Polyols: Molecular Weights and Applications
Polyols are organic compounds carrying multiple hydroxyl (−OH) groups. They range from small triols like glycerol up to polymeric polyether and polyester chains of tens of thousands of g/mol, and they are the backbone of the polyurethane (PU) industry, where they react with isocyanates to build foams, coatings, adhesives, sealants, and elastomers (the CASE segment). If you buy polyols for formulation, three numbers matter more than the headline molecular weight: functionality (number of OH groups per molecule), hydroxyl number (OH value in mg KOH/g), and equivalent weight. Get those right and the stoichiometry, crosslink density, and end-product hardness follow. A common myth is worth correcting up front: higher molecular weight does not mean more hydroxyl groups. A polyethylene glycol of 4000 g/mol is still a diol, with exactly two OH groups, the same as a 200 g/mol PEG. Functionality is set by the initiator, not the chain length. That distinction is the whole game in PU formulation.The Three Numbers You Actually Buy On
- Functionality (f): OH groups per molecule. f = 2 (diol) gives linear, flexible chains; f = 3–8 gives crosslinked, rigid networks.
- Hydroxyl number (OH value): mg of KOH equivalent to the hydroxyls in 1 g of polyol. High OH value means more reactive sites per gram, more crosslinking, harder product.
- Equivalent weight: molecular weight divided by functionality (= 56,100 ÷ OH number). It tells you how many grams of polyol react per mole of NCO, which is what sets your isocyanate dose.
Polyols Compared: Functionality, MW, and OH Number
The table below replaces the “low/medium/high MW” framing with the spec set a PU formulator works from. Values are typical reference figures; the supplier CoA governs.| Polyol | CAS | Functionality (OH groups) | Typical MW (g/mol) | Role |
|---|---|---|---|---|
| Glycerol (glycerin) | 56-81-5 | 3 | 92 | Initiator / chain branch |
| Diethylene glycol | 111-46-6 | 2 | 106 | Chain extender, solvent |
| Trimethylolpropane (TMP) | 77-99-6 | 3 | 134 | Crosslinker, alkyd/PU branch |
| Pentaerythritol | 115-77-5 | 4 | 136 | Alkyd/PU crosslinker |
| Sorbitol | 50-70-4 | 6 | 182 | Rigid-foam initiator |
| Polyether polyol (flex) | 25322-69-4 (PPG) | 2–3 | 3,000–6,000 | Flexible foam, CASE |
| Polyester polyol | family | 2–3 | ~500–3,000 | Coatings, elastomers, rigid foam |
| PolyTHF (PTMEG) | 25190-06-1 | 2 | 650–3,000 | Spandex, TPU, elastomers |
| Polycaprolactone diol | 36890-68-3 | 2 (and higher) | 530–80,000 | TPU, hydrolysis-resistant CASE |
Polyether vs Polyester Polyols: The Core Buying Decision
For most PU applications the first fork is polyether or polyester. They are not interchangeable, and the trade-off is concrete:| Property | Polyether polyol | Polyester polyol |
|---|---|---|
| Hydrolysis resistance | Better | Weaker (ester bond) |
| Mechanical strength / abrasion | Lower | Higher |
| Low-temperature flexibility | Better | Lower |
| Viscosity | Lower | Higher |
| Cost | Generally lower | Generally higher |
How the Numbers Map to Foam Type
The functionality/OH-number framework explains the two big foam families directly:- Flexible foam: high-MW (typically 3,000–6,000 g/mol) polyether triols with a low OH number (roughly 28–56 mg KOH/g). Long, lightly crosslinked chains give a soft, resilient foam for mattresses, furniture, and automotive seating.
- Rigid foam: low-equivalent-weight (~60–200) polyols started on high-functionality initiators like sorbitol or sucrose, with a high OH number (roughly 300–550 mg KOH/g). Dense crosslinking gives the dimensional stability and insulation value used in panels, appliances, and spray foam.
The Small High-Functionality Polyols (Initiators and Crosslinkers)
These are bought as building blocks, not bulk backbone:- Glycerol (CAS 56-81-5), f = 3: the classic triol initiator; also a humectant and solvent. Refined grades are specified by USP/FCC for food and pharma; technical grade for industrial routes including alkyds and nitroglycerin.
- Trimethylolpropane (CAS 77-99-6), f = 3: branch point for polyurethane and alkyd resins, synthetic ester lubricants, and coatings; valued for hydrolytic and thermal stability versus glycerol-based esters.
