This is the author version published as: This is the accepted version of this article. To be published as : This is the author version published as:

QUT Digital Repository: http://eprints.qut.edu.au/

George, Karina A. and Schue, Francois and Chirila, Traian and Wentrup-Byrne, Edeline (2009) Synthesis of 4-arm star poly(L-Lactide) oligomers using an in situ-generated calcium-based initiator. Journal of Polymer Science Part A: Polymer Chemistry, 47(18). pp. 4736-4748.

Copyright 2009 Wiley-Blackwell

1

Synthesis of 4-Arm Star Poly(L-Lactide) Oligomers Using an

In Situ-Generated Calcium-Based Initiator

Karina A. George,1 François Schué,2 Traian V. Chirila,1,3,4 and Edeline Wentrup-

Byrne1,5

1 School of Physical and Chemical Sciences, Queensland University of Technology, 2

George St., Brisbane, Queensland 4001 Australia

2 Organisation Moleculaire Evolution et Materiaux Fluores, UMR CNRS 5073 Université

Montpellier 2, France;

3 Queensland Eye Institute, 41 Annerley Road, South Brisbane, Queensland 4101,

Australia

4 Australian Institute for Bioengineering and Nanotechnology (AIBN), University of

Queensland, St. Lucia, Brisbane, Queensland 4072, Australia

5 Tissue Repair and Regeneration Research Program, Institute for Health and Biomedical

Innovation (IHBI), Queensland University of Technology, 2 George St, Brisbane,

Queensland 4001, Australia

Correspondence to: E.Wentrup-Byrne (E-mail: e.wentrupbyrne@qut.edu.au)

2

ABSTRACT: Using an in situ-generated calcium-based initiating species derived from

pentaerythritol, the bulk synthesis of well-defined 4-arm star poly(L-lactide) oligomers

has been studied in detail. The substitution of the traditional initiator, stannous octoate

with calcium hydride allowed the synthesis of oligomers that had both low PDIs and a

comparable number of polymeric arms (3.7 – 3.9) to oligomers of similar molecular

weight. Investigations into the degree of control observed during the course of the

polymerization found that the insolubility of pentaerythritol in molten L-lactide resulted

in an uncontrolled polymerization only when the feed mole ratio of L-lactide to

pentaerythritol was 13. At feed ratios of 40 and greater, a pseudo-living polymerization

was observed. As part of this study, in situ FT-Raman spectroscopy was demonstrated to

be a suitable method to monitor the kinetics of the ring-opening polymerization (ROP) of

lactide. The advantages of using this technique rather than FT-IR-ATR and 1 H NMR for

monitoring L-lactide consumption during polymerization are discussed.

Keywords: poly(L-lactide); star polymers; Raman spectroscopy; kinetics (polym.);

heterogeneous polymerization

INTRODUCTION

Poly(α-esters), including: poly(L-lactide) (PLLA), polyglycolide (PGA), poly(ε-

caprolactone) (PCL) and their copolymers have been extensively studied for use in a

wide range of biomedical devices. Early applications such as surgical sutures have lead to

the development of more sophisticated devices, which include bioresorbable plates,

screws and scaffolds for use in bone repair and regeneration.1 Improved synthesis and

custom tailoring of the properties of poly(α-esters) are necessary for the continued

3

development and improvement of future products in both the biomedical arena2,3 and in

the emerging ‘green polymer’ field4. One of the many approaches used to manipulate the

physical, mechanical and chemical properties of poly(α-ester)s is to create polymers with

a range of architectures, i.e., block, branched, brush, star etc.5-10 The advantage of such an

approach is that the same building blocks and chemistry can be utilized to yield polymers

with tailored properties including a range of end-group concentrations and functionalities.

The lower melt viscosity of star and branched polymers compared to linear polymers of

the same molecular weight means that less thermo-mechanical degradation would be

expected during melt processing.11 Furthermore, the overall polymerization time can be

significantly decreased due to the greater initiator to monomer ratio employed.12

Several papers report the synthesis of 3- to 10-arm star polymers by a ‘core-first’

approach using polyol molecules, such as 1,1,1-tris(hydroxymethyl)propane13,

pentaerythritol12-14, dipentaerythritol and its derivatives15, natural sugars6,

polyglycerines12 etc. as co-initiators together with stannous octoate as initiator. This

approach involves the ring-opening polymerization (ROP) of a cyclic monomer or dimer,

which, under optimized reaction conditions, can proceed as a controlled or living

reaction. Kricheldorf et al.13 used MALDI-TOF mass spectroscopy to study the

transesterification reactions that occur during the polymerization of L-lactide using

stannous octoate and pentaerythritol. No evidence of intramolecular transesterification

(back-biting), i.e. the presence of cyclic oligolactides, were observed for this system.

However, a small fraction of linear oligolactides were identified. This species was

attributed to the presence of linear oligolactide impurities in the recrystallized L-lactide.

