Chapter 7 Polymers
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CHAPTER 7 Synthetic Polymers
Polymers
Large molecules built up by repetitive bonding together of monomers Indicate repeating unit in parentheses Synthetic polymers are classified by the synthesis method:
2
Chain-growth Step-growth
Chain-Growth Polymers
3
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. Common method is radical polymerisation carried out with practically any vinyl monomer.
Cationic polymerisation (acid catalysed)
4
Effective only with vinyl monomers containing an electron-donating group (EDG) that capable to stabilise the chain-carrying carbocation intermediate. E.g. :Isobutylene polymerisation
Anionic Polymerization (basic catalysed)
5
Vinyl monomers with electron-withdrawing substituents (EWG) can be polymerized by anionic catalysts Chain-carrying step is conjugate nucleophilic addition of an anion to the unsaturated monomer
Examples of Anionic Polymerization Products
6
Acrylonitrile (H2C=CHCN), methyl methacrylate [H2C=C(CH3)CO2CH3], and styrene (H2C=CHC6H5) react
Step-Growth Polymers Produced by reactions in which each bond in the polymer is formed independently, typically by reaction between two different functional reactants
7
Nylon 66: reaction between the sixcarbon adipic acid and the carbon hexamethylenediamin e
Nylon 6: made by polymerisation of the six-carbon caprolactam
Polycarbonates
Carbonyl group is linked to two [O=C(OR)2]
E.g.: Lexan, unusually high impact strength – machinery housing, bicycle safety helmets and bulletproof glass
8
OR groups,
Polyurethanes
Urethane - carbonyl carbon is bonded to both an OR group and an NR2 group
Prepared by nucleophilic addition of an alcohol to an isocyanate (R N=C=O) gives a urethane Polyurethane prepared from reaction between a diol and a diisocyanate.
9
Polymer Structure and Physical Properties Polymers experience substantially larger van der Waals forces than do small molecules, producing regions that are crystallites. Crystallites – highly ordered portions in which the zigzag polymer chains are held together by van der Waals forces. 10
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, T g
11
Thermoplastics
Have a high Tg and are hard at room temperature When heated, become soft and viscous can be molded The individual can slip past one another in the melt because it has little or no cross-linking. Some are:
12
amorphous and non-crystalline e.g. poly(methyl methacrylate), polystyrene Partially crystalline e.g. poly(ethylene terephthalate)
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
13
Fibers
Thin threads produced by extruding a molten polymer through small holes in a die, or spinneret then cooled and drawn out orients the crystallite regions along the axis of the fiber and adds considerable tensile strength E.g.: nylon, dacron
14
Elastomers
Amorphous polymers that have the ability to stretch out and spring back to their original shapes. Have low Tg values and small amount of crosslinking to prevent the chains from slipping over one another When stretched, the randomly coiled chains straighten out and orient along the direction of the pull
15
Natural Rubber and Gutta-Percha
The upper structure is rubber, a natural elastomer The lower structure is the nonelastic guttapercha
16
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
17
Polymers
Large molecules built up by repetitive bonding together of monomers Indicate repeating unit in parentheses Synthetic polymers are classified by the synthesis method:
2
Chain-growth Step-growth
Chain-Growth Polymers
3
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. Common method is radical polymerisation carried out with practically any vinyl monomer.
Cationic polymerisation (acid catalysed)
4
Effective only with vinyl monomers containing an electron-donating group (EDG) that capable to stabilise the chain-carrying carbocation intermediate. E.g. :Isobutylene polymerisation
Anionic Polymerization (basic catalysed)
5
Vinyl monomers with electron-withdrawing substituents (EWG) can be polymerized by anionic catalysts Chain-carrying step is conjugate nucleophilic addition of an anion to the unsaturated monomer
Examples of Anionic Polymerization Products
6
Acrylonitrile (H2C=CHCN), methyl methacrylate [H2C=C(CH3)CO2CH3], and styrene (H2C=CHC6H5) react
Step-Growth Polymers Produced by reactions in which each bond in the polymer is formed independently, typically by reaction between two different functional reactants
7
Nylon 66: reaction between the sixcarbon adipic acid and the carbon hexamethylenediamin e
Nylon 6: made by polymerisation of the six-carbon caprolactam
Polycarbonates
Carbonyl group is linked to two [O=C(OR)2]
E.g.: Lexan, unusually high impact strength – machinery housing, bicycle safety helmets and bulletproof glass
8
OR groups,
Polyurethanes
Urethane - carbonyl carbon is bonded to both an OR group and an NR2 group
Prepared by nucleophilic addition of an alcohol to an isocyanate (R N=C=O) gives a urethane Polyurethane prepared from reaction between a diol and a diisocyanate.
9
Polymer Structure and Physical Properties Polymers experience substantially larger van der Waals forces than do small molecules, producing regions that are crystallites. Crystallites – highly ordered portions in which the zigzag polymer chains are held together by van der Waals forces. 10
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, T g
11
Thermoplastics
Have a high Tg and are hard at room temperature When heated, become soft and viscous can be molded The individual can slip past one another in the melt because it has little or no cross-linking. Some are:
12
amorphous and non-crystalline e.g. poly(methyl methacrylate), polystyrene Partially crystalline e.g. poly(ethylene terephthalate)
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
13
Fibers
Thin threads produced by extruding a molten polymer through small holes in a die, or spinneret then cooled and drawn out orients the crystallite regions along the axis of the fiber and adds considerable tensile strength E.g.: nylon, dacron
14
Elastomers
Amorphous polymers that have the ability to stretch out and spring back to their original shapes. Have low Tg values and small amount of crosslinking to prevent the chains from slipping over one another When stretched, the randomly coiled chains straighten out and orient along the direction of the pull
15
Natural Rubber and Gutta-Percha
The upper structure is rubber, a natural elastomer The lower structure is the nonelastic guttapercha
16
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
17
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