Polymer Degradation and Performance - American Chemical Society

Chapter 4

Synthesis and Degradation of Poly(ethyl glyoxylate)

Downloaded by STANFORD UNIV GREEN LIBR on July 30, 2012 | http://pubs.acs.org Publication Date: January 1, 2009 | doi: 10.1021/bk-2009-1004.ch004

B e n j a m i n e Belloncle, F a b r i c e B u r e l * , and C l a u d e B u n e l P B M , UMR 6522-Laboratoire de Matériaux Macromoléculaires-INSA de Rouen, B.P. 08 Place Emile Blondel, 76131 Mont-Saint-Aignan Cedex, France

Poly(ethyl glyoxylate) ( P E t G ) is a new biodegradable polyacetal obtained by anionic polymerization o f ethyl glyoxylate ( E t G ) . End-capping PEtG with an adequate agent, as phenyl isocyanate, is necessary because o f its l o w ceiling temperature. Degradation o f P E t G was evaluated by in vitro hydrolysis using various techniques such as potentiometry, weight loss, gel permeation chromatography ( G P C ) and 1 H nuclear magnetic resonance ( N M R ) . Whatever the p H o f the medium, the degradation mechanism involved ester hydrolysis and chain scissions leading to ethanol and glyoxylic acid hydrate ( G A H ) release. G l y o x y l i c acid is a Krebs metabolite that confers a potential biodegradable character to P E t G .

Though the interest for synthetic biodegradable polymers is still increasing, few o f them are currently available. The best known products are polyhydroxybutyrate, poly(e-caprolactone), poly(glycolic acid), poly(lactic acid) and their copolymer. W e focused our attention on poly(methyl glyoxylate) ( P M G ) w h i c h is a polyacetal obtained by anionic polymerization o f methyl glyoxylate ( M G ) . This previous work (/) has shown that P M G has very interesting properties in terms o f degradation and toxicity. The ultimate state o f its degradation is g l y o x y l i c acid, which is a metabolite o f plant Krebs cycle. However, P M G degradation also releases methanol which is prohibited in pharmaceutical or medical applications. T o overcome this problem we investigated the anionic polymerization o f ethyl glyoxylate ( E t G ) (2). © 2009 A m e r i c a n Chemical Society

In Polymer Degradation and Performance; Celina, M., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2009.

41

42 A s s u m i n g similar degradation as in the case o f P M G , poly(ethyl glyoxylate) ( P E t G ) hydrolysis should lead to glyoxylic acid and ethanol (3), which are both innocuous.

Moreover, as in the case o f P M G , P E t G has a low ceiling

temperature.

Thus, the hydroxyl ends have to be reacted with a capping agent

under m i l d conditions.

For the polymer concerned in this study, phenyl

isocyanate was used. This paper deals with the synthesis and the in vitro degradation study o f PEtG.

Various techniques including weighing, potentiometry, gel permeation


chromatography ( G P C ) and nuclear magnetic resonance ( N M R ) were used to determine the hydrolysis mechanism.

Experimental Materials Dibutyl tin dilaurate ( D B T L , A l d r i c h , 95%) and phenyl isocyanate ( P h N C O , A c r o s , 99%) were used as received. Ethyl glyoxylate ( E t G ) (M=102), in toluene solution (50% w/w) was kindly supplied by Clariant. Triethylamine ( N E t , A c r o s ) was distilled over potassium hydroxide ( 8 8 ° C ) 3

and dichloromethane (Acros) over sodium ( 4 0 ° C ) before use.

Synthesis E t G was distilled under vacuum over phosphorus pentoxide (P2O5). A clear y e l l o w liquid was obtained ( 1 0 0 ° C / 5 0 mbar). A solution o f E t N (initiator) in 3

dichloromethane (2 u L / m L ) was added to the monomer (50% v/v) and the solution was stirred one hour at - 2 0 ° C . obtained.

