Organic Chemistry: Polymers - SS3 Chemistry Past Questions and Answers - page 3

Addition polymerization is typically initiated by a free radical, which is a highly reactive species with an unpaired electron. This radical initiates the polymerization process by attacking a monomer and initiating chain growth through repeated addition of monomers.

Step-growth polymerization involves the reaction of bifunctional or multifunctional monomers that have two or more active sites. Glycol and terephthalic acid are bifunctional monomers, and their reaction results in the formation of a polyester polymer, like polyethene terephthalate (PET).

Ring-opening polymerization is a specialised process where cyclic monomers (e.g., lactones, cyclic ethers) undergo cleavage of the ring structure to form linear polymer chains. This process is used to produce various important polymers like polyethene oxide and polylactide.

Addition Polymerization vs. Condensation Polymerization:

1.    Mechanism:

Addition Polymerization:

Addition polymerization involves the successive addition of monomers without the elimination of any small molecules. It occurs through the breaking of double or triple bonds in monomers, creating free radicals or ionic species. These reactive species initiate the polymerization process and continue to add monomers until the chain termination step occurs.

Condensation Polymerization:

Condensation polymerization, on the other hand, involves the elimination of small molecules, typically water or alcohol, during the polymerization process. Two different bifunctional monomers with active functional groups react, releasing the small molecule and forming a covalent bond between the monomers. The process continues until the monomers are consumed or the chain termination step occurs.

2.    Monomers:

Addition Polymerization:

The monomers involved in addition polymerization are usually unsaturated compounds, such as alkenes (olefins) or alkynes. They have multiple carbon-carbon double or triple bonds that break during polymerization to form the polymer chain. For example, the additional polymerization of ethylene (C2H4) yields polyethene (PE), a widely used plastic.

Condensation Polymerization:

Condensation polymerization involves bifunctional monomers with different reactive functional groups, such as carboxylic acids and alcohols or amines. During polymerization, the reactive groups from different monomers combine and release small molecules like water, alcohol, or ammonia. For instance, the reaction between adipic acid and hexamethylenediamine leads to the formation of nylon-66.

3.    Small Molecule Byproducts:

Addition Polymerization:

In addition to polymerization, no small molecules are eliminated as byproducts. The polymerization process is clean and straightforward.

Condensation Polymerization:

Condensation polymerization produces small molecules (water, alcohol, etc.) as byproducts during each polymerization step. This requires careful control of stoichiometry to achieve high molecular weight and avoid side reactions.

4.    Polymer Structure:

Addition Polymerization:

The resulting polymers in addition to polymerization have a relatively simple structure since they are formed from repeating units of the same monomer. They often have a high degree of crystallinity.

Condensation Polymerization:

Condensation polymers can have more complex structures due to the combination of two different monomers. This can lead to variations in the polymer's properties and make them less crystalline compared to addition polymers.

In summary, addition polymerization and condensation polymerization are two distinct processes with different mechanisms and outcomes. Addition polymerization involves the successive addition of monomers without the elimination of small molecules, while condensation polymerization eliminates small molecules as byproducts during the reaction. Both methods have their unique advantages and produce a wide range of polymers used in various applications.