Polymer Bulletin 14, 491-495 (1985)
Polymer Bulletin
9 Springer-Verlag 1985
Polylactones
5. Polymerization of L,L-LacUde by Means of Magnesium Salts
Ruth Dunsing and Hans R. Kricheldorf*
Institut for Technische und Makromolekulare Chemie der Universitfit Hamburg,
Bundesstrasse 45, D-2000 Hamburg 13, Federal Republic of Germany
SUMMARY
Bulk p o l y m e r i z a t i o n s of L , L - l a c t i d e were c o n d u c t e d at 120
and 180~
but m a i n l y at 150~
M a g n e s i u m oxide, ethoxide, acetate, stearate, and 2 , 4 - p e n t a n e dionate were used as catalysts.
Time c o n v e r s i o n curves show that at least a t e m p e r a t u r e of 150~
and a r e a c t i o n time of 72 h is r e q u i r e d for m a x i m u m conversion.
The h i g h e s t yields (up to 96 %) w e r e o b t a i n e d w i t h m a g n e s i u m
oxide. However, all initiators, in p a r t i c u l a r m a g n e s i u m oxide,
c a u s e d r a c e m i z a t i o n w h i c h i n c r e a s e d w i t h r e a c t i o n time and temperature. P o l y m e r i z a t i o n s in solutions at t e m p e r a t u r e s a r o u n d
IOO~
failed r e g a r d l e s s of the solvent.
INTRODUCTION
Heavy metal c o m p o u n d s such as lead, tin or zinc oxide,
SnCI4, S n ( I I ) o c t o a t e or d i b u t y l t i n d e r i v a t i v e s w e r e f r e q u e n t l y
used as c a t a l y s t s for the p o l y m e r i z a t i o n of L , L - l a c t i d e and
lactones [I-9]. However, w h e n poly(L-lactide)
or c o p o l y e s t e r s
of L - l a c t i d acid are d e s i g n e d for m e d i c a l or p h a r m a c e u t i c a l
purposes, the p o l y e s t e r s need to be p u r i f i e d from there poisonous catalysts. On the other hand, m a g n e s i u m and c a l c i u m ions
p a r t i c i p a t e in the m e t a b o l i s m of the human body, and thus, catalysts based on m a g n e s i u m or c a l c i u m ions do not n e e d to be
r e m o v e d from the p o l y e s t e r s prior to their application. Therefore, it was the purpose of this p a p e r to i n v e s t i g a t e the usefulness of v a r i o u s c o m m e r c i a l l y a v a i l a b l e m a g n e s i u m c o m p o u n d s
as p o l y m e r i s a t i o n c a t a l y s t s for L , L - l a c t i d e (results o b t a i n e d
w i t h c a l c i u m salts will be r e p o r t e d in another part of this
series). It should be m e n t i o n e d that m a g n e s i u m salts are known
to c a t a l y z e the p o l y m e r i z a t i o n of lactones [1,10]. Yet d e t a i l e d
i n v e s t i g a t i o n s on their u s e f u l n e s s in the case of L , L - l a c t i d e
are lacking.
RESULTS
and D I S C U S S I O N
Five c o m m e r c i a l l y a v a i l a b l e m a g n e s i u m c a t a l y s t s were investigated, n a m e l y m a g n e s i u m acetate, stearate, 2 , 4 - p e n t a n e d i o n a t e (acetylacetonate), m a g n e s i u m e t h o x i d e and m a g n e s i u m
oxide. M a g n e s i u m & t e a r a t e and 2 , 4 - p e n t a n e d i o n a t e may be considered as h o m o g e n o u s c a t a l y s t s b e c a u s e they d i s s o l v e d in the
* To whomoffprintrequestsshouldbe sent.
492
in the m o l t e n monomer, w h e r e a s all other c a t a l y s t s were insoluble. In a first series of e x p e r i m e n t s p o l y m e r i z a t i o n of L,Llactide was a t t e m p t e d in v a r i o u s solvents, such as dioxane,
n i t r o b e n z e n e or p y r i d i n e at IOO~
M a g n e s i u m stearate and m a g n e s i u m o x i d e were used as catalysts; yet all these e x p e r i m e n t s
failed.
