the synthesis of lactic-acid-based telechelic prepolymers

The Synthesis of Lactic-Acid-Based Telechelic Prepolymers

KARI HILTUNEN, JUKKA V. SEPPALA

Helsinki University of Technology, Department of Chemical Engineering, Laboratory of Polymer Technology,Kemistintie 1, 02150 Espoo, Finland

Received 21 April 1997; accepted 13 August 1997

ABSTRACT: We have studied how different catalysts and diols affect the properties oflow-molecular-weight (Mw (GPC) õ 49800 g/mol) lactic-acid-based telechelic prepoly-mers. The catalysts and diols were tested separately in our previous studies. In thisstudy, we used the best previously tested diols and catalysts together in order to preparedifferent types of telechelic prepolymers (for example, crystalline or amorphous). Allcondensation polymerizations were carried out in the melt, using different diols anddifferent catalysts. The prepolymers were characterized by differential scanning calo-rimetry, gel permeation chromatography, titrimetric methods, and 13C nuclear mag-netic resonance (13C-NMR). According to NMR, the resulting polymers contained lessthan 1 mol % of lactic acid monomer and less than 5.1 mol % of lactide. Dibutyltindilaur-ate, like tin(II)octoate, produced quite good molecular weights, but the resulting pre-polymers contained exceptionally high amounts of D-lactic acid structures, and, there-fore, these prepolymers were totally amorphous. Antimony(III)oxide produced a high-molecular-weight prepolymer when the diol used was aliphatic. Like DBTL, Sb2O3

produced amorphous prepolymers, which contained a lower amount of D-lactic acidstructures than DBTL prepolymers. 1,8-dihydroxyanthraquinone produced a differentkind of chain structure with Ti(IV)bu and Ti(IV)iso because one prepolymer had highcrystallinity, and the other showed only a slight crystallinity. Sulphuric acid produceda very high-molecular-weight prepolymer with aliphatic 2-ethyl-1,3-hexanediol; andwith aromatic diols, it produced quite good molecular weights, except with 1,8-dihydrox-yanthraquinone. High-molecular-weight prepolymers produced with H2SO4 alsoshowed high crystallinity; and, according to 13C-NMR, they did not contain lactide andD-lactic acid structures. q 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67: 1011–1016,1998

INTRODUCTION In our process the lactic acid is first condensa-tion polymerized to a low molecular weight pre-polymer which contains one kind of end group.For several years we have studied the production

of lactic acid based biodegradable polymers. Typi- The molecular weight is then increased by joiningprepolymer chains together using difunctionalcally high molecular weight lactic acid polymers

are prepared by the special condensation polymer- substances such as diisocyanates as the chain ex-tenders. In our previous studies3,4,5,6,7 we haveization of lactic acid1 or by the ring opening poly-

merization of lactide. In direct polycondensation used 1,4-butanediol and tin(II)octoate in the syn-thesis of hydroxyl terminated prepolymers. Withthe one problem for achieving high molecular

weights is the depolymerization where the lactide this diol/catalyst system it is possible to producedvery good prepolymers for diisocyanate linking.is formed.2

The final poly(ester-urethanes) had a very goodpolymer characteristics and they all are amor-

Correspondence to: J. V. Seppala.phous. In order to use these new materials in a

Journal of Applied Polymer Science, Vol. 67, 1011–1016 (1998)q 1998 John Wiley & Sons, Inc. CCC 0021-8995/98/061011-06 wider range of applications the crystallinity and

1011

4750/ 8e13$$4750 12-01-97 17:44:15 polaa W: Poly Applied

1012 HILTUNEN AND SEPPALA

glass transition temperature control is essential. 1.0 mL/min. The samples were dissolved in chlo-roform at a concentration of 1.0% (w/v). The in-In our latest studies we have tested the effect of

