A homopolymer is made from a single monomer and a copolymer from two or more, and in commercial resins the second monomer is a minor addition made to correct a specific weakness of the homopolymer. In both polypropylene and acetal (POM), the homopolymer is stiffer and stronger and holds short-term heat better, while the copolymer is better at what its comonomer was added for: impact strength or clarity in polypropylene, long-term heat and chemical resistance in acetal.
The same property can run in opposite directions in the two families. PP copolymers are tougher than PP homopolymer, while acetal homopolymer is tougher than acetal copolymer.
What Is a Homopolymer
A homopolymer is a polymer made from one monomer, so its chain repeats a single unit. Polypropylene homopolymer, made from propylene alone, and acetal homopolymer (POM-H) are common examples. Polypropylene is sold both ways, as a homopolymer and as copolymers with ethylene or butene.
Nylon 6,6 and PET, each made by reacting two complementary monomers, are homopolymers too by IUPAC’s polymer glossary, which treats the reacting pair as one implicit monomer.
What Is a Copolymer
A copolymer is a polymer made from two or more monomers. In commercial resins the comonomer is a minor share next to the main monomer: ethylene in most polypropylene copolymers, and ethylene oxide or dioxolane, depending on the producer, in acetal copolymer (POM-C).
The monomers can be arranged along the chain in several ways:
- Random: the units follow no order, and the unit at any point does not depend on its neighbors.
- Alternating: the two units take turns.
- Block: long runs of one unit are joined end to end with long runs of the other.
- Graft: chains of a different makeup hang as side branches from the main chain.
In PP, “copolymer” names two different products, random copolymer and impact copolymer. Random copolymer has a few percent of ethylene scattered along the polypropylene chain. In a published study of one producer’s commercial grades, its main random-copolymer families averaged about 4 and 5.4 wt%.
Impact copolymer, which price reports and many quotes call block copolymer, is a polypropylene matrix with a dispersed ethylene–propylene rubber phase rather than a chain of blocks. It carries more ethylene, about 8 to 14 wt% in total in the same producer’s grades, most of it in the rubber.
A copolymer is not necessarily made by a different polymerization route. PP homopolymer and random copolymer come out of the same reactors on the same catalyst.
The Difference Between Homopolymer and Copolymer
Stiffness, strength and heat deflection temperature are higher in the homopolymer of both PP and acetal. Impact strength favors the copolymer in PP and the homopolymer in acetal, and chemical resistance separates the two acetals far more than the two PPs.
Which way each difference runs follows from what the comonomer was added for. In PP it is there for toughness or clarity, in acetal for stability against heat and chemical attack. In a PP random chain and in an acetal chain the comonomer also makes the chain less regular, so it crystallizes less and the copolymer is less rigid.
Compare homopolymer and copolymer on one producer’s datasheets at similar melt flow, because between producers, grade and test-specimen differences can reverse even the direction of a gap.
Stiffness and Strength
The homopolymer is the stiffer and stronger material in both families. On one producer’s datasheets per family, flexural modulus and yield strength both drop from the homopolymer to each copolymer.
How much stiffness a PP impact copolymer loses depends on the grade. A high-stiffness impact grade from another producer is rated at 1,517 MPa flexural modulus against 1,586 MPa for its homopolymer, with 2.5 times the notched Izod impact strength (ASTM D790 and D256).
In acetal, the homopolymer also has better resistance to creep under sustained load and to fatigue, though long exposure to heat narrows the gap.
Glass fiber reverses the order in acetal, making a glass-filled copolymer stiffer and stronger than a glass-filled homopolymer.
Impact Resistance
Impact resistance is where the two families split: in PP both copolymers are tougher than the homopolymer, while in acetal the homopolymer is the tougher one. The same producers’ datasheets show the split in notched Charpy impact strength at 23 °C.
PP homopolymer turns brittle as it approaches its glass transition. At −20 °C the impact copolymer still tests 6 kJ/m² notched Charpy, more than the homopolymer gives at 23 °C.
Heat Resistance
For short-term heat the homopolymer is better in both families; over long exposure to heat, acetal copolymer holds its properties longer because it resists oxidation better. On the same producers’ datasheets, the homopolymer has the higher heat deflection temperature at 0.45 MPa in both families.
The two PP copolymers differ in melting point. The impact copolymer melts at 165 °C, the same as the homopolymer, and the random copolymer at 146 °C.
Chemical Resistance
In acetal the copolymer is the more chemically resistant; in PP the two resist chemicals alike, apart from organic liquids.
Acetal homopolymer’s own producer rates it unsatisfactory in 10% sodium hydroxide and 10% ammonia, while acetal copolymer is unaffected by alkalis, even hot. That makes the copolymer the safer choice for parts in hot water, especially chlorinated water.
A PP producer’s chemical-resistance chart rates homopolymer and copolymers as one material, except that copolymers swell more than the homopolymer in organic liquids, most of all in non-polar ones such as benzene and petroleum ether.
How to Choose the Right One
Which is better depends on the property the part needs most:
- Stiffness, strength or short-term heat resistance: the homopolymer, in PP or acetal.
- Impact strength, especially below freezing: PP impact copolymer.
- Clarity: PP random copolymer, the grade sold for clear polypropylene parts.
- Hot water, alkalis or long-term heat: acetal copolymer.
- Glass-filled acetal: the copolymer.
Their uses overlap: caps, closures and thin-wall containers are molded from both PP homopolymer and random copolymer, while crates, pails and battery cases are typical impact-copolymer parts.
Density barely differs between homopolymer and copolymer of the same family.
In PP they process alike; one producer gives its homopolymer, random and impact copolymer the same 1–2% molding shrinkage and nearly the same melt-temperature range. In acetal the copolymer has the wider processing window, because it melts lower and tolerates higher melt temperatures than the homopolymer.
Which Is Cheaper
In polypropylene the homopolymer is usually the cheaper resin, by a few percent. In Argus’s weekly polypropylene report, block (impact) copolymer stood 2.0 ¢/lb above injection-grade homopolymer on the US contract market in both January and July 2026. In Western Europe the contract gap was €60/t in both February and August 2026. At those months’ prices, that is about 3 to 5.5%.
Spot prices can close the gap entirely: on 21 August 2026, Argus assessed China-origin homopolymer and copolymer at the same $1,400/t cfr on the west coast of South America.
Argus does not assess PP random copolymer, and the PP premium is no guide to acetal, so price those on actual quotes.