Chemistry Reference
In-Depth Information
linkages. One variety is based on a hydroxyl-ended polytetrahydrofuran poly-
mer (1-31) with a degree of polymerization of about 25, which is reacted with
4,4
0
-diphenylmethane diisocyanate:
H
OCH
2
CH
2
CH
2
CH
2
OH + 2 OCN
CH
2
NCO
25
1-31
(1-12)
H
H
OCN
CH
2
N
C
O
(CH
2
)
4
ON
C
CH
2
NCO
25
O
O
1-32
The isocyanate-ended prepolymer 1-32 is spun into fiber form and is simulta-
neously treated further with ethylene diamine in aqueous dimethylformamide:
H
H
x OCN
CH
2
N
C
O
(CH
2
)
4
ON
C
CH
2
NCO
25
O
O
DMF/H
2
O
+
x+2
H
2
NCH
2
CH
2
NH
2
H
H
H
H
H
H
NCH
2
CH
2
H
H
2
NCH
2
CH
2
N
C
N
CH
2
N
C
O
(CH
2
)
4
ON
C
CH
2
NC
N
H
25
x
O
O
O
O
1-33
(1-13)
The polytetrahydrofuran blocks in the final structure 1-33 are soft segments,
which permit the molecules to uncoil and extend as the fiber is stretched. The
urea linkages produced in reaction (1-13) form intermolecular hydrogen bonds
that are strong enough to minimize permanent distortion under stress. The fibers
made from this block copolymer snap back to their original dimensions after
being elongated to four or five times their relaxed lengths.
1.6
Molecular Architecture
A linear polymer is one in which each repeating unit is linked only to two others.
Polystyrene (1-1), poly(methyl methacrylate) (1-34), and poly(4-methyl pentene-
1) (1-35) are called linear polymers although they contain short branches that are
part of the monomer structure. By contrast, when vinyl acetate is polymerized by
free-radical initiation, the polymer produced contains branches that were not pres-
ent in the monomers. Some repeating units in these species are linked to three or
