Transcript Chemdraw B&W - Pennsylvania State University

Chapter 31. Synthetic Polymers
Based on McMurry’s Organic Chemistry, 6th edition
Polymers
• Large molecules built up by repetitive bonding
together of monomers
Drawing Polymers
• Indicate repeating unit in parentheses
31.1 Chain-Growth Polymers
• Produced by chain-reaction polymerization
• Initiator (radical, acid or anion) adds to a carbon–
carbon double bond of an unsaturated substrate (a
vinyl monomer) to yield a reactive intermediate that
reacts with a second molecule of monomer and so
on
Anionic Polymerization
• Vinyl monomers with electron-withdrawing
substituents (EWG) can be polymerized by anionic
catalysts
• Chain-carrying step is nucleophilic addition of an
anion to the unsaturated monomer by a Michael
reaction
Examples of Anionic Polymerization
Products
• Acrylonitrile (H2C=CHCN), methyl methacrylate
[H2C=C(CH3)CO2CH3], and styrene (H2C=CHC6H5) react
31.2 Stereochemistry of Polymerization:
Ziegler–Natta Catalysts
• Polymerization of a substituted vinyl monomer can
lead to numerous chirality centers on the chain
• A polymer having all methyl groups on the same side
of the zigzag backbone is called isotactic
• If the methyl groups alternate on opposite sides of the
backbone, it is called syndiotactic
• Randomly oriented methyl groups are on atactic
polymers
Ziegler–Natta Catalysts
• Allow preparation of isotactic, syndiotactic, and atactic
polypropylene
• Prepared by treatment of an alkylaluminum with a
titanium compound
• (CH3CH2)3Al + TiCl4  A Ziegler–Natta catalyst
31.3 Copolymers
• Obtained when two or more different monomers
polymerize together
• They can be random or alternating
Types of Copolymers
• The exact distribution of monomer units depends on
the initial proportions of the two reactant monomers
and their relative reactivities
Block copolymers
• Different blocks of identical monomer units alternate
with each other
• Prepared by initiating the polymerization of one
monomer as if growing a homopolymer chain and then
adding an excess of the second monomer to the stillactive reaction mix
Graft copolymers
• Homopolymer branches of one monomer unit are
grafted onto a homopolymer chain of another
monomer unit
• Made by gamma irradiation of a completed
homopolymer chain in the presence of the second
monomer generating radical sites that can initiate
polymerization of the added monomer
31.4 Step-Growth Polymers
• Produced by reactions in which each bond in the
polymer is formed independently, typically by reaction
between two difunctional reactants
Step-Growth Polymer from a
Lactam
• Addition generates new nucleophile
• Polyamide from caprolactam is Nylon 6
Polycarbonates
• Carbonyl group is linked to two OR groups,
[O=C(OR)2]
Polyurethanes
• Urethane - carbonyl carbon is bonded to both an OR
group and an NR2 group
Preparation of Polyurethanes
• Nucleophilic addition of an alcohol to an isocyanate
(RN=C=O) gives a urethane
• Reaction between a diol and a diisocyanate gives a
polyurethane
31.5 Polymer Structure and
Physical Properties
• Polymers experience substantially larger van der
Waals forces than do small molecules, producing
regions that are crystallites
Heat Transitions
• Heating at the melt transition temperature, Tm, gives an
amorphous material
• Heating noncrystalline, amorphous polymers makes
the hard amorphous material soft and flexible at the
glass transition temperature, Tg
Thermoplastics
• Have a high Tg and are hard at room temperature
• Become soft and viscous when heated
• Can be molded
Plasticizers
• Small organic molecules that act as lubricants between
chains
• Added to thermoplastics to keep them from becoming
brittle at room temperature
• Dialkyl phthalates are commonly used for this purpose
Fibers
• Thin threads produced by extruding a molten polymer
through small holes in a die, or spinneret
• Fibers are then cooled and drawn out
Elastomers
• Amorphous polymers that have the ability to stretch
out and spring back to their original shapes
• When stretched, the randomly coiled chains straighten
out and orient along the direction of the pull
Natural Rubber and GuttaPercha
• The upper structure is rubber, a natural elastomer
• The lower structure is the nonelastic gutta-percha
Thermosetting resins
• Polymers that become highly cross-linked and solidify
into a hard, insoluble mass when heated
• Bakelite is from reaction of phenol and formaldehyde,
widely used for molded parts, adhesives, coatings