Upload
melissa-skinner
View
218
Download
1
Embed Size (px)
Citation preview
Chapter 20
Synthetic Polymers
A polymer is a large molecule made by linking togetherrepeating units of small molecules called monomers
1. Addition polymers( 加聚物 ), also called chain-growth polymers, are made by chain reactions ( 链反应 )
2. Condensation polymers( 缩聚物 ), also called step-growth polymers, are made by combining two molecules by removing a small molecule
Classification of polymers
的纶,的确良
20-1. Addition polymers
Chain-growth polymers proceed by one of three mechanisms:
(1) radical polymerization
(2) cationic polymerization
(3) anionic polymerization
Some important chain-growth polymers and their uses
(1) Radical Polymerization
The molecular weight of the polymer can be controlled by a process known as chain transfer
Head-to-tail addition is favored for steric reasons
Groups that stabilize radicals also favor head-to-tail addition
Chain-growth polymerization of monosubstituted ethylenes exhibits a marked preference for head-to-tail addition
Examples of alkenes that undergo radical polymerization
Some radical initiators
Branching of the Polymer Chain
Branched polymers are more flexible
(2) Cationic Polymerization
Chain-initiating step
Chain-propagating steps
The carbocation intermediates formed during polymerization can undergo rearrangement
Monomers that are best able to undergo cationicpolymerization are those with electron-donating substituents
Examples of alkenes that undergo cationic polymerization
(3) Anionic Polymerization
Chain-initiating step
Chain-propagating steps
LiBu H2C CH
C N
CH2 CH
C N
Bu
Examples of alkenes that undergo anionic polymerization
Monomers that are best able to undergo anionicpolymerization are those with electron-withdrawing substituents
Ring-Opening Polymerization
Stereochemistry of Polymerization
substituents on the same side
substituents regularly alternate on both sides
isotactic configuration (same side) 全同立构
syndiotactic configuration (both sides) 间同立构
atactic configuration (both sides) 无规立构
Long, unbranched polymers can be prepared using analuminum–titanium initiator (Ziegler–Natta catalyst)
Stereochemical control of polymerization
Natural and synthetic rubbers
Natural rubber is a terpene ( 萜 )
Vulcanization ( 硫化 ): cross-linking of rubbers
The product resulting from polymerization of different monomers is called a copolymer
Copolymer ( 共聚物 )
Some examples of copolymers and their uses
20-2 Condensation Polymers (resin 树脂 )
Polyamides: Nylon 6
Polyamides 聚酰胺Polyesters 聚酯Polycarbonates 聚碳酸酯Polyurethanes 聚氨酯
O N OHNH2OH
NHrearrangement
OH2O, heat
The incorporation of aromatic rings into polymers improves the physical strength of the polymers
Polyesters 聚酯
Polycarbonates 聚碳酸酯
热塑聚碳酸酯
Epoxy resins are the strongest adhesive known
polyurethane 聚氨酯
Thermosetting 热固性 Polymers
Very strong and rigid materials can be obtained by cross-linking
Oriented Polymers
These polymers are stronger than steel
They can conduct electricity
Biodegradable Polymers
These polymers can be broken into small segments byenzyme-catalyzed reactions
Assignments
• 26-2, 3, 5, 6