Principles of polymer chemistry

Principles of polymer chemistry - theory of polymerization

Classification of monomers and polyreactions

• Monomers with more than two reactive functions → branched or crosslinked polymers.
• Polymerization or polyreaction: the former consists of polyreactions and includes radical operation; classification:
  • A polyreaction must be:
    • Stoichiometric: The product of a step must be the molecules increased in molecular weight (with respect to the previous step).
    • Reversible: Reaction products do a former act because the reagent is stable.
  • Polyaddition: A + B → A - B → ab
  • Polycondensation: A + B → A - B + release
  • Ionic mechanism: Heterolytic breaking of the bond, electronic pairs move to the more electronegative atom. The reactivity will depend strongly on the degree of charge separation (fundamental role of catalyst and solvent).
  • Radical mechanism: Homolytic breaking of the bond with the formation of organics with high energy, impaired electrons. High reactivity (no role of catalyst nor solvent).
• Classification of monomers:
  1. Monomers with electronically independent reactive functions (bifunctional): Examples include acids, diamines, diacids. They generate individual reactions, with the formation of stable intermediates. Kinetic mechanism: Step-growth type. Every reaction step has a high-energy barrier. Slow process. Each species formed is able to react with another. The base chemical mechanism is monomolecular. The base reaction can be condensation. Examples: polycondensation → polyesters, polyamides, polycarbonates. Polyaddition → polyurethanes.
  2. Monomers with electronically interdependent functions (polyfunctional): Examples include unstructured or cyclic monomers (carbon-carbon group, epoxide). They generate interconnected reaction sites through the precipitation of activated intermediates. The only energy barrier is the initiation reaction: once activation is overcome, the process proceeds fast (kinetic mechanism: inhibition is minor). Chain reactions take place through the precipitation of an active center (with few exceptions) of reaction with the monomer M only: Chain reaction: I + M → I M* → I M* + M → I M₂* → I M_n* The base chemical mechanism may be both radical or ionic; the base reaction is addition. Examples: polyethylene, polypropylene, polystyrene, polyvinylchloride, polymethylmethacrylate.
• It is necessary to select monomers with appropriate chemical reactivities, provide them in appropriate stoichiometric ratios, and control thermodynamic aspects.
• The process of condensation: Thermally comparable strength links break and form. Mildly exothermic condensation process and low reversible probabilities, but kinetic of step-growth process (slow) is high. Thus, the reaction is commenced close to them, the reaction pathway modifies a good duct constantly until completed.
• Polyaddition: Disappearance of reagent to form a more stable bond, strongly exothermic condensation, rapid, favored to low T. Problems with high T because the process is also exothermic. There is a threshold value (T in case gt).

Classification of monomers and polyreactions - continued

• Monomer with more than two reactive functions - branched or crosslinked polymer.
• Polymerization or polyreaction involves an element of simplification.
  • A polyreaction must be:
    • Stoichiometric: The product of a step must be the molecule increased in molecular weight with respect to the previous step.
    • Efficiency: The main product of a former act becomes the reagent for the later.
  • Polycondensation: A_a + B_n → A-B + A-B_n-1 etc.
  • Polyaddition: A + B → A-B
  • Ionic mechanism: Heterolytic breaking of the bond, electronic pair moves to the more electronegative atom. The reactivity will depend strongly on the degree of charge separation (fundamental role of catalyst and solvent).
  • Radical mechanism: Homolytic breaking of the bond with the formation of species with high energy, impaired electron. High reactivity (no role of catalyst nor solvent).

Classification of monomers - continued

1. Monomers with electronically independent reactive functions (expect functions). Examples include diols, diamine, diacids. They generate individual reaction acts, with the formation of stable intermediates. Kinetic mechanism: Step-growth type. Every reaction step has a high energy barrier: slow process. Each species formed is able to react with any other. The base chemical mechanism is monomolecular. The basic reaction can be condensation. Examples: polycondensation → polyesters, polyamides, polycarbonates. Polyaddition → polyurethanes.