Polymer Chemistry Notes

Polymers

  • The term "polymer" originates from the Greek words "poly" (meaning many) and "meros" (meaning parts).
  • Polymers are large molecules consisting of repeating structural units (monomers) linked by covalent bonds and having two or more binding sites.
  • Monomers are the repeating units that make up a polymer.
  • Example: Polythene is formed from multiple ethene (ethylene) molecules.

Degree of Polymerization (DP)

  • DP represents the number of repeating units (n) in a polymer chain.
  • High polymers have a large number of repeating units, while oligomers have a lower number.
  • Molecular weight of a polymer can be determined by multiplying the number of repeating units (n) by the molecular weight of the repeating unit.

Functionality

  • Functionality refers to the total number of functional groups, bonding sites, or reactive sites in a monomer.
  • Reactive functional groups include -OH, -COOH, -NH2, -SH, -NCO, etc.
  • Example:
    • CH3CH2OH has one -OH group (monofunctional).
    • HO-CH2-CH2-OH has two -OH groups (bifunctional).
    • HOOC-CH2-CH(COOH)-CH2-COOH has three -COOH groups (trifunctional).
  • Double or triple bonds in a molecule impart polyfunctionality.
  • Example: Ethylene can take on two atoms of hydrogen or halogens due to its double bond.
  • The functionality of monomers determines whether linear, branched, or three-dimensional cross-linked polymers are formed.
  • Polymerisation example:
    n(C2H4) \rightarrow (C2H4)_n
    Ethene to polythene.

Classification of Polymers

Polymers can be classified based on:

  • Origin
  • Structure
  • Methods of formation
  • Response to heat and crystallinity
  • Properties (or applications)

Based on Origin

  • Natural polymers: Obtained naturally (e.g., cellulose, silk, starch).
  • Synthetic polymers: Man-made (e.g., polythene, PVC, polyester).
  • Semi-synthetic polymers: Chemically modified natural polymers (e.g., cellulose acetate, cellulose nitrate, halogenated rubbers).

Based on Molecular Structure

  • Linear: Monomeric units combine linearly.
    • Homopolymer: Identical monomeric units (e.g., -M-M-M-M-M-M-M-…).
    • Copolymer: Non-identical monomeric units (e.g., -M-M1-M-M1-M-M1-M-…).
    • Block copolymer: Long continuous sequences of each type of unit (e.g., …-M-M-M-M-M1-M1-M1-M1-M-M-M-M-…).
  • Branched: Linear polymer with branches.
  • Cross-linked: Polymers connected in three dimensions.

Based on Method of Formation

  • Addition polymers: Formed by self-addition of monomers without byproduct elimination (e.g., polyethylene, synthetic rubbers).
  • Condensation polymers: Formed by reaction between monomers with elimination of small molecules (e.g., water, ammonia, HCl) (e.g., urea-formaldehyde resins, phenol-formaldehyde resins, polyesters).

Based on Response to Heat

  • Thermo softening (Thermoplastic): Soften on heating, can be reshaped, and retain shape on cooling (e.g., polyethylene, nylons, sealing wax). The process can be repeated multiple times.
  • Thermosetting: Undergo chemical change on heating, becoming an infusible mass (e.g., bakelite, egg yolk).

Based on Application and Properties

  • Plastics: Soft enough to be molded at some temperature and harden on cooling (e.g., polystyrene, polyvinyl chloride, poly methyl methacrylate).
  • Elastomers: Polymers with zig-zag or helical chains that undergo elastic changes under force and regain original shape (e.g., natural rubber, silicone rubbers).
  • Fibers: Molecular chains arranged parallel in a spiral or helical pattern, resistant to stretching or deformation, with length at least 100 times its diameter (e.g., nylons, terylene).
  • Resins: Lower molecular weight polymers in liquid or solid form, used as adhesives or moulding powders, with a glossy appearance; a major component of plastics (e.g., Polysulphide sealants, epoxy adhesives).

Polymerization

  • Polymerization is the process of linking monomer molecules to form a large polymer molecule.

