Organic Chemistry · Synthetic Polymers

Copolymers

8 min read

Lab safety note: none specific — monomers are volatile and often toxic (styrene, acrylonitrile); handle with ventilation and PPE per institutional rules (general principles only). The reactivity-ratio definitions, the r₁r₂ classification (≈1 random, ≈0 alternating), the styrene–maleic anhydride alternation, and the living-polymerization requirement for block copolymers are standard textbook polymer chemistry. The styrene/methyl methacrylate reactivity ratios (0.52, 0.46) are literature values for typical free-radical conditions and vary with temperature and medium; treat as illustrative.

On this page 9 sections

In 30 seconds
Why this matters
The college version
Eli explains
Worked example
Key takeaway
Check yourself
Study tools
Sources & references

In 30 seconds

A is a polymer made from two or more different monomers. Whereas a homopolymer such as polyethylene contains only –CH₂–CH₂– repeat units, a copolymer such as styrene–butadiene rubber (SBR) contains both styrene units (–CH₂–CH(C₆H₅)–) and butadiene units (–CH₂–CH=CH–CH₂–) in the same chain. Copolymerization is the polymer chemist's most powerful trick for tuning properties: combining monomers blends their strengths, much as alloying metals does.

Copolymers come in four sequence patterns. In a the two monomers appear in no particular order (AABABBBA...). In an they strictly alternate (ABABAB...). A has long runs of one monomer followed by long runs of the other (AAAA–BBBB–AAAA). A has a backbone of one monomer with side chains of the other attached along it. The sequence is not random chance: it is determined by reactivity ratios, which quantify how much each growing chain end prefers its own monomer over the other. Controlling sequence controls the final material's crystallinity, elasticity, toughness, and glass-transition temperature — which is why SBR (random), ABS plastic (graft-like), and thermoplastic elastomers (block) are such different materials.

Why this matters

Tuning real materials. SBR is the rubber in most car tires; ABS (acrylonitrile–butadiene–styrene) is the tough plastic in LEGO bricks and appliance housings; nitrile rubber (butadiene + acrylonitrile) resists oil and fuels; block copolymers such as styrene–butadiene–styrene (SBS) are thermoplastic elastomers that behave like rubber but can be melted and recycled.
Reactivity ratios explain what you get. Whether two monomers randomize or alternate is not a choice but a consequence of the growing chain end's selectivity. Understanding r₁ and r₂ lets you predict the sequence from monomer structure.
Exam relevance. Expect problems that (a) identify the copolymer type from a sequence drawing, (b) interpret reactivity ratios, and (c) connect sequence to a physical property.

The college version

Core Concepts

The four sequence architectures

Random: A and B distributed irregularly (AABABBBA...). Properties are roughly an average of the homopolymers. Most radical copolymerizations give random copolymers.
Alternating: strict ABABAB... regularity. Formed when the two monomers strongly prefer to cross-react (each chain end adds the other monomer). Alternation maximizes regularity and can give crystalline or highly regular materials (e.g., alternating styrene–maleic anhydride copolymers).
Block: long homogeneous runs (AAAA...BBBB...AAAA...). Made by living polymerization (Topic 1): polymerize monomer A to completion, then add monomer B, which all the still-active chain ends add to. Block copolymers can phase-separate into rubbery and glassy domains, giving thermoplastic elastomers — rubbery at use temperature yet melt-processable.
Graft: backbone of A with B side chains attached at points along it (like a bottle brush). Made by polymerizing B from initiator sites on a preformed A backbone, or by attaching preformed B chains. Grafting modifies surface and impact properties (e.g., rubber-toughened plastics).

Reactivity ratios: the language of sequence control

Define k₁₁ as the rate constant for chain end A adding monomer A, k₁₂ for chain end A adding monomer B, and analogously k₂₂ and k₂₁ for chain end B. The reactivity ratios are:

$r 1 = k 11 k 12, r 2 = k 22 k 21$

Each r compares a chain end's preference for its own monomer over the other. The interpretation:

r₁ ≫ 1: chain end A strongly prefers A → long A runs (blocky tendency).
r₁ ≈ 0: chain end A strongly prefers B → alternating tendency.
r₁ × r₂ ≈ 1: random copolymer.
r₁ × r₂ ≈ 0: alternating copolymer (both ends prefer the other monomer).

