-1,4-dicarboxylate).

Step-by-step reasoning

To deduce the monomer from a vinyl-polymer chain segment:

1. Find the repeating two-carbon pattern along the backbone. 2. Cut out one repeat unit with its side groups. 3. Put a double bond between the two backbone carbons. 4. Name the alkene and write poly(…) around it.

Visual explanation

Draw –CH₂–CHCl–CH₂–CHCl–CH₂–CHCl–. Box one –CH₂–CHCl– unit, with bonds crossing the brackets at each end. Beside it, draw CH₂=CHCl with a curved arrow showing the π bond opening to make the two bonds to neighbouring units.

Real-world analogy

Source-based naming is like naming a cake after its main ingredient ("apple cake"), while structure-based naming is like naming it by its layered structure. Both identify the same cake; one tells you how it was made, the other what it looks like when cut open.

Real-world example

Recycling codes printed on packaging use abbreviations of names: 1 PET, 2 HDPE (high-density polyethylene), 3 PVC (poly(vinyl chloride), i.e. poly(chloroethene)), 4 LDPE, 5 PP (poly(propene)) and 6 PS (poly(styrene)). Each abbreviation is based on a source-based or common name.

Why?

Why do we need structure-based names at all? Some polymers can be made from different starting materials. Poly(vinyl alcohol) cannot be made from "vinyl alcohol" (which is unstable as a monomer) but is made by hydrolysing poly(vinyl acetate). A name describing the structure avoids pretending that a non-existent monomer was used.

Common misconception

"The repeat unit of an addition polymer still contains a C=C double bond." The double bond is used to make the new bonds to neighbouring units, so the backbone contains only single bonds.

Worked example

Question: Draw the repeat unit and give a source-based name for the polymer made from methyl 2-methylpropenoate, CH₂=C(CH₃)COOCH₃.

Reasoning: Open the C=C: backbone –CH₂–C– with the CH₃ and COOCH₃ groups on the second carbon.

Answer: –[CH₂–C(CH₃)(COOCH₃)]ₙ–, poly(methyl 2-methylpropenoate), commonly called poly(methyl methacrylate), PMMA.

Quick check

1. What monomer is used to make a polymer with repeat unit –[CF₂–CF₂]ₙ–, and what is the polymer called? Answer: Tetrafluoroethene, CF₂=CF₂; the polymer is poly(tetrafluoroethene), PTFE.

Exam focus

Always show open bonds extending through the brackets and the subscript n. Keep side groups off the backbone. For condensation polymers, show the full linkage and state the small molecule lost. Examiners often give a chain segment of three repeat units and ask for the monomer.

Advanced insight

IUPAC rules for choosing the CRU specify a seniority order for its subunits and the direction in which the unit is written, so that each polymer has one preferred structure-based name. For polyethylene the CRU is –CH₂–, not –CH₂–CH₂–, because a single methylene is already the smallest unit that repeats.

Summary

Source-based names put poly in front of the monomer name, in brackets when needed; structure-based names describe the constitutional repeat unit. Addition-polymer repeat units come from opening the C=C bond, with side groups kept off the backbone. Condensation-polymer repeat units show the linkage formed and omit the atoms lost as small molecules.

Practice questions

1. Give the source-based name and draw the repeat unit for the polymer of CH₂=CH–C₆H₅. Answer: Poly(styrene), also called poly(phenylethene); repeat unit –[CH₂–CH(C₆H₅)]ₙ–. 2. A polymer chain contains the segment –CH₂–C(CH₃)₂–CH₂–C(CH₃)₂–. Identify the monomer. Answer: 2-methylpropene, CH₂=C(CH₃)₂; the polymer is poly(2-methylpropene), polyisobutylene. 3. Draw the repeat unit of the polyester formed from HO–CH₂CH₂–OH and HOOC–(CH₂)₄–COOH. Answer: –[O–CH₂CH₂–O–CO–(CH₂)₄–CO]ₙ–, formed with loss of two water molecules per repeat unit. 4. Explain the numbers in the name nylon-6,10. Answer: It is made from a diamine with six carbon atoms and a diacid (or diacid chloride) with ten carbon atoms.