4

Structural analysis of star PLLA polymers synthesized using the ‘core-first’ approach

has revealed that in many cases complete conversion of co-initiator hydroxyl groups does

not occur. Kim et al.14 report that when pentaerythritol and stannous octoate were

employed for a study of L-lactide polymerization not all the 4 pentaerythritol hydroxyl

groups were converted into polymeric arms until the feed mole ratio of L-lactide to

pentaerythritol was 32 or greater. This effect was attributed to steric hindrance around the

core. In a study by Korhonen et al.12, secondary hydroxyl groups of the core molecule

were associated with slower initiation activity compared to primary alcohol groups. Their

study using poly(glycerol) 6 and poly(glycerol) 10 co-initiators implied that the

incomplete reaction of the secondary hydroxyl groups resulted in an overall increase in

experimental chain length, relative to the theoretical chain length, of the polymers

obtained.

Although Biela et al.15 have made significant advances in the characterization of star

poly(L-lactide)s using liquid chromatography at critical conditions, a dedicated study to

the understanding of the synthetic complexities resulting from using an insoluble co-

initiator has not been identified. Considering that many of the reported poly(lactide) star

synthesis are heterogeneous, i.e. when pentaerythritol is employed as the co-initiator in

the bulk polymerization of L-lactide, an understanding of the associated limitations and

phenomena that occur during the reaction need to be established. Such knowledge would

enable the ability to predict the co-initiator requirements necessary for the controlled

synthesis of poly(lactide)s of complex architecture using the ‘core-first’ approach.

Stannous octoate has traditionally been used as an initiator for the living ROP of

poly(α-esters) and has been approved by the U. S. Food and Drug Administration (FDA).

5

However, concerns have been raised in the literature regarding the potential health issues

associated with the toxicity of tin-based residues. Appel16 and Nakanishi17 have published

recent reviews on this topic. Consequently, a number of alternative non-toxic

catalysts/initiators have been investigated including organic18-24 and enzymatic25-27

systems. Of particular interest is the development catalysts/initiators from non-toxic

metal, such as potassium,28-30 lithium,31-33 zinc,34-52 magnesium,24,31,42-45,47,53-57 iron58-62

and calcium36,37,63-71 because these metals are known to take part in normal metabolic

processes. In this study we use a calcium alkoxide initiator, formed in situ from the

reaction of an alcohol with calcium hydride to catalyze the ROP of L-lactide.

Pentaerythritol is employed as the co-initiator to produce well-defined 4-arm star PLLA

polymers. This approach for generating an in situ calcium alkoxide initiator has

previously been reported by Li et al.37 and Rashkov et al.36 for the synthesis of PLLA-

PEG-PLLA triblock copolymers. Their studies showed a linear relationship between nM

and the polymerization time. However, compared to polymers produced using a zinc

metal initiator, their polymers had not only greater PDI values but also had undergone a

greater degree of racemization. No recent literature reports have been identified that

investigate the use of calcium hydride for the ROP of cyclic esters but it constitutes part

of a larger polymer synthesis project which is also exploring the potential of this initiator

for the polymerization of ε-caprolactone have been undertaken.72,73

The second objective of this work is to demonstrate the use of in situ FT-Raman

spectroscopy as an accurate and routine method to quantitatively monitor the conversion

of monomer to polymer during polymerization. There are several reports of the use of

FT-IR spectroscopy to monitor the polymerization of L-lactide or determine the

6

concentration of L-lactide in a polymer sample.74-76 However, these approaches are

complicated by the observed deviation from Beer’s Law and the absence of a suitable

reference band. Messman and Storey75 used an attenuated total reflectance (ATR) probe

to study the polymerization of rac-lactide in situ. The height of the lactide C-O-C

asymmetric vibration at 1240 cm -1 was used to determine the concentration of the lactide

remaining in the polymerization solution.75 This approach required a non-linear

calibration plot in order to produce reliable results. Since no reference band was used,

dependable data could only be obtained if calibration plots were constructed for each of

the systems studied. This would overcome any potential problems associated with

fluctuations in signal intensity caused by changes in the refractive index of the

polymerization solution and scattering of radiation during the polymerization. In a more

recent study, Braun et al.74 used the L-lactide C-CH 3 stretching vibration at 935 cm -1

band to determine the concentration of residual lactide in PLLA samples. Although they

used the 1454 cm -1 antisymmetric methyl deformation band as a reference, deviation

from a linear relationship between conversion and normalized peak area was observed as

a consequence of the fact that at conversions below 75 % the reference band consisted of

two overlapping bands.

In this present study, the complementary vibrational spectroscopic technique, FT-

Raman, was investigated as an alternative and found to be ideal for monitoring

polymerization of L-lactide at 100 C. One major advantage of this method over the ATR

method is that analysis through standard glass vessels is possible, therefore no contact

between the polymerization mixture and a probe is necessary. A linear relationship

between conversion and the normalized peak area of the 603 cm -1 ring deformation

7

band77 was used to produce conversion-time plots that were found to be in excellent

agreement with data obtained using 1 H NMR spectroscopy.

