Addition Polymerisation:

In addition polymerisation, small molecules called monomers combine to form a large chain called a polymer. This process typically involves unsaturated monomers, meaning they contain double or triple bonds. The double or triple bonds in the monomers are broken, and the monomers add to each other in a chain reaction. No atoms are lost in the process; only bonds are broken and formed. Therefore, the molar mass of the polymer is approximately the same as the average molar mass of the monomer. A common by-product is none.

Starting Materials: Unsaturated monomers (e.g., ethylene, propylene, vinyl chloride).

Products: Polymers with no repeating units of the original monomer (e.g., polyethylene, polypropylene, PVC).

By-products: None (ideally). However, impurities can sometimes lead to the formation of small amounts of other compounds.

Examples: Polyethylene (plastic bags), Polypropylene (containers), Polyvinyl Chloride (PVC - pipes).

Condensation Polymerisation:

In condensation polymerisation, monomers combine to form a polymer, and a small molecule, such as water, is removed as a by-product. This process typically involves monomers containing functional groups like carboxyl (-COOH) and amine (-NH2) groups. The monomers react with each other, and the removal of the small molecule (e.g., water) links them together to form the polymer chain. The molar mass of the polymer is less than the sum of the molar masses of the monomers because of the loss of the small molecule.

Starting Materials: Monomers with functional groups (e.g., amino acids, esters, carboxylic acids).

Products: Polymers with repeating units of the original monomer (e.g., nylon, polyester, protein).

By-products: Small molecules like water, methanol, or hydrochloric acid.

Examples: Nylon (clothing), Polyester (clothing, bottles), Protein (muscle, hair), Starch (energy storage in plants).

Here's a table summarizing the key differences:

Primary Structure: The primary structure of a protein refers to the linear sequence of amino acids in the polypeptide chain. This sequence is determined by the genetic code. It is held together by covalent peptide bonds formed between the carboxyl group of one amino acid and the amino group of the next. The primary structure dictates all subsequent levels of protein structure.

Secondary Structure: The secondary structure involves localized, repeating patterns of folding within the polypeptide chain. The most common secondary structures are alpha-helices and beta-sheets.

• Alpha-helices: Formed by hydrogen bonds between the carbonyl oxygen of one amino acid and the hydrogen atom of the amino group four residues further along the chain.
• Beta-sheets: Formed by hydrogen bonds between strands of the polypeptide chain. These strands can run in the same direction (parallel) or opposite directions (antiparallel).

The forces involved are hydrogen bonds between the amide hydrogen and carbonyl oxygen atoms of the peptide backbone. These bonds are not between the R-groups (side chains) of the amino acids.

Tertiary Structure: The tertiary structure is the overall three-dimensional shape of a single polypeptide chain. It arises from interactions between the R-groups (side chains) of the amino acids. These interactions include:

• Hydrogen bonds between R-groups.
• Ionic bonds between oppositely charged R-groups.
• Disulfide bridges (covalent bonds) between cysteine residues.
• Hydrophobic interactions (clustering of non-polar R-groups in the interior of the protein).
• Van der Waals forces (weak attractions between atoms).

These forces collectively determine the protein's unique 3D conformation.

Quaternary Structure: Some proteins consist of two or more polypeptide chains (subunits) held together by weaker bonds. The quaternary structure refers to the arrangement of these subunits in the protein complex. The forces involved are the same as those in tertiary structure: hydrogen bonds, ionic bonds, disulfide bridges, hydrophobic interactions, and van der Waals forces. The quaternary structure is important for protein function, as it can influence the protein's stability and activity.

The role of a catalyst in the polymerisation of ethene is to speed up the reaction without being consumed itself. It provides a surface for the ethene molecules to adsorb, weakening the pi bond and facilitating the radical chain reaction. Without a catalyst, the reaction would be too slow to be practical.

High pressure is necessary for the following reasons:

• Increase the concentration of reactants: Increasing the pressure forces the ethene molecules closer together, effectively increasing their concentration in the reaction mixture. This leads to a higher frequency of collisions between the monomers and the catalyst, increasing the rate of polymerisation.
• Prevent the escape of ethene: Ethene is a gas at room temperature. High pressure prevents the ethene from escaping the reaction vessel, ensuring that it remains available for polymerisation.