Advices For Studying Organic Chemistry

A.2 STEREOCHEMISTRY OF CHAIN-GROWTH POLYMERIZATION

1. Head-to-tail polymerization of propylene produces a polymer in which every other
   atom is a stereocenter.


2. Many of the physical properties of propylene produced in this way depend on the
   stereochemistry of these stereocenters.

CH 2 CH

CH 3

polymerization

(head to tail) CH^2 CHCH^2 CHCH^2 CHCH^2 CH

CH 3 CH 3 CH 3 CH 3

¡ ̄ ¡ ̄ ¡ ̄ ¡ ̄

A.2A ATACTIC POLYMERS

1. The stereochemistry at the stereocenters is random, the polymer is said to be
   atactic (a, without + Greek: taktikos, order).

H

H

H

CH 3

H

H

CH 3

H

H

H

H

CH 3

H

H

CH 3

H

H

H

H

CH 3

H

H

H

CH 3

H 3 CHHH 3 CHCH 3 H 3 CHHCH 3 HCH 3

Figure 10.1 Atactic polypropylene. (In this illustration a “stretched” carbon
chain is used for clarity.

1. In atactic polypropylene the methyl groups are randomly disposed on either side
   of the stretched carbon chain ⇒ (R-S) designations along the chain is random.


2. Polypropylene produced by radical polymerization at high pressure is atactic.


3. Atactic polymer is noncrystalline ⇒ it has a low softening point and has poor
   mechanical properties.

A.2B SYNDIOTACTIC POLYMERS