different catalysts8 and different diols9 on the jection volume was 200 mL.properties of the prepolymers. Based on these For nuclear magnetic resonance (NMR) mea-studies, we have selected the best catalysts and surements, the samples were dissolved in chloro-diols to be used together in lactic acid poly- form-d1 in 5 mm NMR tubes at room temperature.merization. In this article, we report how these The sample concentration was about 10% bycatalyst/diol systems effect the properties of the weight. Proton-decoupled 13C-NMR spectra withfinal prepolymers. Our purpose was to develop dif- nuclear Overhauser effect (NOE) were recordedferent catalyst/diol systems for preparing prepoly- on a Varian Unity 400 NMR spectrometer, work-mers with different properties. We want to control ing at 100.577 MHz for carbon-NOE-13.the molecular weight, glass transition tempera- Differential scanning calorimetric (DSC) mea-ture and crystallinity of the prepared prepoly- surements were made on a PL Thermal Sciencesmers. The basic synthesis and characterization DSC. The measurements were run from 050 towork of prepolymers was presented in our previ- 2007C, at a heating rate of 107C/min and at aous study.7 cooling rate of 17C/min. The glass transition tem-

peratures and crystallinity data were determinedfrom the second heating period. The crystallinityof different polymers was evaluated from the heatEXPERIMENTALof fusion. By integrating the normalized area ofthe melting endotherm, determining the heat in-Materialsvolved, and rating it to the reference 100% crystal-

L-Lactic acid (LA) from Fluka (Switzerland, line polymer (93.6 J/g),10 the relative crystallin-Buchs) was a 90% aqueous solution of the mono- ity of the polymer was assessed. In this article,mer, 99% optically pure according to the manufac- the relative degree of crystallinity is referred to asturer. The excess water was removed before use by crystallinity, and Tm is the melting temperature.distillation under reduced pressure at 1007C. The Acid number was determined by a standard ti-following products were used without further treat- trimetric method DIN 53 402. Every titration wasment: 2-ethyl-1,3-hexanediol, 1,5-dihydroxynaphta- repeated four times, and the reported result is anlene, 1,4- and 1,8-dihydroxyanthraquinone, 4,4-iso- arithmetic average of these titres.propylidenediphenol, 4-hydroxyphenyl sulphone,and 4,4*-dihydroxybiphenyl from Fluka. Sulphuricacid (H2SO4) and dibutyltindilaurate (DBTL) werefrom Fluka; antimony(III)oxide (Sb2O3), titan- Synthesis of Lactic Acid Polymers(IV)butylat [ti(IV)bu], titan(IV)isopropylat [Ti-(IV)iso] were from Aldrich Chemie (Germany, In preparing telechelic polymers with hydroxylSteinheim); and chloroform and chloroform-d1, with end groups, L-lactic acid was condensation-poly-the deuteration degree not less than 99.5%, were merized with 2 mol % of different diols and withfrom Fluka. 0.1 wt % of different catalysts. All polymerizations

were carried out in the melt and under vacuumusing a Buchi Rotavapor equipped with an oil

Characterizations bath. A continuous nitrogen gas flow was main-tained under the surface of the melt. The rotationMolecular weights (MV n and MV w ) and polydispersi-speed was approximately 100 r/min. Over a periodties (MV w /MV n ) were determined with respect toof 8 h, the temperature of the oil bath was raisedpolystyrene standards by gel permeation chroma-from 160 to 2107C, and the pressure was loweredtography (GPC). The system from Waters Associ-from 500 to 30 mbar. After polymerization, theates used was equipped with a Waters 700 Satel-molten polymer was poured into an aluminiumlite wisp injector, a Waters 510 high-performancepan and placed in a desiccator to cool down. Afterliquid chromatography (HPLC) solvent pump,cooling, the resulting polymer was pulverized andfour linear PL gel columns (104, 105, 103, andanalysed. The total polymerization time in every100 A) connected in series, and a Waters 410 dif-experiment was 24 h, and one sample was takenferential refractometer. All samples were ana-after 8 h of polymerization at the final polymeriza-lyzed at room temperature. Chloroform was used

as the eluent and was delivered at a flow rate of tion temperature.


