Polycaprolactone Diol: Properties, Uses & FAQs

Polycaprolactone diol is a semicrystalline, hydroxyl-terminated polyester polyol synthesized by ring-opening polymerization of ε-caprolactone, widely valued for its biodegradability, low glass-transition temperature, and versatility as a soft-segment building block in polyurethanes and other advanced materials.

Chemical Class

Hydroxyl-terminated aliphatic polyester polyol

Synthesis Route

Ring-opening polymerization of ε-caprolactone initiated by a diol

Functional Groups

Two terminal primary hydroxyl groups (–OH), hydroxyl number varies with Mn

Typical Molecular Weight Range

Commercially available from ~530 Da to ~10,000 Da (number-average)

Biodegradability

Enzymatically and hydrolytically degradable; fully compostable under industrial conditions

Key Physical Property

Semi-crystalline solid or viscous liquid at room temperature depending on Mn; Tg approximately –60 °C

What Is Polycaprolactone Diol?

Polycaprolactone diol (PCL diol) is an aliphatic polyester with a hydroxyl group at each chain end, giving it a difunctional architecture suitable for further reaction with isocyanates, anhydrides, epoxies, and other electrophiles. It is produced commercially by the catalytic ring-opening polymerization (ROP) of ε-caprolactone (a seven-membered lactone) using a short-chain diol—commonly 1,4-butanediol or 1,6-hexanediol—as the bifunctional initiator. The resulting polymer backbone contains repeating –(CH₂)₅COO– units. Molecular weight is controlled by the monomer-to-initiator ratio, and the dispersity (Đ) of well-controlled preparations is typically below 1.3.

Because the backbone is free of aromatic units and contains only aliphatic ester linkages, PCL diol combines good flexibility with susceptibility to hydrolytic and enzymatic degradation, properties that are highly desirable in biomedical, packaging, and green-chemistry contexts.

Key Properties and Grades

Molecular weight and physical state: Low-Mn grades (≤1,000 Da) are typically clear, viscous liquids at room temperature, whereas higher-Mn grades (≥2,000 Da) are waxy solids that melt in the range of 40–60 °C. Melting point and degree of crystallinity increase with molecular weight.

Thermal properties: PCL diol exhibits a glass-transition temperature (Tg) near –60 °C and a melting temperature (Tm) that rises with Mn, broadly 40–60 °C. These values make PCL-based segments highly flexible at ambient and sub-ambient temperatures.

Solubility: PCL diol dissolves readily in common organic solvents including tetrahydrofuran (THF), dichloromethane (DCM), chloroform, acetone, and dimethylformamide (DMF). It is insoluble in water, aliphatic hydrocarbons, and diethyl ether at room temperature.

Hydroxyl value: The hydroxyl number (mg KOH/g) is inversely proportional to number-average molecular weight (Mn). Buyers should confirm the hydroxyl value by titration if the exact Mn is critical for stoichiometric formulation.

Applications of Polycaprolactone Diol

Polyurethane elastomers and coatings: PCL diol is widely used as the soft-segment polyol in thermoplastic polyurethanes (TPUs) and polyurethane coatings. It imparts low-temperature flexibility, good mechanical properties, and hydrolytic stability superior to polyether polyols in oxidative environments.

Biomedical devices and drug delivery: Because PCL degrades slowly by hydrolysis under physiological conditions and its degradation products are non-toxic, PCL diol is a valued macromonomer for biodegradable sutures, scaffolds for tissue engineering, wound dressings, and controlled-release drug-delivery matrices.

Shape-memory polymers: PCL-based networks exploit the sharp Tm of the PCL crystalline phase as a thermal switching segment in shape-memory polymer systems.

Green packaging and compostable materials: PCL copolymers and blends are under active development for compostable packaging films that meet industrial composting standards.

Research and custom synthesis: PCL diol serves as a macroinitiator or macromonomer for ABA triblock copolymers, graft polymers, and star polymers, giving researchers a modular platform for materials discovery.

Storage, Handling and Safety

Storage conditions: Store PCL diol in a tightly sealed container under an inert atmosphere (nitrogen or argon) in a cool, dry location away from moisture. High-Mn solid grades are best stored at or slightly below room temperature to prevent long-term crystallization-induced brittleness; low-Mn liquid grades should also be kept away from heat sources. Recommended storage temperature is typically 2–8 °C for long-term preservation of hydroxyl-group integrity, though short-term storage at ambient temperature is generally acceptable.

Moisture sensitivity: Ester groups are susceptible to hydrolysis under prolonged exposure to moisture, especially at elevated temperatures. Always reseal containers immediately after use and consider using desiccants.

Handling: Use standard laboratory PPE (gloves, safety glasses, lab coat). Ensure adequate ventilation. Avoid contact with strong acids, bases, and oxidizers, which can accelerate ester hydrolysis or oxidative degradation.

