Addition and Condensation

Introduction

Organic polymers are categorized according to their polymerization mode, which reflects the polymer’s chemical structure and stability.

The two types of polymerization, addition polymerization and condensation polymerization. Monomers can combine in a variety of ways to generate polymeric polymers. One way is to use a condensation reaction, which is a sort of chemical reaction. Two molecules mix with the loss of a smaller molecule, commonly water, alcohol, or acid, in a condensation reaction. 

Condensation Polymers are the polymers that result from a condensation reaction. Water or other tiny molecules are removed from two or more reactant monomers to generate condensation polymers. Polyamides and polyesters are common condensation polymers; the polymer structure comprises polar amide or ester groups, which are biodegradable. Cellulose is a glucose monomer-based natural condensation polymer.

Addition reactions are another mechanism for monomers to unite to generate polymers. In addition to reactions, the electrons of a monomer’s double bonds are rearranged to form single bonds with other molecules. The qualities of various plastics (soft/hard, stretchy/rigid, clear/opaque, chemically inert) are influenced by the interaction between polymer chains. Addition polymers are the polymers that result from an addition process.

Polymerization by addition

At least one double bond is required for additional polymerization, often known as chain reaction polymerization. In addition, no molecule is lost during polymerization, and no by-product is produced. The produced polymer has the same molecular weight as the sum of all monomers used in the polymerization. A chain reaction connects monomers by altering the bonds between them. The reaction to make ethylene, for example, is

n{CH2=CH2}  –> -CH2-(CH2-CH2)-CH2-

In this form of polymerization, there are three steps: initiation, propagation, and termination. The monomer obtains an active site during initiation, allowing it to become a free radical. The initiation process can be triggered by the addition of initiators or other techniques such as heat, light, or irradiation absorption. During propagation, the started monomers rapidly add other monomers. This process continues until the active site is deactivated by chain termination or chain transfer, as the active site is constantly repositioned at the end of the developing chain during propagation. A polymer chain radical reacts with another free radical during the termination process. The usual step-growth polymerization method consists of these three phases. In many circumstances, however, a fourth phase, known as chain transfer, is also involved. The rising activity of a polymer chain is transferred to previously inert species through chain transfer.

Polymerization by condensation

Condensation polymerization is a type of step-growth polymerization in which monomers and/or oligomers combine with each other to generate larger structural units while releasing smaller molecules such as water or methanol as a byproduct. Strong acids, such as sulfuric acid, can speed up condensation polymerization even further. The esterification of carboxylic acids with alcohol is a well-known condensation reaction. The reaction between a dibasic acid and a glycol is a classic step-growth condensation, as demonstrated below:

HOOC–(CH2)n–COOH + HO–(CH2)m–OH HOOC–(CH2)n–COOH HOOC–(CH2)n–COOH HOOC–(CH2)n–CO

HOOC–(CH2)n–COO–(CH2)m–OH + H2O HOOC–(CH2)n–COO–(CH2)m–OH + H2O

Polyester is the name given to the resulting polymer. Because water is reactive with ester bonds and can reverse the reaction, it is continuously removed from the reaction system.

The condensation product is a linear polymer if both moieties are difunctional, and a crosslinked polymer if at least one of the moieties is tri- or tetra-functional (i.e. a three-dimensional network). Adding monomers with only one reactive group to a developing chain will stop it from growing and, as a result, lower the (average) molecular weight. As a result, the average molecular weight and crosslink density of each monomer engaged in condensation polymerization, as well as its concentration in the mixture, will determine their average molecular weight and crosslink density.

Nucleophile addition

A nucleophile addition is a process in which a nucleophile attaches to a compound’s pi bond. Two new sigma bonds are created as a result. This reaction is important in organic chemistry because it allows carbonyl compounds to combine to produce new products with different functional groups.

Because of the polarity of the C=O link, carbonyl compounds undergo nucleophilic addition reactions. Because the polar link allows partial charges on the carbon and oxygen atoms, the nucleophile can attack the 1,2-positions with ease.

Aldehydes and ketones will promote nucleophilic addition reactions the most. Because neither aldehydes nor ketones have a leaving group, it differs from nucleophilic substitution processes.

Nucleophilic Addition Mechanism

The steps in the nucleophilic addition reaction of carbonyl compounds are listed below

Step 1: The nucleophile establishes a connection with the electrophilic C=O carbon atom, which causes the carbonyl carbon to rehybridize from sp2 to sp3. A tetrahedral alkoxide intermediate is formed when electrons in the pi-bond are attracted to the electronegative oxygen atom.

Step 2: An acid is used to protonate the alkoxide, resulting in the formation of alcohol.

RMgX (Grignard reagent), RL, and RCCM are strong nucleophiles that add straight to the C=O bond, generating the intermediate. Weak nucleophiles, such as H2O, ROH, and RNH2, on the other hand, require an acid catalyst.

Conclusion

Condensation polymerization is a type of step-growth polymerization in which monomers and/or oligomers combine with each other to generate larger structural units while releasing smaller molecules such as water or methanol as a byproduct. In addition, no molecule is lost during polymerization, and no by-product is produced. Polystyrene, polyethene, polyacrylates, and methacrylates are examples of additional polymers. Condensation polymers are made by combining bi- or polyfunctional molecules and removing a tiny molecule (such as water) as a by-product.  Examples are  Polyester, polyamide, polyurethane, and polysiloxane.