LACTIC-ACID-BASED TELECHELIC PREPOLYMERS 1013

RESULTS AND DISCUSSION prepolymer contained 46.3 mol % of D-lactic acidstructures and still showed 2.6% crystallinity. Itappears that DBTL and this diol produced a pre-A detailed description of the low-molecular-

weight lactic acid polymer analysis was presented polymer that contains such uniform chains thatsome crystal structure could be formed. The lowin our previous study,7 and the analysis results

for the prepolymers prepared at 2007C are col- melting temperature shows that the chains con-tain a lot of defects (D-lactic acid structures). Thelected in Table I. The GPC was calibrated with

narrow polystyrene standards; and, therefore, molecular weight of the prepolymers decreasedwhen the polymerization was carried out from 8GPC results were used only as a qualitative tool

to check the peak shape and size distribution of to 24 h. Logically, the acid number and glass tran-sition temperature increased at the same time. Itthe different polymers. Table I lists the polymer-

izations, diols used, and characterization results appears that the optimum polymerization time forDBTL and different diols is between 8 and 24 h.for the prepolymers prepared at 2007C. The

amount of lactic acid and lactide was calculated The amount of D-lactic acid structures increasedwhen the polymerization was carried out from 8using the 13C-NMR spectra methine area peak in-

tegrals. to 24 h. This shows that the ester interchangereactions happen during polycondensation; andthe longer the polymerization time, the higher the

Polymerization Results amount of D-lactic acid structures. The amountof lactide formed was less than 2.7 mol % for everyThe polymerization results are collected in Table

I. The results indicate that there are big differ- prepolymer.Antimony(III)oxide (Sb2O3) and 2-ethyl-1,3-ences in how different catalysts and diols work

together in the polycondensation of lactic acid. For hexanediol, and 2,7-dihydroxynaphtalene and1,8-dihydroxyanthraquinone produced prepoly-example, dibutyltindilaurate (DBTL) produced a

high-molecular-weight prepolymer with 1,4-d- mers with a raised weight-average molecularweight when polymerization was continued fromihydroxyanthraquinone; but with 2,7-dihydroxy-

naphtalene, the resulting prepolymer had a very 8 to 24 h. However, the high acid numbers indi-cate that the polymerization time should be longerlow molecular weight. As with molecular weight,

there are very big differences in the amount of D- than 24 h. The polymerization with 2-ethyl-1,3-hexanediol produced a prepolymer with a fairlylactic acid structures and lactide formed during

the polycondensation reactions. The amount of D- low acid number. The molecular weights of theseprepolymers were reasonably high. Only 1,4-dihy-lactic acid structures formed varied from 0 to 66.3

mol %, depending on the diol and catalyst used. droxyanthraquinone and 4-hydroxybiphenyl sul-phone produced prepolymers with molecularThe amounts of lactide formed varied between 0

and 21.9 mol %. The high acid numbers indicate weights higher after 8 h of polymerization thanafter 24 h of polymerization. After 24 h of poly-that the prepolymers were not completely hy-

droxyl-terminated. In some cases, the acid num- merization, the weight-average molecular weightof these prepolymers had decreased, and the acidbers were lower after 8 h of polymerization, indi-

cating that the optimum polymerization times number had increased. This shows that these pre-polymers had reached their highest molecularwere between 8 and 24 h. In most cases, the acid

numbers decreased when the polymerization was weights before 24 h of polymerization. All prepoly-mers prepared with Sb2O3 were amorphous, andcontinued from 8 to 24 h; after 24 h, of polymeriza-

tion the acid numbers were still quite high. This they contained about 30 mol % of D-lactic acidstructures. It appears that Sb2O3 produced pre-behavior indicates that the polymerization time

should be longer since, according to our previous polymer chains where the D-lactic acid structureswere randomly distributed. The glass transitionstudies, the acid number should be close to or

lower than unity. The results presented in Table temperatures of the prepolymers were about457C. The amount of lactide was less than 3.3 molI show that the glass transition temperatures and

crystallinity of the prepolymer can be widely con- % for every prepolymer.Titan(IV)butylat [Ti(IV)bu] produced a high-trolled.

DBTL produced high amounts of D-lactic acid molecular-weight prepolymer with every diol ex-cept 2,7-dihydroxynaphtalene. The molecularstructures with every diol tested. All prepolymers

were amorphous, except the one that was pre- weight of every prepolymer increased dramati-cally when the polymerization was carried outpared with 1,4-dihydroxyanthraquinone. This



Tab

leI

Res

ult

saf

ter

24H

ours

ofP

olym

eriz

atio

nat

2007

Can

dw

ith

2m

ol%

ofD

iol

and

0.1

wt

%of

Cat

alys

t

D-l

acti

cMU

wA

cid

Str

uc

Lac

tide

Aci

d(G

PC

)P

D(13

C-N

MR

)(13

C-N

MR

)N

um

ber

Tg

Tm

Cry

stal

lin

ity

Pol

ymer

Dio

lC

atal

yst

g/m

ol(G

PC

)(m

ol%

)(m

ol%

)(T

itr)

(7C

)(7

C)

(%)

12-

eth

yl-1

,3-h

exan

edio

lD

BT

L68

001.

