Macromolecules

What are macromolecules?

Macromolecules are large molecules build by joining together smaller subunits, “monomers”.

• Synthetic:

  • Polymers

• Biomolecules:

  • Carbohydrates

  • Lipids

  • Proteins

What are synthetic polymers?

There are many different types of synthetic polymer. They are often classified by their properties or use.

• Plastics are large, rigid materials at service temperatures. Plastics are further subdivided into thermoplastics and thermosets.

• Rubbers are flexible, low modulus materials which exhibit long-range elasticity.

What are some of the variables that impact the properties of polymers?

• Choice of monomer

• Molecular weight and molecular weight distribution

• Crystallinity

• Melting point / glass transition temperature

• Branching

• Number and type of crosslinks

What is addition polymerisation?

• Many monomers unit “add” together

• Polymer has same empirical formula as monomer

What is condensation polymerisation?

• Monomers react together by condensation

• Loss of water as small molecule

What are the thermodynamics of polymerisation?

\Delta G^\circ = \Delta H ^\circ -T\Delta S^\circ

• \Delta H^\circ is large and -ve (> -70kJ mol^-1) \sigma bonds stronger than \pi bonds

• \Delta S^\circ is large and -ve (> -100J K^-1 mol^-1) many molecules become one polymer

→ At low T, \Delta G^\circ -ve = polymerisation

→ At high T, \Delta G^\circ +ve = cracking at > 427C^\circ

What is radical polymerisation?

• Initiator energy produces reactive radical species

• Polymer chain continues to grow until:

  • Two radicals combine

  • Transfer of hydrogen radical

End groups have little effect on the properties of polymer formed.

What is cationic polymerisation?

• Requires initial cation

• Produces stable carbocation

What is anionic polymerisation?

• Requires nucleophile

• Nucleophile attacks electrophile

What are the physical properties of polyalkenes?

• R groups on every second carbon

• Large or polar R groups = rigid polymer

• Chain length depends on conditions

• Range of chain lengths always made

What is the effect of temperature on chain length?

T_g is the temperature at which a glassy brittle polymer becomes flexible

T_m is the temperature at which a thermoplastic begins to flow

Between T_g and T_m a polymer is flexible

What are modified polymers?

• Expanded polymers

• Plasticisers

• Copolymers

• Cross-linked polymers

• Side-chain modifications