A transparent viscous medium was

A n excess o f phenyl isocyanate was then added to end-cap the

polymer in the presence o f D B T L as catalyst. The solution was stirred overnight at room temperature.

Purification by precipitation into methanol was realized

and the polymer was dried under vacuum at 5 0 ° C for 5 hours. *H N M R : chemical shifts ( C D C 1 ) : P E t G : 1.2 ppm ( 3 H , C H ) , 4.2 ppm 3

3

( 2 H , C H ) , 5.5 ppm ( 1 H , C H ) , 7.3 ppm ( 1 0 H , phenyl) ; E t G : 1.4 ppm ( 3 H , t, 2

C H ) , 4.4 ppm ( 2 H , q, C H ) , 9.3 ppm ( 1 H , s, C H ) . 3

2


43 NMR

Studies N M R spectra were recorded on Bruker spectrometers 300 M H z .

Molar

masses were determined as follows (Equ. 1), using the integrations o f ethyl ester group protons (Ich2) and phenyl protons ( I ) . Ph


M n = ^ - ^ x l 0 2 + 256 Iph/10 Thin

films

o f polymer were

placed

(1)

into four

sterilized N M R tubes

containing D 0 and D 0 buffered at pH=4, pH=7 and pH=9. N M R tubes were 2

2

immediately sealed and placed in a drying oven at 3 7 ° C and analyzed at different degradation times.

Thermal

Analysis

The

thermal

stability o f P E t G

was obtained

using a Perkin E l m e r

thermogravimetric analyzer ( T G A 7 ) at the heating flow o f 10°C/min and under N . 2

Potentiometry F i l m samples were placed directly in freshly distilled water thermostated at 37°C.

F o r each degradation time, two samples were recovered.

Sodium azide

( N a N ) was added as a biocide to be sure that the decrease o f p H was not due to 3

bacteria growth.

Weight Loss Polymer thin films were placed in aluminum pans. Samples were weighed initially ( m ) and placed in an oven at 3 7 ° C in water-saturated conditions. A t 0

selected times, two samples were dried under vacuum (O.lmbar, 5 0 ° C ) for 48 h and weighed to obtain dry weights ( m ) . d

Weight loss was determined by

comparing the remaining dry weight m with the initial weight m , (Equ. 2). d

0

m —m i % Weight Loss = — - x 100 m n

0


(2)

44


Chromatography

Studies

T h i n films o f polymer were placed in a drying oven at 3 7 ° C in a saturated water atmosphere. A t different times during degradation, samples were dried under vacuum for 48 h and analyzed. M o l e c u l a r weights were obtained by G P C in dichloromethane (1 m L / m i n ) using a Waters pump model 6000, an injector (Rheodyne) and a refractive index detector ( R J D - 6 A Shimadzu), equipped with a P L gel 5 u m m i x e d - C linear column. The system was calibrated using polystyrene standards with l o w polydispersity.

Results and Discussion Synthesis Ethyl glyoxylate ( E t G ) spontaneously polymerizes at room temperature. A distillation step on P 0 is first necessary to break the oligomers and obtain pure E t G . A previous study on the polymerization o f E t G (2) showed that the optimum conditions are to work in C H C 1 at - 2 0 ° C , with triethylamine as an initiator. A lot o f transfer reactions occur in the medium leading to low molar masses and to a hydroxyl ended P E t G . Because o f its low ceiling temperature, depolymerization o f P E t G occurs at room temperature. That is why a termination step using an adequate agent is necessary. 2

5

2

2

The addition polymerization has not a living character and leads to hydroxyl ended P E t G chains which have to be end-capped similarly to other polyaldehydes (4). A s recommended by the literature, we used phenyl isocyanate (2) as an end-capping agent.

Product supplied by Clariant

Figure 1. Synthesis of Poly (ethyl

glyoxylate).

