Ücretsiz Kullanmaya Başla

Condensation polymers Knowledge-Based Study

Chemistry · Organic Chemistry · Chemistry · Polymers & Biomolecules

Chemistry topic explanations on Condensation polymers.

Study notes

  1. What are Condensation Polymers?

    Condensation polymers are macromolecules formed by step-growth polymerization reactions between monomers containing two or more reactive functional groups. Unlike addition polymerization, which involves the opening of multiple bonds without the loss of atoms, condensation polymerization involves the elimination of a small byproduct molecule—most commonly water (H2OH_2O) or hydrogen chloride (HClHCl).

    Key Characteristics:

    • Difunctional or Polyfunctional Monomers: Each monomer must possess at least two functional groups (e.g., −COOH-COOH, −NH2-NH_2, or −OH-OH) located at different positions on the molecule so that chain extension can proceed at both ends.
    • Step-Growth Mechanism: Chains grow by reactions between any two molecular species (monomers, dimers, oligomers, or longer chains) throughout the reaction mixture.
    • Heteroatoms in the Backbone: The resulting polymer backbone typically contains functional groups containing heteroatoms (such as oxygen or nitrogen), notably ester or amide linkages.

    Formation of Polyamides

    Polyamides are condensation polymers characterized by repeating amide linkages (−CO−NH−-CO-NH-). They are formed by the reaction between a dicarboxylic acid (or a diacyl dichloride) and a diamine, or through the self-condensation of amino acids.

    Example: Nylon-6,6

    Nylon-6,6 is synthesized by reacting hexanedioic acid (adipic acid, a six-carbon dicarboxylic acid) with hexane-1,6-diamine (hexamethylenediamine, a six-carbon diamine):

    n HOOC−(CH2)4−COOH+n H2N−(CH2)6−NH2ightarrow[−CO−(CH2)4−CO−NH−(CH2)6−NH−]n+(2n−1) H2On\,\text{HOOC}-(CH_2)_4-\text{COOH} + n\,\text{H}_2\text{N}-(CH_2)_6-\text{NH}_2 ightarrow \left[ -\text{CO}-(CH_2)_4-\text{CO}-\text{NH}-(CH_2)_6-\text{NH}- \right]_n + (2n-1)\,\text{H}_2\text{O}

    Key Properties and Uses:

    • Extensive hydrogen bonding between adjacent polymer chains confers high tensile strength and elasticity.
    • Widely used in textiles, climbing ropes, engineering plastics, and automotive parts.
    • Kevlar: Formed from benzene-1,4-dicarboxylic acid and benzene-1,4-diamine; rigid aromatic rings and aligned hydrogen bonding give it extraordinary strength-to-weight ratios used in bulletproof equipment.

    Formation of Polyesters

    Polyesters contain recurring ester linkages (−COO−-COO-) along the main polymer chain. They are formed through condensation reactions between a dicarboxylic acid (or diacyl chloride) and a diol, or from hydroxy-carboxylic acids containing both functional groups on the same molecule.

    Example: Polyethylene Terephthalate (PET / Terylene)

    PET is produced by the condensation of benzene-1,4-dicarboxylic acid (terephthalic acid) with ethane-1,2-diol (ethylene glycol):

    n HOOC−C6H4−COOH+n HO−CH2CH2−OHightarrow[−CO−C6H4−COO−CH2CH2−O−]n+(2n−1) H2On\,\text{HOOC}-\text{C}_6\text{H}_4-\text{COOH} + n\,\text{HO}-CH_2CH_2-\text{OH} ightarrow \left[ -\text{CO}-\text{C}_6\text{H}_4-\text{COO}-CH_2CH_2-\text{O}- \right]_n + (2n-1)\,\text{H}_2\text{O}

    Properties and Applications:

    • Strong dipole-dipole interactions and van der Waals attractions between chains yield high dimensional stability and chemical resistance.
    • Extensively used for beverage bottles, synthetic textile fibers (Dacron/Terylene), and food packaging films.
    • PLA (Polylactic acid): An aliphatic polyester synthesized from lactic acid (2-hydroxypropanoic acid) that is both renewable and biodegradable.

    Chemical Reactivity and Environmental Impact

    Because condensation polymers contain polar functional linkages in their main backbones, their chemical and physical behaviors differ sharply from addition polymers (such as polyethene or polystyrene).

    Hydrolysis:

    • Susceptibility: The ester and amide linkages in polyesters and polyamides can be cleaved by hydrolysis in the presence of strong aqueous acids or bases.
    • Acid Hydrolysis: Yields the original dicarboxylic acid and the protonated diamine or diol.
    • Alkaline Hydrolysis: Produces the dicarboxylate salt along with the free diamine or diol.

    Comparison with Addition Polymers:

    Unlike addition polymers, which feature chemically inert non-polar carbon-carbon backbones that persist indefinitely in landfills, many condensation polymers can be degraded under specific environmental or industrial composting conditions, making them important candidates for recycling and sustainable materials engineering.