Functionalized Chitosan Derivatives

Chitosan is a good starting point and a difficult finished material. The amino groups that make it cationic only stay that way while they are protonated, and the literature most often places the pKa of the glucosamine unit near 6.3 to 6.5. Move past that point and the charge goes, the chain coils, and the polymer leaves the solution it was supposed to be carrying. Every formulation built on the untreated polymer inherits the same constraints: a dilute acid as the solvent, a narrow working window, and a viscosity that drifts as that acid is consumed. Functionalisation rewires those constraints deliberately. A group is attached to the backbone on purpose, and from then on the charge, the solubility and the reactivity belong to the molecule rather than to the beaker it happens to be standing in.

The families we keep in stock differ mainly in which group goes on and where it lands. Carboxymethylation introduces acidic groups and makes the chain amphoteric. Quaternisation puts a permanent positive charge on the amino nitrogen. Thiolation hangs reactive sulfhydryl groups off the backbone. Methacrylation installs carbon-carbon double bonds, so a shape can be set and then locked with light. Around them sit the salt forms that dissolve without any acid at all, and a set of grafted and surface-modified grades where the chain carries a second polymer, a tracer or a metal. Each is offered across a range of substitution levels, because two batches of the same derivative at different degrees of substitution behave like two different materials.

The three reactive sites, and what each one buys you

There are only three positions on a glucosamine repeat unit where a reaction can usefully start, and they are far from equivalent. The primary hydroxyl on C6 sits at the end of a flexible arm and is the easiest to reach. The secondary hydroxyl on C3 is the most crowded. The amino group on C2 is the one that carries the charge, and it is also the one whose character changes most once it has been substituted. Picking a site is therefore the first design decision, not a footnote to the synthesis.

Degree of substitution is the number that travels with the batch

Degree of substitution counts the average number of substituent groups carried by each repeat unit, so the ceiling is three and commercial material usually lands between 0.4 and 1.2. The figure is not cosmetic. Where the amino nitrogen is the site being substituted, it sets a hard solubility boundary: products with more than roughly half of their amino groups substituted stop dissolving below pH 5, while those in the 20 to 50 percent range still go into acid between pH 2 and 5. Two samples that both read carboxymethyl chitosan on the label can therefore behave nothing alike in the same buffer. That is why we quote substitution as a band, and why the assay behind the band is named on the certificate.

Powder split under an inert atmosphere, where moisture-sensitive grades are weighed and portioned.

Carboxymethyl chitosan, in three substitution patterns

Carboxymethyl chitosan is the workhorse of the family, and it is really three products wearing one name. In alkali, sodium chloroacetate will attack the hydroxyls, the amino group, or both, and the three outcomes are not interchangeable. A study that prepared all three side by side reported degrees of substitution of 0.58 for the hydroxyl-substituted form, 1.18 for the dual-substituted form and 1.26 for the amino-substituted form, and found the dual-substituted product by far the most water-soluble at close to 68 grams per litre. The practical reading is simple enough: if the material has to keep a positive charge while dissolving at neutral pH, the mixed pattern is the one to specify.

Our bulk carboxymethyl grades follow the common industrial format: degree of substitution at 0.8 or above, purity of 90 percent or more on a dry basis, moisture at 10 percent or below, ash from 1 to 5 percent depending on the grade required, pH between 6.0 and 8.0 in a one percent solution, and heavy metals reported as lead at 10 ppm or below. Viscosity is quoted at a stated concentration and temperature, since the same polymer can read an order of magnitude apart under different shear.

Through the sight glass of an alkalisation vessel: swollen polymer paste ahead of carboxymethylation.

Quaternised grades: charge that does not care about pH

Quaternisation is the answer when the working pH is neutral or alkaline and the charge still has to be there. The trimethyl derivative, N,N,N-trimethyl chitosan, carries a permanently positive nitrogen and needs no protonation step. Its usefulness scales with the degree of quaternisation. Conventional reductive methylation with formaldehyde and formic acid under Eschweiler-Clarke conditions runs hot and slow and typically lands between 40 and 90 percent substitution, depending on reagent ratios and reaction time. Greener routes are now published as well, among them lipase-catalysed methylation with dimethyl carbonate in a ternary deep eutectic solvent, which reached roughly 12 to 16 percent in a much shorter and more solvent-light process. Solubility follows the substitution upward: at a degree of quaternisation near 62 percent, reported solubility at pH 7.4 is about 98 mg/mL, and heavier substitution pushes it further. The quaternised nitrogen is easy to confirm spectroscopically through the C–N stretch close to 1480 cm⁻¹.

