CY 1050: Macromolecules as Engineering Materials

• The course is CY 1050: Macromolecules as Engineering Materials.
• The instructor is Dr. R.L. Gardas, located in office CB 214 (First Floor).
• The instructor can be contacted via email at [email protected] or by phone at 4248.
• The course is worth 9 credits and is in the A slot.
• Class times are Monday at 8:00 am, Tuesday at 1:00 pm, and Thursday at 11:00 am.
• Assessments include Quiz I & II and an End-Semester exam.
• The course covers concepts (6-7 hours), synthesis, thermodynamics, and kinetics (12-14 hours), characterization (9-10 hours), and applications (9-10 hours).
• Key topics include definitions, nomenclature, polymer classification, types of polymerizations (chain growth, step growth, living), molecular weights, and distribution.
• It an elementary tour of physical methods for determining molecular weights and distribution is provided.
• Synthesis of macromolecules includes thermodynamics and kinetics of chain polymerization related to industrial polymers: polyethylene, polypropylene, polystyrene, poly(vinyl chloride).
• Step polymerization thermodynamics and kinetics are covered with reference to specialty polymers like PET, Nylon, PC, and PU.
• Step growth polymerizations involving crosslinking (gelation) or insoluble polymer mass formation are discussed.
• Determination of polymer structure is facilitated via IR and NMR spectroscopies.
• Characterization covers polymer structure in the solid state like characteristics of amorphous and semicrystalline polymers
• Viscoelasticity, glass transition temperature, and rubber elasticity are also covered.
• Applications include engineering and specialty polymers and high-performance fibres such as Kevlar.
• Course covers applications of composite materials (BMC and SMC) and conducting plastics and also polymers for separation science, biomedical devices, electronics, and photonics.
• The course focuses on the chemistry of nonbiological polymers like the synthetic materials used for plastics, fibres, and elastomers.
• A few naturally occurring polymers, such as rubber, wool, and cellulose, are also included.
• A polymer is a large molecule made of repeating small chemical units held together by chemical bonds.
• Repetition can be linear (chain) or branched/interconnected (3D networks).
• The repeat unit of a polymer is nearly equivalent to the monomer (starting material).
• The repeat unit for poly(vinyl chloride) is -CH2CHCl-.
• Its monomer is vinyl chloride, CH2=CHCl.
• Repeating units constituting the polymer molecule are called constitutional repeat units (or CRU).
• A monomer needs at least bifunctionality, which will arise from one double bond or two reactive functional groups, to undergo polymerisation.
• Two major polymerisation methods convert small molecules (monomers) into polymers: Addition polymerisation and Condensation polymerisation
• Unsaturated monomers usually follow Addition polymerisation
• Those containing functional groups undergo Polycondensation
• Polyethylene and polypropylene are addition polymers.
• Nylons and terylene are condensation polymers.
• Each carbon has three sp² hybridised orbitals
  • These form three sigma (σ) bonds.
  • The remaining p orbital forms the pi (π) bond.
• The double bond between 2 is made of one sigma bond and one pi bond.
  • C=C bond length is 1.34 Å.
  • Dissociation energy is 611 kJ/mol.

Addition versus Polycondensation

• Addition polymerisation (Chain growth)
  • Monomers are unsaturated.
  • Involves opening the double bond by active species (free radical or ion).
  • Nothing is eliminated as a result of this type of polymerisation.
  • Polymer molecular weight is equal to DP x molecular weight of monomer.
  • High molecular weight in polymer is attained at once.
  • Only monomers and polymers are present during the course of polymerisation.
  • Examples include polyolefins, polydienes, vinylpolymers and acrylic polymers.
  • Can be done by bulk, solution, suspension, and emulsion polymerisation techniques.
  • Can quickly lead to a polymer with very high molecular weight.
• Polycondensation (Step-growth polymerisation)
  • Monomers contain two or more functional groups.
  • Involves reaction between functional groups.
  • Usually, small molecules like H2O, CH3OH, HCl, etc., are eliminated.
  • High molecular weight is attained only at very high conversions.
  • All possible molecular weight species (dimers, trimers, tetramers and multimers etc.) are present.
  • Examples include polyesters, polyamides, and polycarbonates.
  • Process can be achieved in melt, solution, as well as at interfacial boundary between two liquids.
  • Slow stepwise addition process

Additional Info on Polymerisation

• In chain polymerisation after the initiation occurs, the polymer chains form quickly i.e. in the time scale of 10⁻¹ to 10⁻⁶ s.
• The catalyst concentration needed is very low so, during the course of polymerisation, only monomers and polymers are typically present.
• The process is usually exothermic.
• High polymers with molecular weights of 10,000 to over 10 million can be obtained.
• Monomers need to have a minimum bifunctionality to get polymerised.
• Two main types of polymerisation methods: Addition polymerisation and Condensation polymerisation.
• C=C (alkene/ vinyl) and C=O (aldehydes and ketones) monomers undergo chain polymerization.
• Initiators used in the process are typically Free radical / cation / anion

Mechanism of Polymerisation (Addition Polymerisation)

