Polymers for gene therapy

A polyphosphazene comprising:

• at least one hydrocarbon chain (A);
• and
• at least one hydrocarbon chain (B) of between 6 and 24 members, wherein between 1 and 4 members are heteroatoms independently selected from O, N and S, and the terminal group of the chain is a group of formula —NH2,
• wherein the hydrocarbon chain (A) has one of the following structures:
• wherein m can have the value 0, 1 or 2,
• n can have the value 1, 2 or 3,
• q can have the value 0 or 1,
• each X1, X2 and X3 are independently selected from the NH, S and O atoms,
• Alk is a linear or branched C1-C6 alkyl, optionally having a substituent selected from a hydroxyl or thiol group;
• or