PolyDADMAC (polydiallyldimethylammonium chloride) is a synthetic, water-soluble cationic polyelectrolyte composed of repeating units derived from the monomer diallyldimethylammonium chloride (DADMAC).
Chemical formula: C₈H₁₆ClN
Structure:
Two allyl groups (CH₂=CH–CH₂–)
Two methyl groups (–CH₃)
A central quaternary ammonium nitrogen (N⁺)
Counterion: Chloride (Cl⁻)
Formed by free-radical polymerization of DADMAC monomers.
The double bonds in the allyl groups open up and link together to form a long carbon backbone.
Repeating unit:
[ ext{–[CH}_2 ext{–CH(CH}_2 ext{N}^+ ext{(CH}_3)_2 ext{CH=CH}_2) ext{]–} quad ext{Cl}^- ]
However, due to the nature of allylic polymerization, the actual structure is more complex. The polymer chain typically has pendant quaternary ammonium groups attached via methylene (–CH₂–) spacers.
Simplified repeating unit (common representation):
[ left( ce{[C8H16N+]_n cdot n Cl^-} ight) ]
Or structurally:
[ ce{-(CH2-CH2)_n-} ext{ with } ce{-CH2-N^+(CH3)2-CH2-CH=CH2} ext{ side groups (idealized)} ]
⚠️ Note: Because DADMAC is an allylic monomer, its polymerization can lead to regioirregular structures (e.g., 1,2- vs 1,4-addition), but the key feature is that every repeat unit carries a permanent positive charge on a quaternary nitrogen, balanced by a chloride ion.
| Feature | Description |
|---|---|
| Backbone | Hydrocarbon chain (from polymerized allyl groups) |
| Functional Group | Quaternary ammonium (–N⁺(CH₃)₂R) — permanently cationic |
| Counterion | Chloride (Cl⁻); sometimes partially exchanged for other anions |
| Charge Density | Very high — nearly 100% of monomer units are charged |
| Molecular Weight | Typically medium (100,000 – 1,000,000 g/mol), lower than polyacrylamides |
| Solubility | Highly soluble in water due to ionic nature |
Pure PolyDADMAC is rarely used as-is. Commercial formulations typically contain:
10–50% active PolyDADMAC (by weight)
Water (major component in liquid products)
< 0.1% residual DADMAC monomer (regulated due to toxicity)
Stabilizers or salts (to control viscosity or shelf life)
Example: A common liquid product might be 40% PolyDADMAC, meaning 40 g of polymer per 100 g of solution, with the rest being water and trace impurities.
The permanent positive charge enables strong electrostatic attraction to negatively charged particles (clay, organics, microbes).
No pH dependence: Unlike metal coagulants, it works at any pH because the charge isn’t hydrolytic.
Low molecular weight + high charge = excellent for charge neutralization, not bridging (unlike high-MW CPAM).
PolyDADMAC is a homopolymer made entirely from DADMAC monomers, with a chemical backbone derived from allyl groups and pendant quaternary ammonium chloride groups that give it a strong, permanent positive charge. This unique composition makes it one of the most effective charge-neutralizing coagulants in modern water treatment.
