انجمن شیمیدانان جوان

Polymer Modification

Why Polymer Modification?

• Turn one polymer into another!
• Fundamental premise: Reactivity is unaffected by polymeric state. Often true, but not always (especially for heterogeneous reactions).
• Yield is really conversion.
• If conversion < 100%, the product is a copolymer. You cannot separate and remove unreacted repeat units or attached by-products.
• Common themes: Make an insoluble polymer from a soluble one, or vice-versa.
• Often performed on natural polymers.

Modification of Natural Polymers

Vulcanization of Rubber

• Discovered in 1839 (!) by C. Goodyear.
• Very complicated mechanism, thought to be electrophillic rather than radical.
• Crosslinks via sulfur chains of various lengths. Sulfur alone gives long polysulfide bridges between polyisoprene chains, with z=45 to 50. Modern catalysts produce very short bridges of 1 or 2 sulfur atoms.
• Essential to the performance of rubber. Without crosslinking, natural rubber is sticky and not really elastic, like chewing gum.

Processing of Cellulose via Modification - Formation of xanthate derivative

• Typical average: 0.5 xanthates per repeat unit.
• Xanthate is soluble in water.
• Functionalized on C-2, C-3, and C-6.
• Can cast fibers into acidic water, which reverses reaction and reforms cellulose.
• Slight degradation in MW.
• Regenerated Cellulose, Cellophane, Viscose Rayon.

Cellulose --> Cellulose nitrate

• Typically 2 to 2.5 nitrate esters per repeat unit.
• Very flammable material.
• Used for early photographic and motion picture film.
• Still used for table tennis balls and wood varnishes.

Cellulose --> Cellulose triacetate

• Acetate content can be varied, but 3 per repeat unit is possible.
• Inexpensive, widely used material.
• Plastic sheet on which 35 mm film is coated.
• Cannot be melted without decomposition, but soluble in organic solvents. Can be cast from solution into sheets.
• Many other esters are available, including mixed esters (copolymers).

Modification of Synthetic Polymers

Unsaturated Polyester

• Common, very tough polymer.
• Known as “molding compound.”
• Especially valuable in composites with glass fiber.
• Used to make inexpensive, rugged chairs. Also for some automobile body panels.

Crosslinking of carboxylates

• Usually in copolymers (often with polyethylene) with low concentration of acid groups.
• Toughens polymer very much.
• Ionic bond is reversible at high temperatures; polymer can be molded. Ionic bond reforms as polymer cools.
• Popular in sporting goods, especially golf ball covers.

Poly(vinyl alcohol) --> Poly(vinylbutyral) ("Butvar")

• Make indirectly an “impossible” polymer, poly(vinyl alcohol) (PVOH).
  (Vinyl alcohol is not a stable molecule.)
• PVOH is water soluble. Useful as a thickener and to make "slime."
• Can make useful derivatives, such as polyacetal shown above.
• The polymeric acetal shown above is known as polyvinylbutyral, a tough polymer used to make automobile safety glass. Other side chain acetals are also commercially available.

Pyrolysis of Polyacrylonitrile

• Pyrolysis of acrylonitrile fibers in the absence of ocygen gives ring closure initially, then further condensations to produce graphitic fibers.
• Basis for cabon fibers used in aerospace and sporting goods.