Polymer Chemistry Free Radical Polymerization

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Polymer ChemistryFree Radical Polymerization

• Jihperng (Jim) Leu

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Chain Polymerization

• Free-radical polymerization
• What is free-radical
• Each addition reproduces the reactive group.
• most widely practised method of polymerization
• Three-Stage
• Initiation
• Propagation
• Termination

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Step Polymerization Chain Polymerization
Any two molecular species can react Growth occurs only by addition of monomer to active chain end.
Monomer disappears early Monomer is present throughout, but its concentration decreases.
Polymer MW rises throughout. Polymer begins to form immediately.
Growth of chains is usually slow (minutes to days). Chain growth is rapid (second to microseconds).
Long reaction times increase MW, but yield of polymer hardly changes. MW and yield depend on mechanism details. Usually long reaction times increase the polymer yield, but not the molar mass of polymer
All molecular species are present throughout. Only monomer and polymer are present during reaction.
Usually (but not always) polymer repeat unit has fewer atoms than had the monomer. Usually (but not always) polymer repeat unit has the same atoms as had the monomer
Activation energies are moderately high and reactions are not excessively exothermic High MW is formed immediately the reaction begins
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Initiation

• The formation of free radicals
• The addition of one of these free radicals to a
  molecule of monomer
• Paths to generate free radicals
• Homolysis of a single-bond
• Application of heat
• Peroxide (-O-O-) or azo (-NN-) linkage
• 50-100 C
• Examples
• Application of radiation (UV) - photoinitiators
• Single electron transfer to or from an ion or
  molecule (redox reaction)
• Widely used when a low-temperature polymerization
  is desired
• An active center is created when a free radical
  generated from an initiator attacks the pi-bond
  of a molecule
• Examples

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Initiation

1. Thermal initiator
2. Redox initiators
3. Photochemical
4. Ionizing radiation
5. Self-initiation

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Thermal Initiators

• Most common
• Unimolecular decomposition.
• First order kinetics.
• Most common examples peroxides or azo compounds.
• Peroxides azo compound

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Others

• Photochemical
• Thin Films
• Examples

• Ionizing Radiation (hv)
• X-ray, gamma-ray.
• Random destruction leads to radical formation.
• Used only in very special cases.

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Propagation

• The Steady State
• Initiation is relatively slow but continuous.
• Termination speeds up as active radical
  concentration builds.
• Termination removes (kills) active radicals.
• Results a steady-state concentration of radicals
  is established early in the reaction.
• The concentration of radicals is very small (ca.
  10-8 M) and nearly constant throughout.
• Propagation is Fast!
• Time needed to reach 10E6 in MW
• Styrene7.6 s
• Methyl methacrylate 1.5 s
• Vinyl chloride0.13 s

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• The following graph illustrates the effect for
  PMMA polymerization in benzene solution at
  various conecntrations. Note that at low
  concentreations (40 or more dilute in this
  example), the polymerization proceeds smoothly
  with no unusual effects. However, at higher
  concentrations, a pronounced rate acceleration
  after partial conversion of monomer to polymer.
  The more concentrated the solution, the earlier
  the acceleration occurs.

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• To explain the effect, recall that initiation,
  propagation, and termination are completely
  different chemical reactions with different
  responses to conditions. Termination involves the
  reaction between two chain ends. However, in
  concentrated solutions, the viscosity of the
  reaction mixture becomes high as polymer chains
  form. This high viscosity hinders the diffusion
  of chains because of entanglements, so the rate
  of termination slows considerably. However, the
  diffusion of small molecular monomers is hardly
  affected by viscosity, so propagation proceeds as
  before. In addition, initiator continues to add
  more free radicals to the system. The rates of
  initiation and propagation come out of balance.
  What was once a low, steady state concentration
  of radicals gives way to increasing
  concentration. Chains grow without termination,
  so the conversion is rapid and the MW is high.
• In dilute solutions, the viscosity never builds
  up to the point where the diffusion of chains is
  slowed, so autoacceleration does not occur.
• For neat monomer (i.e., 100 in the graph), often
  in cases where the polymer formed is a high Tg
  material, there can come a point at which even
  the diffusion of monomer is slow. The mixture has
  become a hard glass, and unreacted radicals
  become trapped inside. The reaction shuts down at
  less than 100 conversion, as depicted in the
  curve.

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Chain Transfer

• The Essence of Chain Transfer
• Chain termination occurs when two radical species
  (each odd-electron) react to form one or two new
  molecules without radical (even-electron). Chain
  transfer occurs when a radical species reacts
  with a nonradical species. The result must be at
  least one radical species. In the most common
  occurence, the chain end radical attacks a weak
  bond. An atom gets transferred to the chain end.
• After this happens, the current chain is
  terminated. A new chain may start or not,
  depending on reactivity of new radical.