• If monomers contain at least a double bond, the polymerisation has three steps:
  • Initiation
  • Propagation
  • Termination
• The initiation step involves a reaction of active species, which is generated from an initiator (or catalyst by its decomposition).
  • The active species generated from an initiator may be a free radical, a cation, an anion or a coordination complex.
  • These species on reaction with the monomer form new free radicals such as a carbocation, a carbanion or a coordination complex.
• Propagation: The active species adds to another monomer like the initiation step, repeating over and over until the process ends (termination).
• Termination- the growing chain reacts with other growing chains or decomposes at the active site.
  • Termination types: Combination and Disproportionation.
• Initiators for chain polymerisation:
  • Free radical chain polymerisation
  • Cationic chain polymerisation
  • Anionic chain polymerisation

Initiators commonly used for generating active species include

• Free radical initiators:
  • Peroxides e.g. benzoylperoxide BPO
  • Azo compounds e.g. azobisisobutyronitrile, AIBN)
• Redox systems e.g. persulphate + bisulphite
  • Light or ionising radiation
• Cationic Initiators:
  • Proton or Lewis acids, carbocations, oxonium ions, high energy radiations
• Anionic
  • Lewis bases, organic alkalis e.g. sodium + ammonia, butyl lithium and naphthalene + sodium
• Coordination
  • Transition metal complexes
• A particular monomer can be converted to polymer based on thermodynamic and kinetic considerations.
• Polymerization only occurs if the free-energy difference (ΔG) between monomer and polymer is negative: ΔG = ΔH – TΔS.
• Thermodynamically feasible polymerization is possible if the process proceeds at a reasonable rate under a proposed set of reaction conditions (type of initiation, temperature, etc.).
• While the polymerization of a wide variety of unsaturated monomers is thermodynamically feasible, very specific reaction conditions are required to achieve kinetic feasibility.
• Kinetics is the speed of a reaction under a given set of conditions
  • These will influence the polymer molecular weight, molecular weight distribution, chemical composition, and extent of polymer conversion.
• The carbon-carbon double bond in alkene monomers and the carbon-oxygen double bond in aldehydes and ketones are the two main types of linkages that undergo chain polymerization.

Kinetics of Chain Polymerisation

• The polymerisation of the carbon-carbon double bond is far more important.
• Carbonyl groups are not prone to polymerization by free radical initiators due to polarisation
• Aldehydes and ketones are polymerized by both anionic and cationic initiators
• Unlike the carbonyl linkage,the carbon-carbon double bonds can undergo polymerisation by both radical and ionic initiators.
• The difference arises because the p-bond can respond to the initiator species by homolytic or heterolytic bond breakage:
• Whether a vinyl (-CH=CH2) monomer polymerizes by radical, anionic, or cationic initiators depends on the inductive and resonance characteristics of the substituent(s) present.
• Chain Polymerisation has three key steps: Initiation, Propagation, and Termination.
• The rate of decomposition of an initiator increases with the temperature and concentration of the initiator.

Free Radical Chain Polymerisation

• To initiate the reaction, free-radicals are generated by the homolytic decomposition of an initiator.
• I is initiator, R• is free-radicals formed, and kd is the initiator decomposition rate constant.
• The value of f is usually between 0.6 and 1.0.
• Initiation rate is proportional to [M]• (the concentration of reactive free-radical sites) as well as [M] which is the the concentration of the monomer
• Rate of polymer formation is proportional to the monomer concentration and also to the square-root of the initiator concentration.
• Rate of polymer formation is proportional to the monomer concentration and the square-root of initiator concentration.
• The kinetic chain length (v) is the average number of monomer molecules consumed by each effective free-radical generated by the initiator.
• As the kinetic chain length gives the average number of monomer molecules present in a growing chain at the time of termination, the average degree of polymerisation, D subscript p, can be correlated as follows: D subscript p= 2v, when termination occurs by coupling;
• D* subscript p= v, when disproportionation occurs.

Chain Transfer

• Termination of growing polymer chain can take place through chain transfer.
• M˚ + RH yields MH+R˚
• ktr is the chain transfer rate constant and RH is the chain transfer agent
  • Many initiators (not all) can encourage the chain transfer reaction.
  • Chain transfer reactions are of greater significance with the solvents.
• Chain transfer constant (C): describes the ability of solvent/substance to act as chain transfer agent.
• C=ktr/ Kp. The last term in Eqn. governs termination by coupling and/or by disproportionation.
• Cs, CI, and CM are the chain transfer constants for the solvent, the initiator and the monomer, respectively.
• The Mayo Equation describes chain transfer:
  • 1/DP=1/DP₀+Ctr[transfer agent]/[monomer]
• DP₀ the DP in the absence of chain transfer agent. -DP is the same in the presence of chain transfer agent.

Polymer Architectures

• Homopolymers: Polymers with same repeating unit (A-A-A-A-A-)
• Copolymers: Polymers with two or more repeating units
  • Alternating: A-B-A-B-A-B-
• Random: -A-B-B-A-A-A-B-A-
• Block (two block):-A-A-A-A-B-B-B-B-
• Graft: A main chain with branches of another type (A-A-A-A with B branches)
• **Terpolymers:**Three or more repeating units arranged in various patterns.

Copolymerisation and Kinetics

• Copolymerisation: The process of polymerising two or more monomers together.
• Copolymers: Addition polymers that mixes well possessing similar properties with homopolymers.
• Instead of searching new monomers polymerisation yields desired properties often achieved by using well known inexpensive monomers like styrene, ethylene, propylene, butadiene, vinyl chloride, and acrylic monomers.
• SBR and NBR copolymers (butadiene with styrene/acrylonitrile) are commercially important elastomers and possessing superior properties to polybutadiene rubber.
• Block copolymers of styrene and butadiene are excellent thermoplastic elastomers.