Thiolated grades: reactivity you switch on by changing pH

Thiolation adds a handle rather than a property. Thiol groups grafted onto the backbone through thioglycolic acid, or through a cystamine route with carbodiimide coupling, will oxidise into disulfide bonds, and the rate of that reaction follows the pH closely. Reported thiolated trimethyl chitosans, prepared with degrees of quaternisation from 25 to 54 percent and thiol contents of 5 to 7 percent, oxidised rapidly at pH 7.4 into disulfide-crosslinked networks while remaining comparatively stable at pH 4.0. That gap is the point of the grade. A homogeneous solution can be prepared under mildly acidic conditions and crosslinking triggered later simply by raising the pH, with no added initiator and no heat. Thiol content is normally confirmed by an Ellman's assay, and the quaternisation by proton NMR.

Methacrylated grades: set the shape, then freeze it with light

Methacrylated chitosan, sold under shorthand names such as ChiMA or chitosan methacrylate, is the entry point for anyone who has wanted to cure a chitosan formulation on demand. Methacrylic anhydride writes photo-reactive double bonds onto the backbone, and those bonds will undergo free-radical crosslinking under visible or ultraviolet light once a photoinitiator is present. Catalogue grades span a wide substitution range, from 5 to 20 percent on the research side up to 20 to 40 percent, with weight-average molecular weights reported around 400 to 600 kDa for the common purified lyophilisates. The material arrives as a lyophilisate to be held at -20 °C, kept from light and moisture, and dissolved in 20 mM acetic acid or a buffered saline at around 0.5 to 1 percent with an initiator such as LAP, before exposure near 405 nm. Grades keeping residual methacrylate at or below 100 ppm are available where unreacted monomer has to be held down.

A cast film of a methacrylated grade under tensile load during release testing.

Grafted chains, labelled tracers and metal-loaded carriers

Not every modification is about charge or light. Grafting a second polymer onto the backbone, polyethylene glycol being the usual choice, changes how the material disperses and reduces non-specific adsorption onto surfaces. Attaching a fluorophore turns the polymer into its own tracer, which is why labelled grades turn up in uptake and distribution work. Loading the free amino and hydroxyl groups with a metal ion produces either a supported catalyst or a convenient model system for chelation study; a copper-loaded chitosan carrying 1.3 mmol of copper per gram is a representative catalogue figure. Grafting ratios and metal loadings are reported per gram of dry polymer, established by elemental analysis or a spectroscopic assay.

Surface morphology of a cast film, read at the nanoscale on a scanning probe stage.

What our specification sheet carries

Every batch leaves with the same document, and the fields on it are the ones that decide whether the material will behave as expected in your process.

How the substitution is proven

A stated degree of substitution is worth exactly as much as the method behind it, and different derivatives call for different instruments. The same sample can also be checked by more than one route, which is how a supplier and a customer settle a disagreement over a number without repeating the whole synthesis.

The glass train used to liberate and trap nitrogen from a weighed sample.

Where these grades are used

Dissolution, storage, and the small print

• Salt forms and carboxymethyl grades go into cold water with stirring. Acid is not needed, and adding it is more likely to bring the polymer back out of solution than to help it in.
• Keep powders sealed, dry and out of direct light. Moisture is the main road to degradation, because it drives hydrolysis, which lowers molecular weight and with it the viscosity the specification was written around.
• Lyophilised methacrylated grades are held at -20 °C and kept from light. Photoinitiator is added to the portion being used, not to the whole vial.
• Dissolve what you need on the day. Aqueous solutions of these derivatives drift in viscosity with time, and thiolated grades will begin crosslinking themselves once the pH sits above their stable window.
• Keep stainless steel and metal dust away from dissolution vessels where chelation or catalysis is the intended function, or the sites you paid for will already be occupied.

Reaction and dissolution jars held at a fixed temperature in a circulating bath.

Close to the catalogue line, and past it

Most of the enquiries that reach us are not for a catalogue line but for something close to one. Those we take on directly. A substitution level can be set anywhere in the achievable range for the derivative; the substitution pattern can be steered where the chemistry allows it; the counter-ion can be the hydrochloride, lactate or glutamate salt; molecular weight can be cut to a target band; endotoxin can be held to the lower limit; and material can be supplied sterile, in solution, or as a powder. Pack sizes run from laboratory quantities to full drums. Each shipment carries a certificate of analysis and a safety data sheet, and where a project needs a specification that does not yet exist, the useful next step is a conversation about target values rather than a catalogue number.

For Research or Industrial Raw Materials, Not For Personal Medical Use!