Overall, this study contributes to developing solutions to two important challenges

faced in the development of commercially-viable PLA polymers: the ability to produce

tailored biodegradable systems using reproducible synthetic protocols and the improved

polymer production afforded by the use of on-line user-friendly monitoring systems.

EXPERIMENTAL:

Materials. Calcium hydride (99.99 %), L-lactide (98 %), pentaerythritol (≥ 99 %) and

d-chloroform (99.8 atom % D) were purchased from Sigma-Aldrich. L-lactide was

recrystallized from toluene and then sublimed under vacuum. Pentaerythritol was purified

by sublimation under vacuum. All reagents were stored under an argon atmosphere in a

drybox. The L-lactide was used within 2 weeks of purification. Toluene, chloroform,

dichloromethane and n-hexane were all A.R. grade and purchased from Ajax Finechem

except for dichloromethane which was purchased from Australian Chemical Refiners

(ACR).

Polymerization. A typical polymerization involved the addition of predetermined

quantities of L-lactide (LL) and pentaerythritol (PE) to freshly ground calcium hydride

under an argon atmosphere in a dry box. A CaH 2 / PE ratio of 2 was used for all reactions

and a LL / PE ratio of 13, 40 or 66.7 was used to produce polymer with theoretical

molecular weights of 2000, 5900 or 9700 g / mol. The polymerization vessels were

closed with a Teflon stopper before flame-sealing under vacuum. The polymerization was

performed either by full immersion of the vessel in a preheated oil bath or, for the in situ

8

FT-Raman experiments, by placing the entire vessel in a preheated aluminum block. The

temperature was maintained at 100 °C throughout all reactions.

Reaction vessels were removed from the heating medium at set times, rapidly cooled in

liquid nitrogen and the contents dissolved in chloroform. Once gas evolution had ceased

and the polymer was fully dissolved, the solution was filtered and the solvent removed by

rotary evaporation before being dried to constant weight under vacuum. The crude

polymer was purified by repeated dissolution in dichloromethane and precipitation in n-

hexane until no evidence of L-lactide could be detected by 1 H NMR. The purified

polymers were stored under vacuum.

Characterization

FT-Raman Spectroscopy. A Perkin-Elmer System 2000 NIR FT-Raman spectrometer

was used for the in situ monitoring of the lactide polymerization and for obtaining

reference spectra. Samples were placed in the preheated, thermostated aluminum heating

block (with magnetic stirrer) that was mounted on the moveable sample stage within the

sample compartment. Spectra were collected every 10 minutes over a 24 hour period

using a macro generated with Macro Mania, version 5.0. All spectra were collected in the

range 200 - 3800 cm -1 with 64 co-added scans at 8 cm -1 resolution. The laser power used

depended on the calcium hydride content and was typically in the 100 – 250 mW range.

Reference spectra were collected using the same set-up at either 100 °C or room

temperature.

MALLS-GPC. A Shimadzu system with a Wyatt DAWN EOS multiangle laser light

scattering detector (690 nm, 30 mW) and a Wyatt OPTILAB DSP interferometric

9

refractometer (690 nm) were used for MALLS-GPC analysis. HPLC grade THF was used

as the eluent with three Phenomenex phenogel columns (500, 10 4 and 10 6

Å porosity; 5

μm bead size) operated at 1 mL / min with column temperature set at 30 °C. Astra

software (Wyatt Technology Corp.) was used to process the data using known dn / dc

values to determine the molecular weight or an assumption of 100 % mass recovery of

the polymer where the dn / dc value was unknown.

NMR Spectroscopy. Samples were prepared at a concentration of approximately 0.5 w

/ v % in CDCl 3. All spectra were measured on a Bruker Avance FT-NMR spectrometer

(9.39 Tesla, 400.162 MHz for 1 H and 100.631 MHz for 13 C). All spectra were referenced

to TMS using the CHCl 3 residual resonances at 7.26 ppm (for 1 H NMR) and 77.0 ppm

(for 13 C NMR) as an internal calibration.

FT-IR-ATR Spectroscopy. FT-IR-ATR spectra were collected using a Nicolet Nexus

spectrometer equipped with a Nicolet Smart Endurance single bounce diamond ATR

accessory. Each spectrum consisted of 64 co-added scans obtained over the 4000 cm -1 to

525 cm -1 region at a resolution of 4 cm -1.

Raman Microspectroscopy. A Renishaw InVia Raman microscope equipped with a

Leica microscope and a frequency-doubled, diode-pumped Nd-YAG laser ( = 532 nm)

was used. Spectra were acquired in the 4000 – 4300 cm -1 range using a long working

distance × 50 objective, a laser power of 120 mW and 8 spectral accumulations at 60 s

per spectrum. Grams/32 AI (version 6.00) software was used for spectral analysis.