*H N M R in D M S O - r f was achieved on the end-capped polymer. Three main peaks were observed corresponding to the ethyl ester group ( C H 1.20 ppm and C H : 4.2 ppm) and to the acetalic one ( C H : 5.3 ppm). The integration values are close to our expectations (3/2/1). The signal o f the phenyl moities is 6

3:

2



7.5

7.0

6.0

l

5.5

5.0

4.5

4.0 (ppm)

3.5

3.0

Figure 2. H NMR (300 MHz) of Poly (ethyl glyoxylate)

6.5

c

in

2.5

6

DMSO-d .

2.0


1.5

CO

1.0

0.5

46 observed at 7,3 ppm. Assuming that P E t G is a linear chain, two phenyl groups are present in each chain and molecular weights can be determined from Equation 1. M o r e o v e r no signal corresponding to E t G can be detected thus proving that end-capping o f P E t G was effective.


Thermogravimetric analysis is another way to determine whether P E t G is well end-capped or not. Indeed, a P E t G which is not end-capped or not properly end-capped w i l l not be stable even at 5 0 ° C since chain depolymerization could occur, while end-capped P E t G is stable up to 2 0 0 ° C . A l l experiments were conducted on the same P E t G whose end-capping was checked prior to the degradation study.

Degradation P E t G hydrolysis is a heterogeneous reaction since P E t G is non soluble in water, contrary to its hydrosoluble byproducts.

Thus, to limit any possible

surface dependence degradation, the samples used were all 3mm thick films.

Potentiometry Experiments were conducted at 3 7 ° C for potential medical application purposes. A c c o r d i n g to the results obtained for the in vitro hydrolysis o f P M G (5), ester cleavage o f P E t G is expected leading to carboxylic function, and that is why a drop in p H is expected. Results are presented in Figure 4. During the first five days, no relevant p H variation was observed. F r o m day six on, a p H drop was observed and attributed to an auto acceleration due to the presence o f carboxylic groups coming from glyoxylic acid hydrate ( G A H ) and/or poly(glyoxylic acid) segments. F r o m day eight on, a plateau was observed at p H = 2.30 which corresponds to the p H o f glyoxylic acid monohydrate ( G A H ) in the same conditions.

Weight Loss P E t G was placed in an oven at 3 7 ° C in saturated humidity atmosphere. A t selected times, two samples were taken, dried under vacuum (0.1 m b a r / 5 0 ° C ) for 48 hours and then weighed. A n y weight loss was attributed to the degradation o f P E t G , (Figure 5). D u r i n g the first nine days, less than 1% o f weight loss could be detected. W i t h i n fifteen days only 5% o f weigh loss was observed. Then, up to thirty days, a significant weight decrease was observed leading to 2 7 . 5 % weight loss. O v e r thirty days, the polymer turned to a sticky viscous liquid but no further weight loss was observed. Thus according to the plateau at 72.5%, only ethanol


47 120

s


0 -

500

200

Temperature (°C) Figure 3. Thermogravimetric

analysis of PEtG not end-capped

and end-capped

5

(-)

(—) under N . 2

10

15

Degradation time (days)

Figure 4. Evolution of pH during PEtG degradation. with permission from reference 5. Copyright 2008

(Reproduced Elsevier.)


48 elimination was observed, the remaining product being glyoxylic acid and hydrolyzed oligomers.

Chrom

atography

P E t G molecular weights were measured at different degradation times to demonstrate chain scissions. A shift o f chromatograms to lower molecular weight and a widening o f the peak are observed from the 4 day until the 18 day. After 18 days, no peak can be detected on the chromatogram. S E C results showed that chain scissions are present from the beginning o f P E t G hydrolysis, (Figure 6). G P C data are resumed Table 1.


th

100

th

T

90 ^ 80 '$ 70 60 50 -I 0

1

1

10

1

20

1

30

40

Degradation time (days) Figure 5. Weight loss (%) during PEtG degradation

and (

) corresponding

the only loss of ethanol

Table 1. G P C data during degradation. Days

0

7

10

12

14

15

Mn PDI

73100

61800

27800

16300

5900

6100

2.6

3.8

7.6

9.5

16.0

21.9


to


550

750

950

Elution time (s)

850

1050

1150

Figure 6. GPC chromatogram evolution of PEtG during degradation. (Reproduced with permission from reference 5. Copyright 2008 Elsevier.)