Types of Polymerization

  • Addition (chain growth) polymerization: Self-addition of olefinic monomers without byproduct elimination.
  • Condensation (step growth) polymerization: Monomers with two or more reactive functional groups (hydroxyl, carboxyl, amino) condense with each other.

Distinguishing Features

  • Addition Polymerization:
    • Monomers undergo self-addition without loss of byproducts.
    • Follows a free radical mechanism (chain mechanism).
    • Unsaturated vinyl compounds undergo addition polymerization.
    • Monomers are linked through C–C covalent linkages.
    • High polymers are formed quickly.
    • Linear polymers are produced with or without branching.
    • Examples: polystyrene, plexiglass, PVC.
  • Condensation Polymerization:
    • Monomers undergo intermolecular condensation with continuous elimination of byproducts (H2O, NH3, HCl, etc.).
    • Follows a step mechanism.
    • Monomers containing functional groups (-OH, -COOH, -NH2) undergo this polymerization.
    • Covalent linkages are through their functional groups.
    • The reaction is slow, and polymer molecular weight increases steadily.
    • Linear or cross-linked polymers are produced.
    • Examples: nylons, terylene, PF resins.

Stereo Regular Polymers (Tacticity)

  • Classification based on the position of substituent groups in the polymer chain.
    • Isotactic: All substituent groups are on the same side of the polymer chain.
    • Syndiotactic: Substituent groups alternate regularly along the chain.
    • Atactic: Substituent groups are randomly arranged along the chain.

Structure and Properties of Polymers

  • Polymer structure influences properties like crystallinity, tensile strength, elasticity, chemical resistance, and plasticity.

Strength

  • Forces of Attraction:
    • Strength is determined by the magnitude and distribution of attractive forces between polymer chains.
    • Primary (covalent) and secondary (intermolecular) forces.
    • Straight and branched-chain polymers have weak intermolecular forces.
    • Strength increases with chain length (molecular weight); mechanical strength is attained if the chain length is greater than 150-200 carbon atoms.
    • Polar groups (carbonyl, hydroxyl) increase intermolecular forces.
    • Cross-linked polymers have greater strength due to covalent forces.
  • Slipping Power:
    • Slipping power is the movement of molecules over each other.
    • Polyethylene has high slipping power due to its simple, uniform structure, resulting in lower strength.
    • PVC has bulky chlorine atoms that restrict movement, resulting in higher strength compared to polyethylene.
    • Cross-linked polymers have restricted movement due to covalent bonds, making them strong, rigid, and tough.

Plastic Deformation

  • Permanent deformation in shape under stress (heat, pressure).
  • Slippage is more significant in linear molecules due to weak intermolecular forces.
  • Van der Waals forces weaken at high pressure and temperature.
  • No slippage in cross-linked polymers due to strong covalent bonds; However, excessive force or temperature can cause destruction.

Crystallinity

  • Amorphous state: Random arrangement of molecules.
  • Crystalline state: Regular arrangement of molecules.
  • Crystallization depends on the ease of aligning chains in an orderly arrangement.
  • Crystalline regions are formed by linear chains without branching or bulky substituents, closely arranged parallel to each other.
  • Chains are held together by van der Waals forces, hydrogen bonding, or polar interactions.
  • High crystallinity results in high tensile strength, impact and wear resistance, high density, and high fusion temperature.
  • Polymers with long repeating units or low symmetry do not crystallize easily (e.g., polystyrene).
  • Crystallization leads to denser packing and increased intermolecular forces, resulting in a sharp softening point, greater strength, and rigidity (e.g., PVC, Polypropylene).
  • Polymers are generally amorphous with some degree of crystallinity.

Chemical Resistance

  • Depends on the chemical nature of monomers and their molecular arrangement.
  • Polymers are more soluble in structurally similar solvents.
  • Polar polymers (containing -OH, -COOH) dissolve in polar solvents (water, alcohol) but are chemically resistant to non-polar solvents.
  • Non-polar compounds (hydrocarbons) dissolve in non-polar solvents (benzene, toluene).
  • Solubility decreases with increasing molecular weight.
    • High molecular weight polymers yield high viscosity solutions.
    • Crystalline polymers exhibit higher resistance than less crystalline ones.
    • Greater crystallinity results in lesser solubility.
  • Chemical resistance of plastic bottles is important to prevent drug-polymer interactions when storing drugs and essential oils.