Why reactivity ratios exist: stability logic

A chain end adds whichever monomer gives the more stable new radical. Electron-rich monomers (styrene, butadiene) give stabilized benzylic or allylic radicals; electron-poor monomers (maleic anhydride, acrylonitrile) give less stable radicals but are themselves more reactive toward electron-rich radicals (polar effects). This is why the electron-rich/electron-poor pair styrene + maleic anhydride alternates: the styryl radical prefers the anhydride, and the anhydride radical prefers styrene, so r₁ r₂ ≈ 0.

The copolymer equation (qualitative use)

The instantaneous copolymer composition depends on the monomer feed ratio and the reactivity ratios through the copolymer equation. For study purposes, the key qualitative consequences matter more than the algebra: (1) the faster-reacting monomer is consumed first, so the feed composition drifts as the reaction proceeds; (2) azeotropic compositions (feed = copolymer composition) exist when r₁ = r₂ = 1; (3) block copolymers require living conditions, not radical conditions, because radical termination ends chains before a second block can form.

From sequence to properties

Alternating and block structures pack more regularly and can crystallize; random sequences inhibit crystallization and give amorphous, often rubbery materials. Block copolymers phase-separate on the nanometer scale: hard glassy domains (polystyrene) reinforce soft rubbery domains (polybutadiene), which is why SBS feels like rubber but melts like plastic.

Common Confusions

Do Not Confuse	With	Difference
Random copolymer	Alternating copolymer	Random: irregular sequence (r₁r₂ ≈ 1); alternating: strict ABAB (r₁r₂ ≈ 0)
Block copolymer	Graft copolymer	Block: long runs in the main chain (AAAA–BBBB); graft: B side chains attached to an A backbone
Reactivity ratio r₁	Copolymer composition	r₁ is a property of chain end 1 (rate ratio); composition is the outcome in the polymer
Copolymer	Homopolymer	Copolymer has ≥2 monomers; homopolymer one. Blending two homopolymers is not the same as copolymerizing them
Thermoplastic elastomer	Thermoset rubber	SBS melts and reprocesses (thermoplastic); vulcanized rubber is cross-linked and cannot be remelted

Eli explains

The same idea, in plain words

Explain it like I’m 10

A copolymer is like a bracelet strung with two colors of beads. You can string them randomly (random), strictly alternating red-blue-red-blue (alternating), in big color blocks (block), or put blue beads on a red string like a bottle brush (graft). Which pattern you get depends on how much each bead type "likes" to sit next to the other — the reactivity ratios.

Worked example

Example 1: Interpreting reactivity ratios

Problem. For the copolymerization of styrene (monomer 1) with methyl methacrylate (monomer 2), the reactivity ratios are r₁ = 0.52 and r₂ = 0.46. What type of copolymer forms, and what does each ratio mean?

Step 1 — Interpret each ratio. r₁ = 0.52 means the styryl radical adds methyl methacrylate almost twice as fast as styrene; r₂ = 0.46 means the methacrylate radical also slightly prefers styrene.

Step 2 — Take the product. r₁ r₂ = 0.52 × 0.46 ≈ 0.24.

Step 3 — Classify. The product is well below 1 but not near 0, so the copolymer is random with a slight alternating tendency — both ends cross-react preferentially, but each still adds its own monomer often enough to randomize the sequence.

Answer. A random copolymer with a mild alternation tendency (r₁ r₂ ≈ 0.24).

Example 2: Predicting alternation from structure

Problem. Predict the copolymerization behavior of styrene (electron-rich) with maleic anhydride (electron-poor), and justify with stability logic.

Step 1 — Identify the radicals. The styryl radical is benzylic (stabilized); the maleic anhydride radical is less stabilized but electrophilic.

Step 2 — Apply cross-preference. The stabilized styryl radical adds the electron-poor anhydride readily (polar attraction); the anhydride radical adds styrene readily. Each chain end strongly prefers the other monomer.

