# **Polymers: Classification and Synthesis** Polymers are large macromolecules formed by the repetitive bonding of many smaller molecules called *monomers*. They are broadly classified into two main types based on their formation mechanism: **Addition Polymers** and **Condensation Polymers**. ## **Addition Polymers** Addition polymers are formed when unsaturated monomers (typically alkenes) add to one another in a chain reaction, without the loss of any atoms. The entire monomer unit is incorporated into the polymer chain. * **Formation Mechanism:** Monomers containing double or triple bonds (e.g., alkenes) open their multiple bonds to form new single bonds, linking together to create a long polymer chain. * **Reaction Conditions:** * Often catalyzed by peroxides or acids. * Traditional methods required high pressures (1000 atm) and high temperatures ($200^\circ C$). * *Ziegler-Natta catalysts* (e.g., trialkylaluminum $\mathrm{Al(R)_3}$ and titanium tetrachloride $\mathrm{TiCl_4}$ in an inert solvent) allow for lower temperatures and pressures. * **Polymer Backbone:** Consists solely of carbon atoms, originally derived from the multiple bonds of the monomers. * **Characteristics:** No small molecules are eliminated during the polymerization process. ### **Example: Polyvinyl Chloride (PVC)** * **Monomer:** Vinyl chloride ($\mathrm{CH_2=CHCl}$). * **Monomer Synthesis:** Vinyl chloride is obtained by treating acetylene with $\mathrm{HCl}$ in the presence of $\mathrm{HgCl_2}$. * **Polymerization:** Vinyl chloride monomers add to form PVC. $$ n \mathrm{CH_2=CHCl} \xrightarrow{\text{catalyst}} \mathrm{[-CH_2-CHCl-]} _n $$ * **Uses:** Artificial leather, floor coverings, corrugated roofing materials, gramophone records. ## **Condensation Polymers** Condensation polymers are formed by the reaction of monomers with the elimination of small molecules, such as water ($\mathrm{H_2O}$), ammonia ($\mathrm{NH_3}$), or simple alcohols ($\mathrm{ROH}$). This process typically involves monomers each having at least two functional groups. There are two main types of condensation polymers: **Polyamides** and **Polyesters**. ### (a) Polyamides Polyamides are condensation polymers characterized by the presence of repeating *amide linkages* ($-\mathrm{CO-NH-}$) in their backbone. They are typically formed from the reaction of diamines and dicarboxylic acids, or from amino acids. #### (i) Nylon-6,6 * **Monomers:** Adipic acid (a dicarboxylic acid with 6 carbons) and hexamethylenediamine (a diamine with 6 carbons). The "6,6" in its name refers to the six carbon atoms in each monomer. * Adipic acid: $\mathrm{HOOC(CH_2)_4COOH}$ * Hexamethylenediamine: $\mathrm{H_2N(CH_2)_6NH_2}$ * **Formation:** A hydroxyl ($-\mathrm{OH}$) group from the dicarboxylic acid reacts with a hydrogen atom from an amine ($-\mathrm{NH_2}$) group, forming water and an amide linkage. Since both monomers have two functional groups, polymerization continues to form a long chain. * **Reaction Equation:** $$ n \mathrm{HOOC}(\mathrm{CH_2})_4\mathrm{COOH} + n \mathrm{H_2N}(\mathrm{CH_2})_6\mathrm{NH_2} \xrightarrow{\text{200}^\circ C, \text{ N}_2} \mathrm{[-OC}(\mathrm{CH_2})_4\mathrm{CONH}(\mathrm{CH_2})_6\mathrm{NH-]} _n + 2n \mathrm{H_2O} $$ $$ \mathrm{HOOC}\left(\mathrm{CH}_{2}\right)_{4} \mathrm{COOH}+\mathrm{HNH}\left(\mathrm{CH}_{2}\right)_{6} \mathrm{NH}_{2} \rightarrow \mathrm{[-CO}\left(\mathrm{CH}_{2}\right)_{4} \mathrm{CONH}\left(\mathrm{CH}_{2}\right)_{6} \mathrm{NH-]} _n $$ * **Uses:** Synthetic fibers for stockings, apparel, fishing lines, ropes, brush bristles, and molded objects like gears and bearings. #### (ii) Kevlar * **Monomers:** Benzene-1,4-dicarboxylic acid (terephthalic acid) and benzene-1,4-diamine (p-phenylenediamine). These monomers are aromatic. * **Formation:** Similar to Nylon-6,6, amide linkages are formed