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Chain Transfer agent

• In many cases, a chain transfer agent is added
  deliberately to the reaction mixture. Many
  compounds work well for this purpose, but
  mercaptans (also known as thiols) are the most
  general. Example for styrene butyl mercaptan
• The sulfur-centered radical reinitiates very
  efficiently. The result is a dimunition of the
  molecular weight without changing the overall
  rate of conversion of monomer to polymer. (Using
  more initiator is another way to decrease MW, but
  the reaction rate would increase proportionally,
  a possibly dangerous situation.)

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Chain Transfer

• Naturally, there are many even-electron species
  present in the reaction mixture (i.e., monomer,
  initiator, solvents, polymer chains, etc.), and
  all of these may participate in transfer
  reactions, depending on the relative reactivities
  of the structures involved. Here is an example of
  transfer to initiator featuring acrylonitrile and
  benzoyl peroxide (BPO)
• One chain is terminated, but another one
  initiates. This particular reaction reduces MW
  and wastes initiator (i.e., an initiator molecule
  is consumed, but no new chains are begun).
  Sometimes this process is called induced
  decomposition of the initiator. It is a common
  side reaction for the peroxy initiators, but
  happens less often with the azo initiators.

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Inhibition and Retardation

• Inhibitor
• Retarder
• Most commercial monomers are packaged with traces
  of inhibitor to prevent premature polymerization.
  The inhibitor can be removed prior to
  polymerization by distillation, chromatography,
  or extraction. In many cases, it is simply left
  alone, and additional initiator is used to
  overwhelm the inhibitor.
• Inhibitors are added in minute quantities to many
  other chemicals (e.g., ether, THF) to interrupt
  radical chain reactions that lead to
  decomposition. They are also used in foods to
  slow oxidation that leads to spoilage.

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The role of Oxygen

• Molecular oxygen presents peculiar behavior
  toward free radical polymerization.
• O2 is a very potent inhibitor for most common
  vinyl monomers. Polymerization media are usually
  thoroughly purged with inert gas to remove O2,
  otherwise the reaction may not work.
• Traces of O2 can initiate free radical vinyl
  polymerization (indirectly). O2 can react with
  some monomers or with trace impurities to form
  peroxy compounds that are thermal initiators.

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Termination

• Combination
• Two radicals at the chain termini simply join to
  form a single bond, as shown here in an example
  with styrene
• Disproportionation
• The radical at the end of one chain attacks a
  hydrogen atom at the second-to-last carbon atom
  in the second chain, as shown here in an example
  with methyl methacrylate (MMA)

Replaced with correct example
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Rate of Polymerization

• Degree of Polymerization

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Polymerization Process

• Bulk with monomer only
• High rate of polymerization and DP
• High MW and purity
• High viscosity and heat removal issue
• Solution in a solvent
• Lower the viscosity, better heat transfer, avoid
  autoacceleration
• DP (reduced conc. And chain transfer)
• Used in solution

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Polymerization Process (cont.)

• Suspension with monomer dispersed in an aqueous
  phase
• Better heat transfer
• Reaction mixtures suspended as droplets in an
  inert medium (agitation, dispersion stabilizers)
• High surface area in droplets (0.1-2 mm diameter
  as mini-reactors)
• PMMA, PVC and PS
• Water insoluble monomer.
• Water insoluble initiator.
• Suspending agent (optional).
• Emulsion
• Initiator must not soluble in monomers, but only
  in the aqueous dispersion medium (Water insoluble
  monomer Water soluble initiator)
• 0.05-1 um polymer particle
• Complicated mechanism.

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Comparison
Advantages Disadvantages
Suspension Simple, few ingredients, cheap. Reaction medium is mostly water, which absorbs the hear of polymerization. Produces beads that have technological uses (xerographic toner, catalyst carriers, ion exchange resins, substrates for combinatorial synthesis, etc.) Autoacceration will still occur. Isolation of the polymer can be laborious if you didn't want beads. May need to purify polymer from suspending agent.
Emulsion Make very high MW polymer quickly. Reaction medium is mostly water, which absorbs the heat of polymerization. Creates very tiny particles of polymer that have technological uses (paint, coatings, drug delivery, etc.). Isolation of the polymer can be laborious if you didn't want tiny particles. May need to purify polymer from surfactant.
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Important Vinyl Polymers prepared by free
radical polymerization

• LDPE
• PVC
• PS
• Polychloroprene (neoprene rubber)
• PMMA
• PVA
• Poly(vinylidene chloride)
• Polyacrylamide
• Polytetrafluoroethylene
• SBR
• ABS
• SAN
• SMA
• EVA (ethylene-vinyl acetate copolymer)
• Acrylonitrile-vinyl chlorodie copolymer