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ChemWiki: The Dynamic Chemistry E-textbook > Organic Chemistry > Polymers > Addition Polymers

Addition Polymers

Polymers are long chain giant organic molecules are assembled from many smaller molecules called monomers. Polymers consist of many repeating monomer units in long chains. A polymer is analogous to a necklace made from many small beads (monomers). Many monomers are alkenes or other molecules with double bonds which react by addition to their unsaturated double bonds.

Introduction

The electrons in the double bond are used to bond two monomer molecules together. This is represented by the red arrows moving from one molecule to the space between two molecules where a new bond is to form. The formation of polyethylene from ethylene (ethene) may be illustrated in the graphic on the left as follows. In the complete polymer, all of the double bonds have been turned into single bonds. No atoms have been lost and you can see that the monomers have just been joined in the process of addition. A simple representation is -[A-A-A-A-A]-. Polyethylene is used in plastic bags, bottles, toys, and electrical insulation.

  • LDPE - Low Density Polyethylene: The first commercial polyethylene process used peroxide catalysts at a temperature of 500 C and 1000 atmospheres of pressure. This yields a transparent polymer with highly branched chains which do not pack together well and is low in density. LDPE makes a flexible plastic. Today most LDPE is used for blow-molding of films for packaging and trash bags and flexible snap-on lids.  LDPE is recyclable plastic #4.  
  • HDPE - High Density Polyethylene: An alternate method is to use Ziegler-Natta aluminum titanium catalysts to make HDPE which has very little branching, allows the strands to pack closely, and thus is high density. It is three times stronger than LDPE and more opaque. About 45% of the HDPE is blow molded into milk and disposable consumer bottles. HDPE is also used for crinkly plastic bags to pack groceries at grocery stores.  HDPE is recyclable plastic #2.  

Other Addition Polymers

  • PVC (polyvinyl chloride), which is found in plastic wrap, simulated leather, water pipes, and garden hoses, is formed from vinyl chloride (H2C=CHCl). The reaction is shown in the graphic on the left. Notice how every other carbon must have a chlorine attached.
  • Polypropylene: The reaction to make polypropylene (H2C=CHCH3) is illustrated in the middle reaction of the graphic. Notice that the polymer bonds are always through the carbons of the double bond. Carbon #3 already has saturated bonds and cannot participate in any new bonds. A methyl group is on every other carbon.
  • Polystyrene: The reaction is the same for polystrene where every other carbon has a benzene ring attached.  Polystyrene (PS) is recyclable plastic #6.  In the following illustrated example, many styrene monomers are polymerized into a long chain polystyrene molecule.  The squiggly lines indicate that the polystyrene molecule extends further at both the left and right ends.  
  • Polystyrene_formation.PNG
  • Blowing fine gas bubbles into liquid polystyrene and letting it solidify produces expanded polystyrene, called Styrofoam by the Dow Chemical Company.  
  • Polystyrene with DVB: Cross-linking between polymer chains can be introduced into polystyrene by copolymerizing with p-divinylbenzene (DVB).  DVB has vinyl groups (-CH=CH2) at each end of its molecule, each of which can be polymerized into a polymer chain like any other vinyl group on a styrene monomer.  

Other addition polymers

Table 1: Links to various polymers with Chime molecule - Macrogalleria at U. Southern Mississippi

Monomer Polymer Name Trade Name Uses
F2C=CF2 polytetrafluoroethylene Teflon

Non-stick coating for cooking utensils, chemically-resistant specialty plastic parts, Gore-Tex

H2C=CCl2 polyvinylidene dichloride Saran Clinging food wrap
H2C=CH(CN) polyacrylonitrile Orlon, Acrilan, Creslan Fibers for textiles, carpets, upholstery
H2C=CH(OCOCH3) polyvinyl acetate   Elmer's glue - Silly Putty Demo
H2C=CH(OH) polyvinyl alcohol   Ghostbusters Demo
H2C=C(CH3)COOCH3 polymethyl methacrylate Plexiglass, Lucite Stiff, clear, plastic sheets, blocks, tubing, and other shapes

Addition polymers from conjugated dienes

Polymers from conjugated dienes usually give elastomer polymers having rubber-like properties.  

 

Table 2. Addition homopolymers from conjugated dienes
Monomer Polymer name Trade name Uses
H2C=CH-C(CH3)=CH2 polyisoprene natural or some synthetic rubber applications similar to natural rubber
H2C=CH-CH=CH2 polybutadiene polybutadiene synthetic rubber select synthetic rubber applications
H2C=CH-CCl=CH2 polychloroprene Neoprene chemically-resistant rubber

Ring opening polymerization

In this kind of polymerization, molecular rings are opened in the formation of a polymer.  Here epsilon-caprolactam, a 6-carbon cyclic monomer, undergoes ring opening to form a Nylon 6 homopolymer, which is somewhat similar to but not the same as Nylon 6,6 alternating copolymer.  

Nylon 6 formation.png

Demonstrations with Polymers

  • Silly Putty - polyvinyl acetate
  • Ghostbusters - polyvinyl alcohol
  • Ghosbusters Slime - Polyguar gum
  • Peanut Race - polystyrene
  • Disappearing Cup - polystyrene
  • Solid Water - sodium polyacrylate

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Last Modified
13:07, 26 Dec 2013

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