RESULTS AND DISCUSSION:

10

Synthesis of Hydroxyl-Terminated 4-Arm Star PLLA Oligomers. The results

summarized in Table 1 demonstrate that by strictly controlling the reaction time, three

different molecular weight oligomeric star PLLA species were successfully synthesized

using the in situ-generated calcium alkoxide of pentaerythritol. The integral of the

polymer and monomer methine proton resonances in the 1 H NMR spectra of the crude

polymerization mixtures indicate that conversions between 98 and 100 % were achieved

in all cases. These polymers were shown to possess well-defined structures with PDI

values of 1.03 to 1.07. nM values were determined by both MALLS-GPC and 1 H NMR

and were found to be in close agreement with the theoretical values predicted from the

feed ratio. Conversion of pentaerythritol hydroxyl groups to polymeric arms, determined

from 1 H NMR spectroscopy, was close to the theoretical maximum of 4, i.e. all

pentaerythritol hydroxyl groups were consumed. The presence of very small bands

around 169.27 ppm in the 13 C NMR spectrum of sample C provides evidence that some

racemization may have occurred during the polymerization. However, quantification of

the degree of heterotacticity was not possible due to the structural complexity of the star

polymer and 13 C NMR spectrum.

Using eqs 1 - 3, nM and the number of arms per molecule were calculated from the

integrated 1 H NMR spectra of the purified polymer. A typical 1 H NMR spectra of a

purified star polymer (sample B in Table 1) is presented in Figure 1. Resonance

assignment was made in accordance with the work of Korhonen et al.12 and Rashkov et

al. As L-lactide is a dimer, not a monomer, the 2 in the denominator of eq 2 is necessary

to calculate the average degree of polymerization of L-lactide units instead of L-lactic

acid units.

11

4d'd

d'arms of no.

(1)

a"2

''a'a"aDP arm n,

(2)

136144arms) of (no.DPM arm n,star n, (3)

The close agreement in the values of the integrals of the junction methine group (a’’)

and the end methine group (a’’’) in the 1 H NMR suggests that the polymeric arms are

terminated exclusively with hydroxyl groups (within the detection limits of 1 H NMR).

This conclusion is also supported by an absence of resonances of carboxylated lactyl end

groups at 4.9 – 5.0 ppm36 as well as the good agreement between the nM values

calculated by MALLS-GPC and 1 H NMR.

The results shown in Table 1 demonstrate that using the in situ-generated calcium

alkoxide initiator both conversion of L-lactide to polymer and the number of polymeric

arms per molecule were comparable to literature results for star PLLA polymers. The

former were synthesized using stannous octoate as initiator and either pentaerythritol12,14

or erythritol6 as co-initiator. The PDI values obtained in our current work were

significantly less than those reported for these other 4-arm star polymers.

Polymerization Scheme and Evidence of Initiator Formation. The proposed

reaction scheme for the formation of initiator and for the polymerization is shown in

Figure 2. This scheme was derived from a study by Zhong et al.65 for the polymerization

of cyclic esters using an in situ-generated calcium-based initiator derived from

bis(tetrahydrofuran)calcium bis[bis(trimethylsilyl)amide] and an alcohol.

When Li et al.37 and Rashkov et al.36 studied the synthesis of PLLA-PEO-PLLA

triblock polymers using calcium hydride as the initiator they assumed that the in situ-

12

generated calcium alkoxide was responsible for initiating polymerization. However, no

evidence was presented to support this assumption nor did they eliminate the possibility

that the presence of calcium hydride itself or trace impurities, such as calcium hydroxide,

were not, in fact, responsible for the initiation. In this study we have carried out control

experiments, where either one or both species were absent, to assess whether both

initiator and co-initiator are required for polymerization to proceed. For each control

experiment, the reagents were heated in vacuum-sealed tubes at 100 °C for 24 hours. No

evidence of any polymerization was detected by 1 H NMR for the systems containing

only L-lactide or both pentaerythritol and L-lactide. However, in the system containing L-

lactide and calcium hydride (no pentaerythritol), some polymerization was observed. The

molecular weight and PDI values of these products are given in Table 2. For these

reactions the conversion of L-lactide to polymer was relatively slow, compared to the

systems where both calcium hydride and pentaerythritol were present. An increase in the

concentration of calcium hydride in the reaction mixture resulted in an increase in

conversion of the L-lactide. No trend was evident between concentration of calcium

hydride and molecular weight of the polymer formed. The actual initiating species for

this polymerization remains unknown but one possible explanation is the presence of

trace impurities in the L-lactide. Water, lactic acid or lactoyllactic acid (dilactic acid)

could react with the calcium hydride to form potential calcium alkoxide initiating species.

