PEG molecular weight: what the number after the name tells you

The number in a grade name such as PEG-400 or PEG-4000 is not a brand index. It is the approximate average molecular weight of the polymer chain, and it predicts almost everything else about the product: what it looks like at room temperature, how many hydroxyl groups sit in a given mass, how viscous it is, and which job it can do. This note explains what polyethylene glycol molecular weight actually refers to, how it shows up in the analytics you receive with a certificate, and how to specify a grade so that two suppliers quote the same thing.

What the number actually refers to

Polyethylene glycol is made by polymerising ethylene oxide onto a starting glycol, so the chain is a string of ethylene oxide units capped at both ends. The grade number is the average molar mass of that chain: a 400 grade averages roughly 400 g/mol, a 4000 grade roughly 4000 g/mol. Because it is an average and not a single species, real grades are sold as ranges rather than exact figures. Our PEG-4000 is specified as molecular weight 3600-4400, for example, which is why a 4000 and a 4200 grade are the same family rather than different products. Read the range, not the round number.

Molecular weight sets the physical form first

Above a few hundred grams per mole the chain length starts to dominate the physical behaviour. Short chains behave like solvents; medium chains are pastes; long chains are waxy solids. The grade table on our polyethylene glycol page runs PEG-200 and PEG-400 as liquids, PEG-1000 as a paste and PEG-2000 upwards as waxes, which is simply the same chemistry at increasing chain length. That single trend - liquid to paste to solid - is the fastest way to sanity-check a grade you have not used before.

Molecular weight shows up again in the hydroxyl value

Every PEG chain ends in hydroxyl groups, and the hydroxyl value measures how many of them there are per unit mass. Longer chains mean fewer chain ends per gram, so hydroxyl value falls as molecular weight rises - a monotonic relationship that makes the certificate self-checking. Two published examples from our range make the point: PEG-4000 (molecular weight 3600-4400) carries a hydroxyl value of 26-32 mgKOH/g, while PPG-400 (molecular weight 360-440) carries 255-312 mgKOH/g. The tenfold difference in chain length shows up as a tenfold difference in hydroxyl value. If a supplier quotes a long chain with an implausibly high hydroxyl value, or a short chain with an implausibly low one, the quoted molecular weight deserves a second look.

Note the last row carefully. Random Polyether BPE-1600 is published with colour, acid value, content and pH, but its molecular weight is a series-level reference rather than a grade-specific figure, so it is stated as not published for this grade. That is the honest answer, and it is also the practical lesson: molecular weight is the one number worth confirming in writing before you build a formulation around a grade.

Two axes, not one: chain length and backbone chemistry

Molecular weight is only one axis of a polyether. The other is what the chain is made of. PEG is ethylene-oxide based and water-soluble; PPG is propylene-oxide based and markedly more hydrophobic, which is why PPG-400 (CAS 25322-69-4) appears where water resistance or lower surface activity is wanted rather than solubility. Random polyethers distribute the two units along one chain instead of in blocks, so a single molecule can be tuned for wetting, levelling or low foam by changing the ratio. Two grades of the same molecular weight but different backbones will not substitute for each other, and two grades with the same backbone but different chain lengths usually will - within a working window. Specify both axes, or the comparison is not a comparison.

How to specify a PEG grade properly

• Give the average molecular weight as a range, not as the round grade number, so the supplier's certificate can be checked against it directly.
• Add an identity or purity test that moves with chain length - hydroxyl value is the usual one, and it is quick to run.
• State the water content and colour limits you can live with, since both drift with storage and both matter in cosmetic, pharmaceutical and food-adjacent applications.
• Name the standard the grade is made to, so the specification is anchored to a document rather than to a conversation.
• State the end use. The same molecular weight can be a binder in one system and a lubricant in another, and the service conditions decide which limits matter.

Four common specifying mistakes

• Treating the grade number as exact. It is an average within a range, and the range is the specification.
• Buying on molecular weight alone for a PEG used as a surfactant intermediate, where the end-group chemistry and the ethoxylation distribution matter as much as the chain length.
• Copying a grade name from one standard onto another supplier's product without checking whether the specification behind the name is the same.
• Selecting by viscosity instead of by molecular weight, then finding that the two do not track once temperature changes.

