Principles of polymer chemistry

Principles of Polymer Chemistry - Theory of Polymerization

1. 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: acids, diamines, diacidides. 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: 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’s necessary to select monomers with appropriate chemical reactivities, provide them in appropriate stoichiometric ratios, 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)

PRINCIPLES OF POLYMER CHEMISTRY - THEORY OF POLYMERIZATION

CLASSIFICATION OF MONOMERS AND POLYREACTIONS

• Monomer with more than two reactive functions - branched or crosslinked polymer.
• Polymerization or polyreaction, the former consists of polyeactions and 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
• Ionix 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 (fundamentals 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
  1. Monomers with electronically independent reactive functions (expect functions). Examples: diols, diamine, diacids. They generate individual reaction acts, with the formation of stable intermediates. Kinetic mechanism: step-growth type. Every reaction step have a high energy barrier: slow process. Each species formed able to react with any other.
     • N-H + N= → N₂ + N₂ = N₃ 1ѓN₂ N₂ N=N + N₂ etc.
     The base chemical mechanism is monomolecular. The basic reaction can be condensation. Examples: polycondensation → polyesters, polyamides, polycarbonates.
  2. Polyaddition → polyurethanes
     • Monomers with electronically interdependent f