As Kricheldorf et al.13 point out oligolactides (including lactoyllactic acid) are most

probably present in recrystallized L-lactide in trace quantities and these impurities can act

as co-initiators. In this study, although the L-lactide was sublimed after recrystalization,

the presence of trace quantities of lactoyllactic acid in the purified L-lactide cannot be

13

completely ruled out. The 1 H NMR spectra of the crude polymer mixtures provide some

evidence for this hypothesis: small quartets were observed at ~ 4.9 ppm suggestive of

carboxylated lactyl end-groups. Another possibility is the presence of trace quantities of

metal-based impurities in the calcium hydride. However, since unimodal GPC traces

were obtained from polymers synthesized in the presence of both calcium hydride and

pentaerythritol, the contribution of this much slower reaction was assumed to be

insignificant.

Although direct evidence for the formation of calcium alkoxide proved elusive, indirect

evidence based on the identification of the gas produced from a heated mixture of

pentaerythritol and calcium hydride (sealed under vacuum) as hydrogen was possible

using Raman microspectroscopy. Figure 3 shows the 4 bands observed in the 4125 –

4165 cm -1 region that are characteristic of the hydrogen Q band splitting.78 These bands

were observed in the head-space of sealed tubes which contained all three reagents (in the

quantities given for the synthesis of 2000 and 5900 g / mol polymers) and had been

heated at 100 °C for at least 4 hours. The H2 bands were not observed in tubes that did

not contain both pentaerythritol and calcium hydride, even though polymerization was

observed in tubes that contained only L-lactide and calcium hydride. This last

observation is consistent with impurities initiating the polymerization, as the amount of

H2 gas evolved is below the detection limit of the technique.

Due to the impracticality of introducing a suitable reference gas, quantitative

determination of the hydrogen gas produced proved impossible in this case.

Consequently, the concentration of calcium alkoxide in the reaction mixture also

remained unestimated. However, the observation that hydrogen gas evolved during the

14

work-up is strong evidence that unreacted calcium hydride was still present at the end of

the reaction and thus the molar concentration of calcium alkoxide present must have been

less than the initial molar concentration of calcium hydride, even though stoichiometric

quantities of reagents were used. However, the synthesis of polymers with both low PDI

values and with molecular weights that were comparable to their theoretical values,

confirms that reversible transfer between hydroxyl groups and alkoxide groups must

occur at a rate that is at least comparable to the rate of propagation.

Since the reaction between calcium hydride and alcohols is favorable and very fast, it

can be assumed that, provided the hydroxyl groups on the pentaerythritol are accessible,

the formation of calcium alkoxide should occur rapidly. Assuming that there was no

significant change in reaction rate when the ratio of reagent quantity to vessel size was

changed means that the formation of alkoxide species is not limited by the pressure of

hydrogen gas inside the tube. However, because the system is heterophasic and relies on

the reaction of two solids in a molten solution, the incomplete consumption of calcium

hydride may, to some extent, be due to the physical restraints of the system preventing

reaction of all calcium hydride molecules with the pentaerythritol molecules.

Evaluation of Controlled Nature of Polymerization using FT-Raman and 1 H

NMR. The use of in situ FT-Raman to monitor the progress of polymerizations in real-

time is routinely used for free radical polymerizations. However, the use of this technique

has not yet been published for the ROP of lactide, or other cyclic esters, although FT-IR

spectroscopy has been used to study the polymerization of L-lactide in situ.27,75 Among

the many drawbacks and limitations of this FT-IR approach are: overlapping spectral

bands that cause a non-linear relationship between the measured intensity / area and the

15

concentration of L-lactide; the necessary use of an ATR probe to circumvent the IR

absorbance of glass; a potential change in atmosphere in the reaction vessel thus limiting

the use of vacuum flame-sealed tubes. Consequently, septums and / or alternative

atmospheres would be needed to prevent water from entering. The use of FT-Raman

spectroscopy avoids these limitations and produces comparable spectral information.

Studies in our laboratory have confirmed that this technique is ideal for monitoring ε-

caprolactone polymerization.72,73

Figure 4 shows the FT-Raman spectra of the three reactants, and purified star polymer

of nM = 9700 g / mol (sample C in Table 1). In order to use the spectra to quantify the

concentration of L-lactide in the reaction mixture during the polymerization, a suitable

unique band of either L-lactide or poly(L-lactide) need to be identified. The spectra in

Figure 4 show that the L-lactide ring breathing vibration at 603 cm -1 does not overlap

with any vibrational bands of the other reactants or purified product. Although the L-

lactide band at 656 cm -1 is more intense than the 603 cm -1 band it could not be used due

to the presence of small bands between 680 and 630 cm -1 in the spectra of the purified

polymer. To determine the concentration of L-lactide, a linear relationship between the

area of the 603 cm -1 band and the proportion of L-lactide was used. To eliminate any

effects caused by laser power fluctuations and changes in scattering during the course of

the reaction, the data was normalized using the area of the CH 2 and CH 3 deformation

band at 1454 cm -1.

