Chapter 21 Polymers: Macromolecules 21 Hein * Best * Pattison * Arena Version 1.0 Colleen Kelley Chemistry Department Pima Community College © John Wiley and Sons, Inc.

What is a “mer”? The terms polymer and monomer are part of our everyday speech. Poly = many Mono = one “Mer” is derived from the Greek meros, meaning “part.” So, a monomer is a “one part” and a polymer is a “many part.”

Macromolecules There exist in nature some very large molecules (macromolecules) that contain tens of thousands of atoms. Starch, glycogen, cellulose, proteins, silk, and DNA Synthetic macromolecules touch every phase of our lives.

Polyethylene Polyethylene is an example of a synthetic polymer. Ethylene, derived from petroleum, is made to react with itself to form polyethylene.

Polymerization polymerization - The process of forming very large, high-molar-mass molecules from smaller units. The large molecule, or unit, is called the polymer. The small unit is called the monomer. Ethylene is a monomer; polyethylene is a polymer Because of their large size, polymers are often called macromolecules or plastics.

Classification of Synthetic Polymers Rubberlike materials or elastomers Flexible films Synthetic textiles and fibers Resins (or plastics) for casting, molding, and extruding Coating resins for dip-, spray-, or solvent-dispersed applications Miscellaneous (e.g. hydraulic fluids, foamed insulation, ion-exchange resins)

Polymer Types

Polymer Types Addition Polymer Condensation Polymer Produced by successive addition reactions Polyethylene is an example of an addition polymer. Condensation Polymer Formed when monomers combine and split out water or some other simple substance. Essentially a substitution reaction Nylon is a condensation polymer.

Polymer Types Thermoplastic Polymers Soften on reheating

Thermosetting Polymer Set to an infusible solid and do not soften on reheating

Addition Polymerization

Addition Polymerization Addition polymerization starts with monomers that contain C=C bonds. When these bonds react, each alkene carbon bonds to another monomer and the double bond is lost every time.

Free radicals catalyze or initiate many addition polymerizations Free radicals catalyze or initiate many addition polymerizations. A free radical has one or more unpaired electrons and is VERY reactive. It is represented as something with a single ‘dot’, meaning an unpaired electron. Example, a hydroxyl free radical looks like this: Chlorine can break into two free radicals:

Organic peroxides (ROOR) are frequently used for either initiating or catalyzing a polymerization reaction. An organic free radical can be represented as RO.

The reaction proceeds in three steps: Free-radical formation Propagation of polymeric chain Termination

Step 1: Free-radical Formation The peroxide splits into free radicals: RO:OR  2 RO.

Step 2: Propagation of polymeric chain The initial free radical adds to ethylene to form a new free radical. The chain continues to elongate (polymerize) as long as free radicals continue to add to ethylene: RO. + CH2=CH2  ROCH2CH2. ROCH2CH2. + CH2=CH2  ROCH2CH2CH2CH2. ROCH2CH2CH2CH2. + CH2=CH2  RO(CH2CH2)n.

Step 3: Termination Polymerization stops when the free radicals are used up. This occurs when free radicals combine to form a stable compound: RO. + RO(CH2CH2)n.  RO(CH2CH2)nOR RO(CH2CH2)n. + RO(CH2CH2)n.  RO(CH2CH2)n (CH2CH2)nOR

Condensation polymerization Each will involve combining two separate molecules while losing something small (often water, or HCl or HBr, or something like that) Example:

Addition Polymers: Use and Reuse (Recycling)

Recycling Unfortunately, the C-C bonds of common addition polymers, like polyethylene, are different from those of most natural polymers and cannot be metabolized by many microorganisms. Recycling seems to be the best solution to the problem of disposing of these long-lived addition polymers.

Butadiene Polymers

Butadiene Polymers Natural rubber is a polymer of isoprene (2-methyl-1,3-butadiene).

In general, butadiene polymerization looks like this:

Copolymers A copolymer contains two different kinds of monomer units.

Vulcanized Rubber Vulcanization extends the useful temperature range of rubber products and imparts greater abrasion resistance to them. The vulcanization process is usually accomplished by heating raw rubber with sulfur and any other auxiliary agents.

Geometric Isomerism in Polymers

Geometric Isomerism in Polymers The recurring double bonds in isoprene and butadiene polymers make it possible to have polymers with specific spatial orientation as a result of cis-trans isomerism.

Geometric Isomerism in Polymers Natural rubber is cis-polyisoprene with all-cis configuration about the carbon-carbon double bonds.

Gutta-perch, also obtained from plants, is a trans-polyisoprene with an all-trans configuration

Chicle comes from the sapodilla tree and goes into making chewing gum. Ziegler –Natta catalysts (C2H5)3Al/TiCl4) are used to make “synthetic natural rubber”, which is all-cis. Without these catalysts, synthetic rubber is very inferior because it is mix of cis and trans.

Geometric Isomerism in Polymers