- Pentaerythritol (CAS 115-77-5), f = 4: tetraol for alkyd resins, intumescent fire-retardant coatings, and the explosive PETN.
- Sorbitol (CAS 50-70-4), f = 6: hexol initiator for rigid-foam polyols; also a humectant and bulk sweetener in food grades.
- Diethylene glycol (CAS 111-46-6), f = 2: a chain extender and solvent; note DEG is toxic if ingested and must never be used in food or oral pharmaceutical products.
The Polymeric Polyols (Backbone)
- Polyether polyols (PPG/PEG, CAS 25322-69-4 / 25322-68-3): propylene-oxide or ethylene-oxide chains; the largest-volume PU feedstock for flexible foam and CASE.
- Polyester polyols (family): diol-plus-diacid condensates; tougher and more chemical-resistant, used in coatings, adhesives, elastomers, and rigid foam.
- PolyTHF / PTMEG (CAS 25190-06-1): a diol giving outstanding abrasion resistance and elasticity; the soft segment in spandex, TPU, and high-performance elastomers.
- Polycaprolactone diol (CAS 36890-68-3): tough, hydrolysis-resistant polyester used in TPU, coatings, and adhesives; low-MW grades also serve technical (e.g. modeling/orthotic) markets.
- Polybutadiene polyols (family): hydrophobic hydrocarbon backbone for moisture-resistant sealants, encapsulants, and specialty elastomers.
Handling and Safety Notes
Most bulk polyols are combustible liquids of low acute toxicity, but the family spans real hazards: diethylene glycol is toxic by ingestion; isocyanate co-reactants (MDI/TDI) used with these polyols are respiratory sensitizers and drive the PU process hazard. Always work from the specific SDS, control mist and vapor, and use compatible storage (stainless steel or lined tanks; many polyols are hygroscopic and pick up water that throws off NCO stoichiometry). Food/pharma grades of glycerol and sorbitol are specified to USP/FCC and must not be substituted with technical grade.Sourcing and RFQ Guidance
Quoting a polyol by name alone gets you the wrong grade. State on your RFQ:- Functionality (diol, triol, or higher) for the network you need.
- Hydroxyl number (mg KOH/g) and acceptable window.
- Molecular weight / equivalent weight target.
- Backbone (polyether, polyester, PTMEG, PCL, polybutadiene).
- Viscosity at use temperature and water content limit.
- Grade/cert (technical, USP, FCC) if food/pharma.
- Packaging and volume (bulk, IBC tote, drum) and annual tonnage.
FAQs about Polyols
Does higher molecular weight mean more hydroxyl groups?
No. Functionality (number of OH groups) is set by the initiator, not the chain length. A PEG 4000 and a PEG 200 are both diols with two OH groups. Higher molecular weight at fixed functionality means a lower hydroxyl number.
What is the hydroxyl number of a polyol?
The hydroxyl number (OH value) is the milligrams of KOH equivalent to the hydroxyl groups in 1 gram of polyol. It relates to molecular weight by OH number = 56,100 × functionality ÷ molecular weight, and it sets crosslink density and isocyanate dose.
What is the difference between polyether and polyester polyols?
Polyether polyols resist hydrolysis and stay flexible at low temperature; polyester polyols give higher strength, abrasion, and chemical resistance but are more prone to hydrolysis. Choose polyether for moisture exposure, polyester for toughness.
Which polyols are used for rigid versus flexible foam?
Flexible foam uses high-MW polyether triols with a low OH number (about 28–56 mg KOH/g). Rigid foam uses low-equivalent-weight, high-functionality polyols (sorbitol- or sucrose-initiated) with a high OH number (about 300–550 mg KOH/g).
What is the CAS number of glycerol?
Glycerol (glycerin) is CAS 56-81-5. Other common polyols: sorbitol 50-70-4, trimethylolpropane 77-99-6, pentaerythritol 115-77-5, diethylene glycol 111-46-6, polyethylene glycol 25322-68-3, and PTMEG 25190-06-1.
What is equivalent weight and why does it matter?
Equivalent weight equals molecular weight divided by functionality (also 56,100 ÷ OH number). It tells you how many grams of polyol react per mole of isocyanate, so it directly sets your NCO:OH stoichiometry and the cured product’s properties.
§ Sourcing Desk
Sourcing this chemistry in bulk?
Search 1,300+ industrial chemicals by name or CAS, or send us your spec — we quote by the drum, tote, or container.
Browse the Chemical Index → Request a Quote