Although Braun et al.74 used the same reference band to determine the lactide

concentration of PLLA samples using FTIR-ATR and reported changes in both the peak

shape and maximum with L-lactide concentration, we encountered no such problems in

16

this study. This was attributed to neither polymer or monomer being present in a

crystalline form during the polymerization. The absence of the characteristic crystalline

PLLA bands at 926 and 510 cm -1 79 throughout the course of all the polymerization

reactions studied was evidence that, even at the low reaction temperature of 100 C,

neither the polymer or the cyclic dimer were crystalline. Although the reference band

does overlap with the C-H scissor vibration of pentaerythritol,80 the relative concentration

of combined L-lactide and PLLA to pentaerythritol, and the respective peak intensities of

these bands, any effect caused by this overlap is assumed to be negligible.

Conversion-time plots were constructed for the three systems studied using eq 4.

t,1454cm

t,656cm

1

1

Area

Area(f) Conversion

(4)

For the t = 0 data, the FT-Raman spectra were obtained from reaction mixtures that were

in every other way identical to those used in the polymerizations except they did not

contain calcium hydride. These ‘t = 0 tubes’ were allowed to equilibrate at 100 C before

spectra were measured using identical conditions to the polymerizations.

Figure 5 shows the conversion versus time plots obtained using both the in situ FT-

Raman method and 1 H NMR measurements for the synthesis of star PLLA polymers

with feed LL / PE ratios of 13, 40 and 66.7. In addition to the excellent agreement

between these two techniques, the application of this in situ technique requires minimal

user input for the collection of kinetic data at narrow time intervals (in this case very 10

minutes) makes in significantly advantageous over more conventional off-line methods

such as 1 H NMR spectroscopy, GPC, or GC-MS.

17

The first-order kinetic plots shown in Figure 6 were derived from the in situ FT-Raman

spectroscopy data. For the three systems studied, a linear relationship was found to exist

between ln (Mo / Mt) and time (until 80 % conversion), consistent with a first-order

polymerization process. As termination reactions are believed to be uncommon in these

systems, the presence of the linear (steady-state) region implies that the calcium hydride

participated in the process only for a very short time at the beginning of the

polymerization. Once a critical amount of alkoxide was formed, the calcium hydride

ceased to be involved in any further reaction that could cause an increase in the number

of calcium alkoxide groups (initiating species). This conclusion is strongly supported by

the presence of unreacted calcium hydride even after full conversion of L-lactide, as

discussed earlier, by the controlled polymerizations for two out of the three systems

studied (see below), as well as by the short induction period observed for the system that

containing the lowest concentration of pentaerythritol and calcium hydride.

Effect of the Heterophasic System on Polymerization: Insolubility of Initiator and

Co-initiator. Plots of nM versus conversion are shown in Figure 7. These plots show

that for LL / PE ratios of 40 and 66.7, a linear relationship between nM and conversion

exists that agrees well with the theoretical linear relationship that is representative of a

living or controlled system. However, for the system with a LL / PE ratio of 13, the nM

versus conversion plot was non-linear and the experimental nM was consistently greater

than the theoretical nM until after full conversion of L-lactide was reached.

The nonlinearity of the nM versus conversion plot for this system was attributed to the

insolubility of pentaerythritol in molten L-lactide and PLLA. Figure 8 shows a plot of the

18

molar ratio of L-lactide units to chloroform-soluble pentaerythritol units in the

polymerization mixture after different reaction times. The molar ratio was obtained from

the integrals of the lactide and polylactide methine resonances and pentaerythritol

methylene resonances in the 1 H NMR spectra of the crude polymers. Because

pentaerythritol is insoluble in chloroform, it was assumed that only those pentaerythritol

molecules that had reacted with L-lactide are accounted for in this plot. The absence of a

carbonyl stretching band in the FT-IR-ATR of the chloroform-insoluble particles was

used to confirm that all lactic acid groups were accounted for in the 1 H NMR analysis.

The plot shows that for the polymerizations with feed LL / PE ratios of 40 and 66.7, the

ratio of L-lactide units to pentaerythritol was initially greater than the feed ratio of the two

components by ~ 20 %. However, within 10 minutes this ratio had decreased to the feed

ratio. For the system with an added LL / PE ratio of 13, the concentration of

pentaerythritol in the polymer remained below the feed ratio by ~ 50 – 100 % throughout

the polymerization. Only after 250 minutes, five times the time taken to reach full

conversion of L-lactide, did the ratio of L-lactide to pentaerythritol became equivalent to

the feed ratio. These results strongly suggest that the insolubility of the pentaerythritol

only affects the polymerization in the very early stages of the reaction when the feed LL /

PE ratios are 40 or greater. However, for systems with smaller feed ratios, the greater

concentration of pentaerythritol and the rapid increase in solution viscosity with the

consumption of L-lactide resulted in a slow incorporation of the pentaerythritol into the

polymer.