All further i n v e s t i g a t i o n s were c o n c e n t r a t e d on bulk polym e r i z a t i o n s of m o l t e n L,L-lactide. When m a g n e s i u m stearate was
used at 120~
no polymer could be isolated after 8 h, and even
after 96 h the yield only reached 35 % (Nos. I-2, Tab. I). At
150oc the yield i n c r e a s e d to 65 % after 96 h. However, the time
c o n v e r s i o n curve (Nos. 3-12, Tab. I) c l e a r l y indicates that the
c o n v e r s i o n levels off after 72 h, still far b e l o w the thermod y n a m i c a l l y a c h i e v a b l e m a x i m u m (ca. 97 %). Three o b s e r v a t i o n s
support the h y p o t h e s i s that the low yields result from desact i v a t i o n of the c a t a l y s t (e.g. decarboxylation)
and not from
d e g r a d a t i o n of poly(L-lactide)
due to b a c k - b i t i n g of the active
chain end. First, the v i s c o s i t i e s increase steadily w i t h inc r e a s i n g conversion. Second, longer r e a c t i o n times favour racem i z a t i o n i n d i c a t i n g side r e a c t i o n s due to p r o t o n t r a n s f e r of the
acidic C-~ p r o t o n of the monomer. Third, a further increase of
the r e a c t i o n t e m p e r a t u r e s t r o n g l y favours r a c e m i z a t i o n and lowers
the y i e l d (No. 13, Table I). The yields o b t a i n e d w i t h m a g n e s i u m
acetate (Nos. 14-18, Tab. I) are s l i g h t l y higher, yet the degree of r a c e m i z a t i o n is also higher. I n t e r e s t i n g l y , the viscosities increase w i t h conversion, i n d i c a t i n g a living c h a r a c t e r
of the chain growth. Also w i t h m a g n e s i u m 2 , 4 - p e n t a n e d i o n a t e
t i m e - c o n v e r s i o n curves w e r e m e a s u r e d at 120, 150 and 180~
(Tab. 2)' A g a i n a r e a c t i o n t e m p e r a t u r e of 150~
was found to be
o p t i m u m and again the yields leveld off after 72 h, far b e l o w
the t h e o r e t i c a l maximum. Furthermore, r a c e m i z a t i o n p r o c e e d e d
w i t h i n c r e a s i n g r e a c t i o n time and temperature. M a g n e s i u m ethoxide
(Nos. 13-16, Tab. 2) gave slightly higher yields; yet the extent
of r a c e m i z a t i o n was not higher, despite the greater b a s i c i t y of
the e t h o x i d e groups.
Finally, three d i f f e r e n t grades of m a g n e s i u m oxide w e r e
used as c a t a l y s t s (Tab. 3). All three grades y i e l d e d similar
results, and thus, do not need separate discussion. However, the
results o b t a i n e d w i t h m a g n e s i u m oxides d i f f e r from those of
other c a t a l y s t s in three aspects. First, yields up to 96 % were
found, d e m o n s t r a t i n g that m a g n e s i u m oxides are the m o s t effective c a t a l y s t s of this study
despite its h e t e r o g e n e o u s charactez
Second, w h e r e a s the yield steadily increases w i t h the r e a c t i o n
t i m e , t h e m o l e c u l a r w e i g h t s are i n d e p e n d e n t on the conversion.
In this r e s p e c t the m a g n e s i u m oxide c a t a l y z e d p o l y m e r i z a t i o n s
of L , L - l a c t i d e resemble the radical p o l y m e r i z a t i o n of vinylmonomers, and not an anionic living p o l y m e r i z a t i o n . Third, the
extent of r a c e m i z a t i o n is greater than that of all other catalysts.
The p r e s e n t study enables two i m p o r t a n t conclusiones. First,
M a g n e s i u m c o m p o u n d s are not suited as catalysts, if o p t i c a l l y
pure poly(L-lactide)
or other p o l y l a c t o n e s w i t h acidic protons
493
are to be p r e p a r e d . This is l i k e w i s e true for c a l c i u m comp o u n d s as will be d e m o n s t r a t e d in a n o t h e r p a r t of this series.
Second, w h e n m a g n e s i u m c o m p o u n d s are u s e d as c a t a l y s t s for
D , L - l a c t i d e or o p t i c a l l y i n a c t i v e lactones, m a g n e s i u m o x i d e
(or hydroxide) is o b v i o u s l y the m o s t e f f e c t i v e catalyst.