751

.31.

96.

632

nda

02

2-et

hyl

-1,3

-hex

aned

iol

Sb 2

O3

21,1

001.

727

.63.

44.

646

nd

03

2-et

hyl

-1,3

-hex

aned

iol

Ti(

IV)b

u24

,700

1.6

27.7

3.6

1.6

45n

d0

42-

eth

yl-1

,3-h

exan

edio

lT

i(IV

)iso

19,4

001.

623

.12.

53.

544

nd

05

2-et

hyl

-1,3

-hex

aned

iol

H2S

O4

49,8

006.

30

0.3

8.9

5016

146

.26

2,7-

dih

ydro

xyn

aph

tale

ne

DB

TL

3700

1.6

66.3

2.1

32.4

36n

d0

72,

7-di

hyd

roxy

nap

hta

len

eS

b 2O

316

,800

2.2

33.4

3.3

8.7

45n

d0

82,

7-di

hyd

roxy

nap

hta

len

eT

i(IV

)bu

10,0

002.

121

.45.

111

.742

nd

09

2,7-

dih

ydro

xyn

aph

tale

ne

Ti(

IV)i

so98

002.

021

.92.

912

.127

nd

010

2,7-

dih

ydro

xyn

aph

tale

ne

H2S

O4

18,4

002.

00

08.

060

164

33.3

111,

4-di

hyd

roxy

anth

raqu

inon

eD

BT

L23

,200

1.5

46.3

2.7

5.9

4013

52.

612

1,4-

dih

ydro

xyan

thra

quin

one

Sb 2

O3

11,1

001.

436

.93.

020

.144

nd

013

1,4-

dih

ydro

xyan

thra

quin

one

Ti(

IV)b

u16

,000

1.5

31.8

2.4

10.9

4112

04.

614

1,4-

dih

ydro

xyan

thra

quin

one

Ti(

IV)i

so17

,800

1.6

26.7

2.3

13.1

4212

53.

515

1,4-

dih

ydro

xyan

thra

quin

one

H2S

O4

21,7

001.

60

08.

850

167

54.1

161,

8-di

hyd

roxy

anth

raqu

inon

eD

BT

L20

,200

1.6

47.3

2.5

11.4

41n

d0

171,

8-di

hyd

roxy

anth

raqu

inon

eS

b 2O

313

,800

1.5

34.6

3.1

18.3

47n

d0

181,

8-di

hyd

roxy

anth

raqu

inon

eT

i(IV

)bu

18,4

001.

68.

22.

818

.553

156

38.8

191,

8-di

hyd

roxy

anth

raqu

inon

eT

i(IV

)iso

18,0

001.

621

.42.

111

.245

140

9.4

201,

8-di

hyd

roxy

anth

raqu

inon

eH

2SO

429

001.

30

057

.621

nd

021

4-h

ydro

xybi

phen

ylsu

lfon

eD

BT

L13

,200

2.1

48.2

2.9

31.6

44n

d0

224-

hyd

roxy

biph

enyl

sulf

one

Sb 2

O3

9000

1.9

32.1

2.8

37.9

46n

d0

234-

hyd

roxy

biph

enyl

sulf

one

Ti(

IV)b

u21

,300

2.1

28.1

4.2

27.8

47n

d0

244-

hyd

roxy

biph

enyl

sulf

one

Ti(

IV)i

so20

,800

2.0

25.6

3.3

30.8

47n

d0

254-

hyd

roxy

biph

enyl

sulf

one

H2S

O4

19,6

002.

40

030

.753

157

36.5

an

dÅ

not

dete

ctab

le.