The nM values calculated from MALLS-GPC versus conversion showed this same

trend that was observed in the 1H NMR study (Figure 7). The synthesis of the two star

19

polymers with the largest molecular weight have PDI values above 1.09 in the first half

of the polymerization which decrease as the polymerization continues to values of 1.03 to

1.06. Unimodal molecular weight distributions were observed for all GPC traces for these

two systems. These observations are consistent with literature studies of linear PLLA by

Zhong et al.66 and complete reaction of pentaerythritol occurred in the early stages of the

reaction. In contrast, the system with LL/PE ratio of 13 displayed a substantial increase in

PDI after full conversion of monomer was reached. The increase in PDI is followed by

the development of a bimodal molecular weight distribution and a decrease in the average

molecular weight of the polymer. These observations are consistent with the

interpretation of the 1H NMR data and suggest that the incorporation of pentaerythritol

into the polymer after full conversion of L-lactide had occurred was via

transesterification.

Both methods of analysis showed that for the system with a LL/PE ratio of 13 the

controlled nature of the polymerization was lost even though at first glance this appears

contradictory to the narrow PDI values reported. Szymanski81 notes that the measured

PDI of star polymers are not a measure of the actual polydispersity of the polymeric arms

themselves, which is typically much greater. Although it is possible to estimate the PDI

of the polymeric arms based on that of the star polymer, the assumption that all arms are

initiated at the same time is necessary. Consequently, this method cannot be applied to

our star polymers.

CONCLUSIONS

20

An in situ-generated calcium alkoxide of pentaerythritol has been successfully used to

synthesize star PLLA oligomers with hydroxyl end groups in a controlled polymerization.

The generation of hydrogen gas confirmed the formation of the alkoxide. However,

Raman microspectroscopy studies revealed that only a fraction of the available calcium

hydride actually reacted with the pentaerythritol. Compared to star PLLA polymers of

similar molecular weight synthesized using stannous octoate and pentaerythritol or a

tetraol molecule as co-initiator, the polymers synthesized in this study exhibit lower PDI

vales and a similar number of polymeric arms and exhibit lower PDI values.

The synthesis of star polymers with feed LL / PE ratios of 40 and 66.7 proceeded in a

controlled, pseudo-living fashion with a linear increase in nM as a function of time.

Appreciable loss of control resulting from the insolubility of pentaerythritol during

polymerization was only observed when the feed LL / PE ratio was 13. For this particular

system, polymers of a nM higher than predicted were obtained due to incomplete

incorporation of pentaerythritol during the polymerization.

In situ FT-Raman spectroscopy was both effective and efficient for collecting kinetic

data for the ROP of L-lactide. The resulting first-order kinetic plots provided evidence

that the calcium alkoxide was generated only in the initial stages of the polymerization.

ACKNOWLEDGEMENTS

The authors thank Dr Llewellyn Rintoul, Prof Graeme George and Dr John Colwell for

useful discussions and advice on instrumental analysis and Dr James Wiltshire and Assoc

Prof Greg Qiao for the MALLS-GPC analysis.

21

REFERENCES

1. Albertsson, A.-C.; Varma, I. K. Biomacromolecules 2003, 4, 1466-1486.

2. Ouchi, T.; Ohya, Y. Polym Prepr 2002, 43, 648-649.

3. Wentrup-Byrne, E.; George, K. In Biomaterials and Regenerative Medicine in

Opthalmology; Chirila, T. V., Ed.; Woodhead Publishing: Cambridge, UK, in press.

4. Williams, C. K.; Hillmyer, M. A. Polym Rev 2008, 48, 1-10.

5. Finne, A.; Albertsson, A.-C. Biomacromolecules 2002, 3, 684-690.

6. Hao, Q.; Li, F.; Li, Q.; Li, Y.; Jia, L.; Yang, J.; Fang, Q.; Cao, A. Biomacromolecules

2005, 6, 2236-2247.

7. Tasaka, F.; Ohya, Y.; Ouchi, T. Macromolecules 2001, 34, 5494-5500.

8. Atthoff, B.; Trollsås, M.; Claesson, H.; Hedrick, J. L. Macromol Chem Phys 1999,

200, 1333-1339.

9. McKee, M. G.; Unal, S.; Wilkes, G. L.; Long, T. E. Prog Polym Sci 2005, 30, 507-539.

10. Zhang, W.; Zheng, S. Polym Bull 2007, 58, 767-775.

11. Kim, S. H.; Han, Y.-K.; Ahn, K.-D.; Kim, Y. H.; Chang, T. Makromol Chem 1993,

194, 3229-3236.

12. Korhonen, H.; Helminen, A.; Seppälä, J. V. Polymer 2001, 42, 7541-7549.

13. Kricheldorf, H. R.; Ahrensdorf, K.; Rost, S. Macromol Chem Phys 2004, 205, 1602-

1610.