T a b l e I: R e a c t i o n c o n d i t i o n s and r e s u l t s of bulk p o l y m e r i z a t i o n s
of L , L - l a c t i d e by m e a n s of Mg s t e a r a t e and Mg a c e t a t e
NO Mg salt
I
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
a)
b)
Stearate
Stearate
Stearate
Stearate
Stearate
Stearate
Stearate
Stearate
Stearate
Stearate
Stearate
Stearate
Stearate
Acetate
Acetate
Acetate
Acetate
Acetate
measured
measured
Mon.
Temp.
Init.
(~
2OO/I
120
200/I
120
200/I
150
200/I
150
2OO/I
150
2OO/I
150
200/I
150
2OO/I
150
200/I
150
200/I
150
200/I
150
2OO/I
150
2OO/I
150
200/I
150
200/I
150
200/I
150
2OO/I
150
200/I
150
at c = 2 g/l
at c = 10 g/l
Time
Yield
~inh a)
[ ~ D20 b)
(h)
(%)
8
0
90
35
0,15
-122
4
3
6
9
8
12
0,11
12
15
16
18
0,15
-139
21
34
O,18
-136
24
36
O,19
-124
48
55
0,25
-110
70
65
0,26
-104
96
65
0,28
-101
96
37
0,22
- 41
16
36
0,18
-107
24
38
0,18
-108
48
58
O,19
- 97
72
71
0,20
- 88
96
74
0,26
- 85
in d i c h l o r o m e t h a n e at 20~
in d i c h l o r o m e t h a n e
EXPERIMENTAL
L , L - l a c t i d e was a gift of B o e h r i n g e r - I n g e l h e i m KG
(W.Germany). It was r e c r y s t a l l i z e d from dry e t h y l a c e t a t e , w a s h e d
w i t h l i g r o i n and d r i e d in a d e s s i c c a t o r over P 4 0 1 0 in vacuo.
M g O of t e c h n i c a l grade and M g O p.a. w e r e p u r c h a s e d from
M e r c k & Co (Darmstadt, W.Germany) and w e r e used w i t h o u t f u r t h e r
p u r i f i c a t i o n . A t h i r d grade was p r e p a r e d by d i s s o l v i n g the
t e c h n i c a l MgO in d i l u t e d p.a. HCL f o l l o w e d by p r e c i p i t a t i o n
w i t h a m m o n i u m h y d r o x i d e . All other c a t a l y s t s w e r e p u r c h a s e d
f r o m L a n c a s t e r S y n t h e s i s Ltd. (Morecambe, England,) and d r i e d
over P 4 0 1 0 in v a c u o at 60~
All p o l y m e r i z a t i o n s w e r e c o n d u c t e d in 50 ml E r l e n m e y e r
flasks w i t h g r o u n d glass joints and s i l a n i s e d glass walls. Both
c a t a l y s t and L , L - l a c t i d e w e r e s u c c e s s i v e l y w e i g h e d into the rea c t i o n flasks u n d e r n i t r o g e n . The r e a c t i o n v e s s e l was c l o s e d
w i t h a glass s t o p p e r and a steel spring and c o m p l e t e l y i m m e r s e d
into the t h e r m o s t a t e d oil baht. Finally, the r e a c t i o n p r o d u c t
was d i s s o l v e d in m e t h y l e n e c h l o r i d e , p r e c i p i t a t e d from c o l d
m e t h a n o l and d r i e d at 60~
in vacuo.
494
Table 2: R e a c t i o n c o n d i t i o n s and r e s u l t s of b u l k p o l y m e r i z a t i o n s
of L , L - l a c t i d e by m e a n s of MgO (three grades) at 150~
No
I
2
3
4
5
6
7
8
9
grade of a)
purity
Mon.
Init.
Time
(h)
technical
technical
technical
technical
technical
purumanalyticum
reprecipitated
reprecipitated
200:1
200:1
200:1
200:1
200:1
200:1
200:1
200:1
200:1
17
24
48
72
96
48
96
48
120
Yield
(%)
57
65
77
87
97
72
96
85
94
~inh b)
0,37
O,41
0,36
0,32
0,32
0,40
0,38
0,30
0,34
20 b)
[~]D
-114
-110
-104
- 98
- 99
-115
-102
-105
-102
a) for c h a r a c t e r i z a t i o n see E X P E R I M E N T A L
b) m e a s u r e d at c = 2 g/l in d i c h l o r o m e t h a n e at 20~
c) m e a s u r e d at c = 10 g/l in d i c h l o r o m e t h a n e
T a b l e 3: R e a c t i o n c o n d i t i o n s and r e s u l t s of b u l k p o l y m e r i z a t i o n s
of L , L - l a c t i d e by m e a n s of M g - 2 , 4 - p e n t a n e d i o n a t e and Mg e t h o x i d e
No M a g n e s i u m
Derivative
Mon.