between the dicarboxylic acid and the diamine, with the elimination of water. * **Properties:** Strong, lightweight, corrosion, and heat resistant. * **Uses:** Bullet and fireproof vests, crash helmets, aircraft bodies, boats. #### (iii) Polypeptides (Proteins) * **Monomers:** Amino acids. Each amino acid contains at least one amine ($\mathrm{-NH_2}$) group and one carboxylic acid ($\mathrm{-COOH}$) group. * **Formation:** Amino acids polymerize by forming *peptide bonds* (a type of amide linkage) between the carboxylic acid group of one amino acid and the amine group of another, with the elimination of water. * **General Reaction (using aminoethanoic acid as an example):** $$ n \mathrm{H_2N-CH_2-COOH} \rightarrow \mathrm{[-NH-CH_2-CO-]} _n + n \mathrm{H_2O} $$ (aminoethanoic acid) $\quad$ (polypeptide) ### (b) Polyesters Polyesters are condensation polymers characterized by repeating *ester linkages* ($-\mathrm{COO-}$) in their backbone. They are typically formed from the reaction of a diol (alcohol with two $-\mathrm{OH}$ groups) and a dicarboxylic acid. #### (i) Terylene (Polyethylene Terephthalate, PET) * **Monomers:** Ethane-1,2-diol (ethylene glycol) and benzene-1,4-dicarboxylic acid (terephthalic acid). * **Formation:** Ester linkages are formed between the hydroxyl groups of the diol and the carboxylic acid groups of the dicarboxylic acid, with the elimination of water. * **Properties:** Readily reacts with alkalis but slowly with acids. * **Uses:** Plastic bottles, clothing, carpets. #### (ii) Polylactic Acid (PLA) * **Monomer:** 2-hydroxypropanoic acid (lactic acid). This is a hydroxycarboxylic acid, meaning it contains both a hydroxyl ($\mathrm{-OH}$) and a carboxylic acid ($\mathrm{-COOH}$) group within the same molecule. * **Formation:** Lactic acid monomers undergo self-condensation to form ester linkages. * **Uses:** Biodegradable plastics, biomedical applications. ## **Possible Questions/Answers** These exercises help reinforce the concepts of polymer classification and monomer identification. * **Q: (CONCEPT ASSESSMENT EXERCISE 13.1) Predict the type of polymerisation reaction for the following monomers.** * **(i) Propylene $\mathrm{CH_3-CH=CH_2}$** * **(ii) $\mathrm{HO-A-OH} + \mathrm{HOOC-B-COOH}$** **A:** * **(i) Propylene ($\mathrm{CH_3-CH=CH_2}$):** This is an alkene monomer with a double bond. Therefore, it will undergo **addition polymerization**. * **(ii) $\mathrm{HO-A-OH} + \mathrm{HOOC-B-COOH}$:** These monomers have multiple functional groups ($\mathrm{-OH}$ and $\mathrm{-COOH}$) that can react to eliminate a small molecule (water in this case) and form an ester linkage. Therefore, it will undergo **condensation polymerization** (specifically, forming a polyester). * **Q: (CONCEPT ASSESSMENT EXERCISE 13.2) A polymer can be made from monomers benzene-1,4-dioyl chloride and benzene-1,4-diamine. Draw its repeat unit.** **A:** * **Monomers:** * Benzene-1,4-dioyl chloride (terephthaloyl chloride): $\mathrm{ClOC-C_6H_4-COCl}$ * Benzene-1,4-diamine (p-phenylenediamine): $\mathrm{H_2N-C_6H_4-NH_2}$ * **Reaction:** An acid chloride reacts with an amine to form an amide linkage, eliminating $\mathrm{HCl}$. * **Repeat Unit:** $$ \mathrm{[-CO-C_6H_4-CO-NH-C_6H_4-NH-]} _n $$ * **Q: (CONCEPT ASSESSMENT EXERCISE 13.3) Draw the monomer that makes the following condensation polymer: $\mathrm{[-O-CH_2-CH_2-CO-]} _n$** **A:** The repeat unit shows an ester linkage formed by condensation. The repeating unit $\mathrm{[-O-CH_2-CH_2-CO-]} _n$ indicates that the oxygen comes from a hydroxyl group and the carbonyl carbon comes from a carboxylic acid group. Since the chain has a hydroxyl and carboxylic group derived from the same unit, it implies a *hydroxycarboxylic acid* underwent self-condensation. * The monomer is **3-hydroxypropanoic acid**: $\mathrm{HO-CH_2-CH_2-COOH}$.