Methylpropanediol (MPO): what it is and where it fits
Technical article · Eapearl Chemical ·
MPO is a small, branched diol that solves two specific problems: polyester chains that hydrolyse too readily, and plants that would rather not melt a solid glycol before every batch. It is a specialty, not a commodity, and it is bought for a reason.
What it is — and what it is not
Methylpropanediol is 2-methyl-1,3-propanediol: a four-carbon chain with hydroxyl groups at both ends and a methyl branch in the middle. That branch is the whole point. It disrupts the regularity of any polymer built from it, which lowers crystallinity, keeps viscosity down, and improves solubility in the systems where it is used.
| CAS number | 2163-42-0 |
|---|---|
| Molecular formula | C4H10O2 |
| Also sold as | MPO · MPDiol · 2-methyl-1,3-propanediol |
| Physical form | Liquid at room temperature |
| Harmonised CLP classification | None — no Annex VI entry applies |
The confusion worth clearing up
MPO is frequently described, including in supplier literature, as being “also known as 2-methylpentane-2,4-diol”. It is not. That name belongs to hexylene glycol, a different substance:
| Methylpropanediol (MPO) | 2-methyl-1,3-propanediol · CAS 2163-42-0 · C4H10O2 · four carbons, hydroxyls at positions 1 and 3 |
|---|---|
| Hexylene glycol | 2-methylpentane-2,4-diol · CAS 107-41-5 · C6H14O2 · six carbons, hydroxyls at positions 2 and 4 |
Different carbon skeleton, different molecular weight, different reactivity, different applications. If a specification, a purchase order or a data sheet uses the two names interchangeably, the safe assumption is that nobody has checked — ask for the CAS number and treat it as the identity, which is how we handle substance identity generally.
Where MPO earns its price
Unsaturated polyester and alkyd resins
Polyesters built with MPO resist hydrolysis better than those built with unbranched diols, because the branch shields the ester linkage. In practice that shows up as a laminate or a coating that survives humidity and hot water exposure longer. It also lowers resin viscosity, which lets a formulator either cut styrene content or improve wet-out at the same viscosity.
Polyurethanes and polyester polyols
As a diol in polyol synthesis and as a chain extender, where the same anti-crystallinity effect gives softer, clearer, more flexible segments.
Waterborne and high-solids coatings
Low viscosity and good solubility make MPO useful where the formulation is being pushed to higher solids or lower co-solvent content. It stays in the resin rather than evaporating, so it does not count against the VOC figure the way a co-solvent does — a point worth confirming against the definition in your market.
Personal care
Used as a solvent and humectant, notably for dissolving actives that resist water and glycerine, and as a booster that improves the performance of some preservative systems. This is a grade question rather than a chemistry question: cosmetic use needs the corresponding specification and documentation, agreed before the order.
The practical argument: liquid beats solid
The alternative most often compared against MPO is neopentyl glycol, which delivers similar branching benefits and is a solid at room temperature. On paper the comparison is about polymer properties. On the plant floor it is about handling:
- a solid glycol needs melting, heated storage or bag handling, and each of those is a cost and a failure point;
- a liquid diol meters, pumps and dispenses like every other liquid raw material in the plant;
- dust exposure disappears from the risk assessment entirely.
Plenty of plants pay more per tonne for MPO and still come out ahead once the melting step is removed. That calculation belongs to your process engineer, not to a price list — but it is the reason the material exists.
Classification and handling
No entry in Annex VI to the CLP Regulation applies to MPO, so the classification field on our product page is empty rather than filled with a supplier’s own reading. That is deliberate and it is the same rule we apply everywhere: where the EU has not harmonised a classification, we say so instead of inventing one.
An absent harmonised classification is not the same as an absence of hazard. Request the safety data sheet for the grade you are considering and use that, together with your own risk assessment, rather than the presence or absence of a pictogram on a web page.
On storage: like all glycols, MPO is hygroscopic. Water pickup shifts the hydroxyl value and takes the polyester stoichiometry with it, so closed containers and controlled sampling matter more here than the ambient temperature does.
What to specify when you order
- Hydroxyl value and water content — the two numbers a polyester plant actually reacts against.
- Colour — relevant if the resin ends up in a clear or light-coloured article.
- Grade and end use — industrial or personal care; they carry different documentation.
- Packaging and destination — see logistics and packaging for the formats we hold.