Init.
Temp.
(~
I Pentanedionate
2 Pentanedionate
3 Pentanedionate
4 Pentanedionate
5 Pentanedionate
6 Pentanedionate
7 Pentanedionate
8 Pentanedionate
9 Pentanedionate
10 P e n t a n e d i o n a t e
11Pentanedionate
12 P e n t a n e d i o n a t e
13 E t h o x i d e
14 E t h o x i d e
15 E t h o x i d e
16 E t h o x i d e
2OO:1
200:1
200:1
200:1
200:1
200:1
200:1
200:1
200:1
2OO:1
200:1
200:1
200:1
2OO:1
200:1
200:1
120
120
120
120
150
150
150
150
180
180
180
180
150
150
150
150
a) m e a s u r e d
Time
(h)
Yield
(%)
24
48
72
96
24
48
72
96
24
48
72
96
24
48
72
96
at c = 10 g/l in d i c h l o r o m e t h a n e
17
20
41
34
49
59
68
68
25
30
48
31
59
65
75
76
20 b)
[~D
-140
-134
-137
-134
-128
-127
-117
-114
- 95,2
- 84
- 81
- 63
-132
-115
-113
-112
495
The v i s c o s i t i e s were m e a s u r e d in an U b b l e h o d e v i s c o s i m e t e r
t h e r m o s t a t e d at 20~
A c o n c e n t r a t i o n of 2 g/l in d i c h l o r o m e t h a n e was used in all cases; ~. I is given in dl/g
The optlcal r o t a t i o n s were m e a s u r e ~ on a Perkin Elmer Md 243.
The specific r o t a t i o n [ e ~ O of o p t i c a l l y pure poly(L-lactide)
is -158+I in d i c h l o r o m e t h a n e .
9
.
i n
"
REFERENCES
I)
2)
3)
4)
5)
6)
7)
8)
9)
10)
J. Kleine, H.-H. Kleine; M a k r o m o l . C h e m .
30 23 (1959)
W. Dittrich, R.C. Schulz; M a k r o m o l . C h e m .
15 109 (1971)
R. V a s a n t h a k u m a r i , M.J. Pennings; P o l y m e r 24 175 (1983)
Ethicon. Inc.Ger: Offen; 2.118.127 (28. 0ct__1971)
Chem. Abstr. 76 73051w (1972)
D.M. Young, F. H o s t e t t l e r and C.F. Horn; Ger. Offen
1.205 586 (11. April 1957) to Union C a r b i d e Corp.
T.C. Snapp and A.F. Blood; US.Pat. 3645 941
(29. Feb. 1972) to E a s t m a n Kodak Co.
H. A m a n n and H. Rauch; G e r . O f f e n 2056 729 (19. Nov. 1970)
to D E G U S S A AG.
H.R. Kricheldorf, J.M. Jont~ and M. Berl; M a k r o m o l . C h e m .
in press (Part 3 of this series)
D.B. Johns, R.W. Lenz and A. Luecke in " R i n g - O p e n i n g Polymerization"
(K.J. Ivin and T. Saegusa eds.) E l s e v i e r N.Y.
1984, Vol. I pp. 461-521
E.I. DuPont; Brit. Pat. 1.101.766 (18. April 1966)
We thank B o e h r i n g e r - I n g e l h e i m for a gift of L , L - l a c t i d e and
the D e u t s c h e F o r s c h u n g s g e m e i n s c h a f t
for financial support.
Accepted November 14, 1985
C
Keep reading this paper — and 50 million others — with a free Academia account
Used by leading Academics
Kenneth Vecchio
University of California, San Diego
Irina Kolesnik
Moscow State University
Martín A. Rodríguez
Universidad Nacional de San Martin
Prof. Dr. rer. nat. Mohammadamin Emami
Isfahan University of Art
