What is a key feature of polymers formed by condensation polymerisation?

Polymers formed by condensation polymerisation are characterized by the loss of small molecules, typically water or methanol, during the polymerization process. In condensation polymerisation, monomer units react with each other, resulting in the formation of covalent bonds and simultaneously releasing small molecules as by-products. This process contrasts with addition polymerisation, where no small molecules are eliminated.

The formation of water or another small molecule is integral to condensation polymerisation, as it drives the reaction forward and allows for the growth of the polymer chains. This key feature influences the properties of the resulting polymer, such as its molecular weight and structure.

The other choices do not accurately describe the characteristics of polymers formed through condensation polymerisation. Polymers from this process can be amorphous or crystalline in structure, but it is not a defining feature. They are not composed of metal ions, as most condensation polymers are organic materials composed of carbon-based monomers. Lastly, they do not remain monomers; rather, they are formed through the linkage of monomers into long chain structures.