Chemistry Reference
In-Depth Information
(CH
2
)
n
+ H
2
O
- H
2
O
H
2
N
(CH
2
)
COOH
n
HN
CO
(CH
2
)
n
HN
(CH
2
)
CO
OH
x
(CH
2
)
COOH
H
H
2
N
n
n
HN
CO
x
T
- H
2
O
H
2
N
(CH
2
)
COOH
H
2
N
(CH
2
)
COOH
+
n
n
HN
(CH
2
)
CO
H
HN
(CH
2
)
CO
OH
n
n
- H
2
O
2
2
O
Me
Me
Me
Me
2
Si
O
SiMe
2
CF
3
SO
3
H
F
3
C
SO
3
Si
2
O
Si
O
Si
OH
2
O
SiMe
2
Me
Me
Me
-
CF
3
SO
3
H
Me
Me
Me
back-biting
fast
F
3
C
SO
3
Si
O
Si
O
Si
O
H
Si
O
x
Me
Me
Me
2
Formula 15.7
zwitterionic polymerizations of cyclic esters even at room temperature. Cyclic
polylactides having Mn's up to 20 kDa (corrected values) [
32
] were isolated.
Those authors presented a kinetic scheme characterized by a high rate of propa-
gation and relatively slow rates of initiation and end-to-end cyclization. On the
basis of these kinetic results and considering the rather low PDs (\1.5), these
zwitterionic ROPs were classified as KCPs. However, the MALDI-TOf mass
spectra of the polylactides revealed almost equal amounts of odd- and even-
numbered cyclics proving that rapid ''back-biting cyclization'' was involved. In
other words, the structure of the cyclic polylactides is thermodynamically con-
trolled. As expected from anionic polymerizations of lactides and lactones [
31
,
32
], the formation of alkoxide anions as active chain ends can cause side reactions,
such as the formation of unsaturated end groups in the case of ß-lactones [
23
-
38
]
or racemization of lactide. The latter side reaction was not reported because only
rac.lactide was used as monomer.
