Page 191 - Carrahers_Polymer_Chemistry,_Eighth_Edition

154 Carraher’s Polymer Chemistry

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The cationic polymerization of vinyl isobutyl ether at −40 C produces stereoregular polymers
(5.21). The carbocations of vinyl alkyl ethers are stabilized by the delocalization of “p” valence elec-
trons in the oxygen atom, and thus these monomers are readily polymerized by cationic initiators.
Poly(vinyl isobutyl ether) has a low T because of the steric hindrance offered by the isobutyl group.
g
It is used as an adhesive and as an impregnating resin.
R
R
O
(5.21)

C
H 3
CH 3
This production of stereoregular structures has been known for some time and is especially
strong for vinyl ethers. Several general observations have been noted. First, the amount of stereoreg-
ularity is dependent on the nature of the initiator. Second, steroregularity increases with a decrease
in temperature. Third, the amount and type (isotactic or syndiotactic) is dependent on the polarity
of the solvent. For instance, the isotactic form is preferred in nonpolar solvents, but the syndiotactic
form is preferred in polar solvents.
Commercial polymers of formaldehyde are also produced using cationic polymerization. The
polymer is produced by ring opening of trioxane. Since the polyacetal, polyoxymethylene (POM),

is not thermally stable, the hydroxyl groups are esterified (capped) by acetic anhydride (Equation
5.22). These polymers are also called poly(methylene oxides). The commercial polymer is a strong
engineering plastic. Engineering plastics typically have higher modulus and higher heat resistance
than general-purpose plastics.

O
H O O O O O
HO O
O O
CH 3 CH 3 (5.22)
Trioxane Polyacetal Capped polyoxymethylene (POM)

Another stable polyacetal (POM; Celcon) is produced by the cationic copolymerization of a mix-
ture of trioxane and dioxolane (Equation 5.23).

R
O
O
+ O
O O (5.23)
O
R
Trioxane Dioxolane Poly(oxymethylene)

Many other cyclic ethers have been polymerized using cationic polymerization. Ethylene oxide
(also called oxirane) polymerizes forming poly(ethylene oxide) (PEO; also known as poly(ethylene
glycol) (Equation 5.24) in the presence of acids such as sulfuric acid, producing a wide range of
chain sized polymers sold under various trade names, including Carbowax and Polyox. PEO is
also used in cosmetics and pharmaceuticals (as water-soluble pill coatings and capsules).

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