For the definitional background, start with what polyethylene glycol is and how the grades differ. For how the same polyether family behaves in foam control and polyurethane systems, see polyether polyols and block polyethers. Representative grades to compare are PEG-4000 for the high-molecular-weight, hydroxyl-poor end, PPG-400 for the propylene-oxide backbone at the same physical form, APEG-1000 for an allyl-terminated thousand-molecular-weight chain, and Random Polyether BPE-1600 for a mixed EO/PO random structure.

The molecular weight of PEG, in the form you can buy it

The sections above explain why the number matters. What a buyer needs next is the form that number actually takes on a purchase order and on a certificate. For any PEG grade it is three things at once: the nominal number carried in the grade name, the published window the grade is sold against, and the measured value on the lot certificate. Keeping them apart is most of the work.

Read the table in order and most of the confusion around 'molecular weight of PEG' resolves. The grade number is a label; the window is the specification; the supporting test is the cross-check; and the certificate is the evidence. A buyer who quotes only the round grade number has quoted the label.

Two suppliers, one number, both correct

Molecular weight on a PEG grade is an average, so it describes the centre of a distribution rather than a single molecule. Two commercial chains with the same published average can sit differently around it, and the difference is not academic: it can show up in melting behaviour, in viscosity at a given temperature and in how the grade behaves on dilution. That is why the same nominal grade from two sources can be inside the same published window and still not be interchangeable in a running formulation.

• The average says where the centre of the distribution sits; it does not describe the spread.
• The supporting tests on the specification are what make two lots comparable in practice - hydroxyl value above all, because it moves monotonically with chain length.
• Where the published window is wide, the two ends of it can be far enough apart to matter, so a second source has to be qualified against its own certificate rather than against the grade name.

The practical consequence is not that molecular weight is unreliable. It is that the number is the beginning of a specification rather than the whole of one.

Bracketing the grade from the property you need

Because molecular weight is a continuum rather than a set of categories, choosing a grade means picking the point on that line where the property you care about most is strongest. The table below reads the same published examples from this range in the other direction - from the need back to the chain length.

The last row is the one that catches buyers out most often. Where a grade exists for its end group or its backbone rather than for its chain length, the molecular weight describes the product rather than explaining why you buy it - so it belongs in the specification as a check, not as the selection criterion.

What to put in an enquiry for a PEG grade

An enquiry that names only a grade number will attract quotations that look identical and are not. The list below is the minimum that makes two suppliers comparable, and every item is something the specification already contains.

• The molecular-weight window, written as a range rather than as the round grade number.
• The backbone chemistry, since an ethylene-oxide chain and a propylene-oxide chain of equal molar mass are different products.
• A supporting test that moves with the chain length, so the window can be checked rather than trusted.
• The physical form and the colour limit you can accept, because both vary within a family.
• Water content and pH limits, and the standard the grade is made to, so the specification is anchored to a document.
• The end use and the service conditions, since the same window can be a binder in one system and a lubricant in another.
• Confirmation that a certificate of analysis accompanies the lot, so the accepted values are the ones that arrived.

Two grades from this range make the point about backbone chemistry concrete. Emulsifier EL-40 (PEG-40 castor oil) is an ethoxylated oil built on a PEG chain, while 400MOP, a PEG400 mono-oleate phosphate ester puts a short PEG backbone into a phosphate-ester emulsifier with a different duty altogether. Same chain family, different product - which is the reason an enquiry has to name the chemistry as well as the number.

Related reading

The class-and-job distinction behind most PEG questions is set out in surfactant vs emulsifier, and the ionic side of the same argument in nonionic vs anionic surfactants. For the definitional background, what an emulsifier is covers the surface activity that a PEG only acquires once its chain is long enough to matter.

Molecular weight is the axis most PEG grades are bought on, but it is rarely the only one. The grades where a different axis leads - an end group, a mixed backbone, a phosphate ester - are the ones where the specification conversation pays for itself.

Next step

Tell us the molecular weight window, the backbone chemistry and the service conditions, and we will shortlist grades, confirm the specification values against the certificate and prepare samples. Send us your application.