Disposal: Dispose of waste in accordance with local regulations for organic polymer waste. Do not discharge into waterways.

Quality, Purity and Analytical Characterization

Molecular weight determination: Gel permeation chromatography (GPC/SEC) calibrated against polystyrene or polyethylene glycol standards is the standard method for determining Mn, Mw, and dispersity (Đ) of PCL diol. End-group analysis by ¹H NMR in CDCl₃ provides an independent determination of Mn for lower-molecular-weight grades.

Hydroxyl functionality: The hydroxyl value (OHV) is determined by acetylation or phthalation titration methods (e.g., ASTM E1899 or DIN 53240). Confirmation of difunctionality is possible by MALDI-ToF mass spectrometry, which resolves individual oligomeric species and verifies the presence of two hydroxyl end groups.

Purity indicators: Residual monomer (ε-caprolactone) content can be measured by GC or ¹H NMR. Residual catalyst (commonly tin(II) 2-ethylhexanoate or aluminium alkoxide) may be present at low levels and should be within specification for biomedical applications.

Lot-to-lot variability: Hydroxyl value and Mn may vary within stated specification ranges between lots. For critical formulations, users should characterize each lot independently rather than relying solely on the certificate of analysis (CoA).

Frequently asked questions

What is the difference between polycaprolactone diol and polycaprolactone triol?

Polycaprolactone diol uses a difunctional diol initiator during ring-opening polymerization, yielding a linear polymer with exactly two terminal hydroxyl groups (functionality f = 2). Polycaprolactone triol is initiated by a triol (e.g., trimethylolpropane), producing a three-arm star architecture with three hydroxyl termini (f = 3). The choice of functionality determines network topology in polyurethane synthesis: diols form linear or lightly chain-extended polymers, while triols generate crosslinked networks with higher modulus and lower elongation.

How should I select the right molecular weight of polycaprolactone diol for my polyurethane formulation?

Molecular weight selection is governed primarily by the desired mechanical and thermal performance of the final polyurethane. Lower-Mn PCL diols (530–1,000 Da) provide a higher hard-segment content for a given isocyanate index, resulting in stiffer, harder materials. Higher-Mn PCL diols (2,000–10,000 Da) contribute longer, more flexible soft segments that lower the glass-transition temperature of the soft phase and improve low-temperature elasticity. A common starting point for elastomeric TPU applications is Mn ≈ 2,000 Da. The NCO/OH stoichiometry must also be recalculated when switching Mn grades because hydroxyl equivalent weight changes.

Is polycaprolactone diol safe for biomedical and tissue engineering applications?

Polycaprolactone and its oligomers have an established record of biocompatibility; PCL-based devices (e.g., Monocryl sutures, certain drug-delivery implants) have received regulatory clearance in several jurisdictions. PCL degrades hydrolytically to 6-hydroxyhexanoic acid and further to common metabolites that are processed via normal metabolic pathways. However, the biocompatibility and regulatory status of any specific PCL diol grade intended for clinical use must be independently confirmed, including testing for residual catalyst, residual monomer, and compliance with relevant ISO 10993 biocompatibility standards. Aladdin Scientific products are supplied for research use only unless otherwise stated.

Why is my solid-grade polycaprolactone diol difficult to dissolve or transfer?

Higher-Mn PCL diol grades are semi-crystalline waxes at room temperature. If the material is unusually hard or lumpy, gentle warming to 60–70 °C (above Tm) in a water bath or oven will melt it to a pourable liquid without degradation. For dissolution, pre-melt the sample before adding solvent, or dissolve at a slightly elevated temperature in THF, DCM, or chloroform with gentle stirring. Avoid prolonged heating above 100 °C, which may accelerate thermal degradation or discoloration.

Can polycaprolactone diol be used in 3D printing or additive manufacturing?

PCL diol itself is a liquid or soft solid and is not directly used as a print filament, but it is a key reactive component in polyurethane-based photopolymer resins and two-component reactive inks for vat photopolymerization and inkjet printing. Researchers have also used PCL diol as a soft-segment macrodiol to create TPU filaments for FFF/FDM printing with shape-memory and biodegradable characteristics. Viscosity, reactivity (with isocyanates or acrylate-functionalized reagents), and pot life need to be optimized for specific printing platforms.

References

1. Labet, M.; Thielemans, W. Synthesis of polycaprolactone: a review. Chemical Society Reviews 2009, 38, 3484–3504. doi:10.1039/b820162p
2. Woodruff, M. A.; Hutmacher, D. W. The return of a forgotten polymer—Polycaprolactone in the 21st century. Progress in Polymer Science 2010, 35, 1217–1256. doi:10.1016/j.progpolymsci.2010.04.002
3. Gnanou, Y.; Fontanille, M. Organic and Physical Chemistry of Polymers; Wiley-Interscience, 2008.

Products are supplied for research and development use only. Not for use in humans, animals, diagnosis, or therapy.