LACTIC-ACID-BASED TELECHELIC PREPOLYMERS 1015

from 8 to 24 h. The acid number of the prepolymer 13C-NMR results, the resulting prepolymers didnot contain D-lactic acid structures and lactide.that was produced with 2-ethyl-1,3-hexanediol,

was 1.6, which is a very good value. The acid num- It appears that sulphuric acid prevents the forma-tion of lactide and catalyzes the ester interchangebers of the other prepolymers indicate that the

polymerization time was not long enough. Only reactions through the breaking of the carbonyl–oxygen bond. The glass transition temperatures1,4- and 1,8-dihydroxyanthraquinone produced

prepolymers that show crystallinity. The prepoly- of the high-molecular-weight prepolymers pre-pared with H2SO4 were clearly higher than thosemer that was produced with 1,4-dihydroxyanthra-

quinone showed only a slight crystallinity, while of the prepolymers prepared with other catalysts.the 1,8-dihydroxyanthraquinone prepolymershowed over 38% crystallinity. Low crystallinity

The Effect of Dioland melting temperature indicate that the 1,4-dihydroxyanthraquinone prepolymer chains are In our earlier work,3–7 we have used 1,4-butaned-

iol as a functionalizing substance. In these stud-not uniform and contain a lot of defects. Accordingto 13C-NMR, these defects must be D-lactic acid ies, we noticed that it is possible to produce hy-

droxyl-terminated prepolymer and that the mo-units. However, these D-lactic acid units must bequite well organized because the amount of D- lecular weight of the prepolymer can be controlled

by the amount of diol. Even in earlier studies, welactic acid units are as high as 31.8 mol %, andthe prepolymer still shows crystallinity. The noticed that the polycondensation of lactic acid

proceeds differently when the diol is added to theamount of lactide was less than 5.1 mol % forevery prepolymer. polymerization mixture. With Sn(II)Oct, the col-

lected results are as follows (at 2007C): no diolTitan(IV) isopropylat [Ti ( IV) iso ] producedquite similar prepolymers to those produced with MV wÅ 12900 g/mol; with 2 mol % of 1,4-butanediol,

MV w Å 12000 g/mol; with 2 mol % of 1,4-dihydroxy-Ti(IV)bu. Both catalysts produced almost equalmolecular weights, and the molecular weight of anthraquinone, MV w Å 26800 g/mol; and even as

high as 71800 g/mol, molecular weights can beevery prepolymer increased dramatically whenthe polymerization was carried out from 8 to 24 achieved. In our recent studies, we have investi-

gated how different catalysts work in the polycon-h. Ti(IV)iso produced slightly crystalline prepoly-mers with 1,4- and 1,8-dihydroxyanthraquinone. densation of pure L-lactic acid8 and the effect if

the catalyst is Sn(II)Oct and the diol is changed.9Both of these prepolymers had low crystallinityand melting temperatures, and they contained In this study, we examined the production of lac-

tic-acid-based telechelic prepolymers with differ-over 20 mol % of D-lactic acid structures. Highacid numbers (except with 2-ethyl-1,3-hexaned- ent catalysts and diols. According to our results,

the diol, catalyst, and, perhaps, lactic acid formediol) indicate that the polymerization time shouldbe longer than 24 h. The amounts of lactide some kind of active center, which polymerized lac-

tic acid faster and more efficiently than if justformed were smaller than for the prepolymersprepared with Ti(IV)bu. catalyst and lactic acid were used. The structure

of this possible center is not yet clear and requiresSulphuric acid (H2SO4) produced high molecu-lar weight and crystalline prepolymers with every further investigation. What is clear is that very

high molecular weights (71800 g/mol) can bediol except 1,8-dihydroxyanthraquinone, whichproduced a very low molecular weight and amor- achieved with different catalyst–diol–lactic acid

systems. If the structure of this possible activephous lactic acid oligomer. With all the other di-ols, the molecular weight of the prepolymer in- center is optimized, the production of high-molec-

ular-weight poly(lactic acid) by condensation po-creased dramatically when polymerization wascarried out from 8 to 24 h. In addition, the crys- lymerization may be achievable.tallinity of the prepolymers did not decrease dur-ing 24 h of polycondensation. The acid numbersindicate that the polymerization time should be a CONCLUSIONSlittle bit longer; and, in particular, the 4-hydro-xybiphenyl sulphone polymerization should be In this study, we used 2 mol % of different diols