Step 3 — Conclude. r₁ ≈ 0 and r₂ ≈ 0, so r₁ r₂ ≈ 0: an alternating copolymer.

Answer. Styrene and maleic anhydride form a strictly alternating copolymer, the textbook example of alternation driven by complementary electronic character.

Example 3: Choosing a method for a block copolymer

Problem. A chemist wants a styrene–butadiene–styrene (SBS) triblock copolymer. Explain why ordinary radical polymerization cannot make it and which method can.

Step 1 — Identify the obstacle. Radical chain growth terminates by combination/disproportionation; once a chain terminates, it cannot add a second block.

Step 2 — Choose the method. Anionic living polymerization (Topic 1): polymerize styrene to completion, then add butadiene, then styrene again. All chains remain active between additions, so each chain grows A–B–A.

Step 3 — Property payoff. The polystyrene end-blocks glassy domains; the polybutadiene middle is rubbery → thermoplastic elastomer.

Answer. Radical polymerization terminates chains and cannot build blocks; living anionic polymerization (sequential monomer addition) produces the SBS triblock, a thermoplastic elastomer.

Key takeaways

Copolymer = polymer from ≥2 monomers; four architectures: random, alternating, block, graft.
Reactivity ratios: r₁ = k₁₁/k₁₂, r₂ = k₂₂/k₂₁. r ≫ 1 → prefers own monomer; r ≈ 0 → prefers the other.
r₁ r₂ ≈ 1 → random; r₁ r₂ ≈ 0 → alternating.
Alternation is favored by electron-rich + electron-poor monomer pairs (styrene + maleic anhydride).
Block copolymers require living polymerization (no termination); radical termination makes blocks impossible.
Graft copolymers = backbone of A with B side chains; used to toughen plastics (e.g., rubber-modified polystyrene, ABS).
Real products: SBR (tires, random), ABS (tough plastic), SBS (thermoplastic elastomer, block), nitrile rubber (oil-resistant).

Check yourself

6 review questions from the chapter. Try each one, then open the answer.

Name the four copolymer architectures and sketch (in words) the sequence of each.
Show answer
Random (AABABBBA...), alternating (ABABAB...), block (AAAA–BBBB–AAAA...), graft (A backbone with B side chains).
What do r₁ = k₁₁/k₁₂ and r₂ = k₂₂/k₂₁ measure, and what does r₁ r₂ ≈ 0 predict?
Show answer
r₁ = rate constant for chain end 1 adding monomer 1 vs monomer 2; r₂ analogous for chain end 2. r₁ r₂ ≈ 0 predicts an alternating copolymer (each end strongly prefers the other monomer).
Why does the styrene–maleic anhydride pair alternate?
Show answer
The styryl radical is stabilized and electrophilic-attracted to the electron-poor anhydride; the anhydride radical prefers the electron-rich styrene — both ends cross-react, so the sequence alternates.
Why must block copolymers be made by living polymerization?
Show answer
Block copolymers need chains that stay alive between monomer additions; radical chains terminate (combine/disproportionate) before a second monomer can be added.
SBR is a random styrene–butadiene rubber used in tires. What property consequence follows from its randomness?
Show answer
Random sequences cannot pack into crystals, so SBR is amorphous and rubbery — ideal for tires, where elasticity is wanted.
What is a , and which copolymer architecture produces one?
Show answer
A rubbery material that melts and reprocesses; made from block copolymers such as SBS, where glassy polystyrene blocks reinforce rubbery polybutadiene segments.

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Study tools & related lessonsKey vocabulary · Related

Key vocabulary

Copolymer: Polymer chain containing two or more monomer types
Random copolymer: Monomers arranged irregularly (AABABBBA...)
Alternating copolymer: Strict ABABAB sequence
Block copolymer: Long runs of each monomer (AAAA–BBBB)
Graft copolymer: Backbone of A with B side chains
Reactivity ratio r: Rate of adding own monomer vs the other (k_ii/k_ij)
Thermoplastic elastomer: Rubber-like material that melts and can be reprocessed

Sources & references

openstax.org — Organic Chemistry

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