Elasticity

  • Results from the uncoiling and recoiling of molecular chains under force.
  • Unstretched elastomers have irregularly coiled and entangled snarls (amorphous state).
  • Stretched state: snarls disentangle and straighten out (crystalline state).
  • Elasticity requires chains not to break even after prolonged stretching, achieved by crosslinking or introducing nonpolar or side groups.

Glass Transition Temperature (Tg)

  • Amorphous polymers do not have sharp melting points but possess a softening point.
  • Tg is the temperature below which an amorphous polymer is brittle, hard, and glassy, and above which it becomes flexible, soft, and rubbery.
    Glassy \ state \rightarrow Rubber \ state
    (Hard brittle plastic) (soft flexible)
  • In the glassy state, neither molecular motion nor segmental motion is possible.
  • Beyond Tg, segmental motion becomes possible, but molecular mobility is disallowed.

Factors Affecting Tg

  • Chain geometry: Regular chain geometry increases Tg.
  • Chain flexibility: Bulky groups on the chain increase Tg (e.g., polyethylene has a low Tg of -110 °C due to no strong intermolecular forces or bulky side groups).
  • Molecular aggregates: Polar groups leading to strong intermolecular hydrogen bonding increase Tg (e.g., nylon 6 has a Tg of 50 °C).
  • Molecular weight: Significantly affects Tg only if the molecular weight is not around 20000. With increased molecular mass, Tg will be higher.
  • Crystallinity: Crystalline polymers have higher Tg than amorphous polymers due to strong forces like H-bonding.
  • Plasticizers: Reduce Tg by reducing cohesive forces (e.g., dibutyl phthalate, diacetyl phthalate).
  • Tg helps in choosing the right processing temperature and is a measure of a polymer's flexibility, thermal expansion, heat capacity, and electrical and mechanical properties.

Molecular Weight of Polymers

  • Polymers consist of molecules of varying molecular weights, so an average value is used.

  • Ethylene gas has a fixed molecular weight of 28, but polyethylene has an indefinite structure --(-CH2–CH2- )n—where 'n' varies.

  • Chain termination is a random process, leading to different numbers of monomer units and molecular weights.

  • Polymer molecular weight is viewed statistically and expressed as an average.

  • Two main methods of averaging:

    • Number-average molecular weight.
    • Weight-average molecular weight.

Number-Average Molecular Weight

  • Total mass of all molecules in a polymer sample divided by the total number of molecules present.

Weight-Average Molecular Weight

  • Sum of the fractional masses that each molecule contributes to the average according to the ratio of its mass to that of the whole sample.
  • Examples: White foam cups, clear plastic cups, and ultrathin fishing lines are made of polystyrene with different average molar masses.
    • Styrofoam cups: polystyrene with an average mass of approximately 15000 g/mol.
    • Clear plastic cups: polystyrene with an average mass of approximately 250000 g/mol.
    • Ultrathin fishing lines: polystyrene with an average mass of approximately 1000000 g/mol.

Application of Average Molecular Mass (AMM)

  • Used to characterize a polymer.
  • Samples prepared under different conditions may have different AMMs.
  • Affects mechanical, solution, and melt properties.
  • Chain length is related to the ease of processing.
  • Longer chains make the melt more difficult to process due to increased entanglement.
  • Polymers must be able to flow for flattening into sheets or moulding into bottles.

Numerical Problems

  1. Calculate the number average & weight average molecular weight of a polymer sample in which 40 % molecules have molecular mass of 25000, 20 % have molecular mass of 30000 & rest have molecular mass of 55000.
  2. A polymer sample contains 1, 2, 3 & 4 molecules having molecular weight 10^5, 2 \times 10^5, 3 \times 10^5 and 4 \times 10^5 respectively. Calculate the number average & weight average molecular weight of the polymer.