and 0.1 wt % of different catalysts in order tomuch longer. The highest weight-average molecu-lar weight (49800 g/mol) was achieved with 2- prepare hydroxyl-terminated prepolymers. In our

latest studies, we have tested different catalysts8ethyl-1,3-hexanediol, and the highest crystallinitywith 1,4-dihydroxyanthraquinone. According to and different diols with Sn(II)Oct.9 In this study,



we tested the best previously tested catalysts diol, and, perhaps, lactic acid seemed to form somekind of active center, which polymerized the lacticand diols in order to produce different kinds ofacid monomer more efficiently than the systemhydroxyl-terminated prepolymers. Dibutyltindi-without diol or catalyst. Some of these possiblelaurate, like tin(II)octoate, produced quite goodcenters produced amorphous, and the other,molecular weights; but the resulting prepolymerscrystalline prepolymers. 2-ethyl-1,3-hexanediolcontained exceptionally large amounts of D-lacticformed the most efficient center if the molecularacid structures, and, therefore, these prepolymersweight was observed with sulphuric acid, with an-were totally amorphous. Antimony(III)oxide pro-timony(III)oxide, and with Ti(IV)bu. DBTL, onduced a high-molecular-weight prepolymer whenthe other hand, formed its most active center withthe diol used was aliphatic. All the aromatic diols1,4-dihydroxyanthraquinone, and Ti(IV)iso withwith antimony(III)oxide produced prepolymers4-hydroxybiphenyl sulphone. If the crystallinitywith clearly lower molecular weights. Like DBTL,was observed, Ti(IV)bu formed the most efficientSb2O3 produced amorphous prepolymers, whichcenter with 1,8-dihydroxyanthraquinone, and sul-contained lower amounts of D-lactic acid struc-phuric acid with 1,4-dihydroxyanthraquinone. Intures than DBTL prepolymers. Both titaniumother words, the structure of the diol and catalystcompounds produced almost equal molecularhad a big effect on the properties of the final pre-weight prepolymers with every diol tested. Onlypolymers.1,8-dihydroxyanthraquinone produced a different

kind of chain structure with Ti(IV)bu and Ti-(IV)iso because the one prepolymer had a high REFERENCEScrystallinity and the other showed only a slight

1. K. Enomoto, U.S. Pat. 5,310,865 (1994).crystallinity. Sulphuric acid produced a very2. E. M. Filachione and C. H. Fisher, Ind. Eng. Chem.,high-molecular-weight prepolymer with aliphatic

36, 223 (1944).2-ethyl-1,3-hexanediol; with aromatic diols, it3. K. Hiltunen, J. V. Seppala, and M. Harkonen, J.produced quite good molecular weights, except

Appl. Polym. Sci., 63, 1091 (1997).with 1,8-dihydroxyanthraquinone. High-molecu-4. K. Hiltunen, J. V. Seppala, and M. Harkonen, J.

lar-weight prepolymers also showed high crys- Appl. Polym. Sci., 64, 865 (1997).tallinity; and, according to 13C-NMR, they did not 5. K. Hiltunen, J. V. Seppala, M. Itavaara, and M.contain lactide and D-lactic acid structures. Harkonen, J. Environ. Polym. Deg., 5, 167 (1997).

Sulphuric acid and all diols tested except 1,8- 6. M. Harkonen, K. Hiltunen, M. Malin, and J. V. Sep-pala, J. Macromol. Sci., Pure Appl. Chem., A32,dihydroxyanthraquinone and Ti(IV)bu and 1,8-857 (1995).dihydroxyanthraquinone produced an ester in-

7. K. Hiltunen, J. V. Seppala, M. Harkonen, and T.terchange mechanism where the possible esterVaananen, Macromolecules, 29, 8677 (1996).interchange reactions happen mainly through the

8. K. Hiltunen, J. V. Seppala, and M. Harkonen, Mac-breaking of the oxygen–carbonyl bond. This romolecules, 30, 373 (1997).mechanism leads to optically pure polyester 9. K. Hiltunen and J. V. Seppala, J. Appl. Polym. Sci.,chains. All other catalyst–diol systems seem to submitted.catalyze ester interchange reactions through the 10. J. W. Leenslag, S. Gogolewski, and A. J. Pennings,

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the synthesis of lactic-acid-based telechelic prepolymers

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