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