650


1250

50 NMR

Studies

P E t G hydrolysis was followed i n water and i n buffered media, with sodium acetate as an internal standard. Because P E t G is insoluble i n water, only the degradation products w i l l be detected. T w o main signals are observed. In D 0 , at first ethanol ( E t O H ) is detected o w i n g to ester group hydrolysis. Then, glyoxylic acid hydrate ( G A H ) is observed, indicating chain scissions, (Figure 7).


2

After one month, a h i g h degradation rate is observed and ethyl glyoxylate hydrate ( E t G H ) , w h i c h is usually h y d r o l y z e d into G A H , may be detected. Similar results are observed at pH=4 and pH=7. A t pH=9, w h i c h is still under investigation, only E t O H release is expected since polyacetals are generally stable in basic media.

1

(d)

( c )

(b)

EtGH j GAH |

EtGH

EtGH

1

3L

EtOH

EtOH

CHaCOONa

(a)

J l

ppm l

Figure 7. H NMR spectra of PEtG in D 0 versus degradation time : t = 0 (a); 22 days (b); 39 days (c); 59 days (d). (Reproduced with permission from reference 5. Copyright 2008 Elsevier.) 2

A c c o r d i n g to these results, we proposed the f o l l o w i n g mechanism o f degradation by hydrolysis w h i c h involves simultaneous chain scissions and further depolymerization together with ester group hydrolysis (Figure 8).

Conclusion A n i o n i c polymerization o f E t G leads to a new synthetic biodegradable polymer, namely poly(ethyl glyoxylate). Because o f a l o w ceiling temperature and transfer reactions, P E t G has h y d r o x y l ends that can be end-capped with phenyl isocyanate.


51 H

H

.

E

t

O

—c—o—c—

O^OEt f PEtG

N

H

H

-

H

— c — o — c —

OEt

O ^ O r O o ^ O 1 Et EtOH


H

2

chain scissions depolymerization

H HO

—c—o—c— I I c c O' O H O * O H N

N

/OH CH i

N

C0 Et EtGH 2

l a n d 2 \

/-EVOH HO

x

/ O H CH C0 H 2

GAH Figure 8. PEtG mechanism

of degradation

by in vitro

hydrolysis.

The in vitro hydrolysis study, by means o f potentiometry, weight loss, S E C and N M R , shows that the degradation mechanism involves simultaneous chain scissions and ester group hydrolysis. Indeed, ester hydrolysis was evidenced by the release o f an acidic group as glyoxylic acid hydrate by N M R , or by the drop o f p H and the loss o f ethanol (weight loss and N M R ) . The decrease in molecular weight observed by S E C and the presence o f small molecules by N M R prove that chain scissions occurred at the same time. The ultimate degradation product is glyoxylic acid hydrate which is a Krebs cycle metabolite and allows us to think that P E t G is biodegradable.

References 1. 2.

Brachais, C. -H.; Huguet, J.; Bunel, C . Polymer 1997, 38, 19, 4959-4964. B u r e l , F.; Rossignol, L.; Pontvianne, P . ; Hartman, J . ; Couesnon, N.; B u n e l , C . e-Polymer 2003, 31.

3.

Brachais, C . -H.; Huguet, J . ; Bunel, C . ; Brachais, L. Polymer

1998, 39,

4, 883-890. 4. 5.

V o g l , O . J. Polym. Sci. A: Polym. Chem. 2000, 38, 13, 2293-2299. Belloncle, B.; B u r e l , F . ; O u l y a d i , H.; B u n e l , C . ; Polymer Degradation Stability; doi: 10.1016/j.polymdegradstab.2008.03.004.


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