14. Kim, S. H.; Han, Y.-K.; Kim, Y. H.; Hong, S. I. Makromol Chem 1992, 193, 1623-

1631.

15. Biela, T.; Duda, A.; Pasch, H.; Rode, K. J Polym Sci Part A: Polym Chem 2005, 43,

6116-6133.

22

16. Appel, K. E. Drug Metab Rev 2004, 36, 763-786.

17. Nakanishi, T. Journal of Health Science 2007, 53, 1-9.

18. Wang, C.; Li, H.; Zhao, X. Biomaterials 2004, 25, 5797-5801.

19. Coulembier, O.; Lohmeijer, B. G. G.; Dove, A. P.; Pratt, R. C.; Mespouille, L.;

Culkin, D. A.; Benight, S. J.; Dubois, P.; Waymouth, R. M.; Hedrick, J. L.

Macromolecules 2006, 39, 5617-5628.

20. Myers, M.; Connor, E. F.; Glauser, T.; Möck, A.; Nyce, G.; Hedrick, J. L. J Polym

Sci Part A: Polym Chem 2002, 40, 844-851.

21. Nederberg, F.; Lohmeijer, B. G. G.; Leibfarth, F.; Pratt, R. C.; Choi, J.; Dove, A. P.;

Waymouth, R. M.; Hedrick, J. L. Biomacromolecules 2007, 8, 153-160.

22. Lohmeijer, B. G. G.; Pratt, R. C.; Leibfarth, F.; Logan, J. W.; Long, D. A.; Dove, A.

P.; Nederberg, F.; Choi, J.; Wade, C.; Waymouth, R. M.; Hedrick, J. L. Macromolecules

2006, 39, 8574-8583.

23. Zhang, L.; Nederberg, F.; Pratt, R. C.; Waymouth, R. M.; Hedrick, J. L.; Wade, C. G.

Macromolecules 2007, 40, 4154-4158.

24. Tang, H.-Y.; Chen, H.-Y.; Huang, J.-H.; Lin, C.-C. Macromolecules 2007, 40, 8855-

8860.

25. Sinnwell, S.; Ritter, H. J Macromol Sci Part A Pure Appl Chem 2007, 44, 1155-1160.

26. Numata, K.; Srivastava, R. K.; Finne-Wistrand, A.; Albertsson, A. C.; Doi, Y.; Abe,

H. Biomacromolecules 2007, 8, 3115-3125.

27. Namekawa, S.; Uyama, H.; Kobayashi, S.; Kricheldorf, H. R. Macromol Chem Phys

2000, 201, 261-264.

23

28. Lemmouchi, Y.; Perry, M. C.; Amass, A. J.; Chakraborty, K.; Schacht, E. J Polym Sci

Part A: Polym Chem 2008, 46, 5348-5362.

29. Lemmouchi, Y.; Perry, M. C.; Amass, A. J.; Chakraborty, K.; Schacht, E. J Polym Sci

Part A: Polym Chem 2007, 45, 3966-3974.

30. Lemmouchi, Y.; Perry, M. C.; Amass, A. J.; Chakraborty, K.; Schué, F. J Polym Sci

Part A: Polym Chem 2007, 45, 2235-2245.

31. Dobrzynski, P.; Kasperczyk, J.; Jelonek, K.; Ryba, M.; Walski, M.; Bero, M. J

Biomed Mater Res Part A 2006, 79, 865-873.

32. Hsueh, M.-L.; Huang, B.-H.; Wu, J.; Lin, C.-C. Macromolecules 2005, 38, 9482-

9487.

33. Ko, B.-T.; Lin, C.-C. J Am Chem Soc 2001, 123, 7973-7977.

34. Schwach, G.; Coudane, J.; Engel, R.; Vert, M. Polym Int 1998, 46, 177-182.

35. Schwach, G.; Coudane, J.; Engel, R.; Vert, M. Biomaterials 2002, 23, 993-1002.

36. Rashkov, I.; Manolova, N.; Li, S. M.; Espartero, J. L.; Vert, M. Macromolecules

1996, 29, 50-56.

37. Li, S. M.; Rashkov, I.; Espartero, J. L.; Manolova, N.; Vert, M. Macromolecules

1996, 29, 57-62.

38. Hiemstra, C.; Zhong, Z.; Li, L.; Dijkstra, P. J.; Feijen, J. Biomacromolecules 2006, 7,

2790 - 2795.

39. Kricheldorf, H. R.; Kreiser-Saunders, I.; Damrau, D.-O. Macromol Symp 2000, 159,

247-257.

40. Williams, C. K.; Breyfogle, L. E.; Choi, S. K.; Nam, W.; Young, V. G., Jr.; Hillmyer,

M. A.; Tolman, W. B. J Am Chem Soc 2003, 125, 11350-11359.

24

41. Huang, M. H.; Coudane, J.; Li, S.; Vert, M. J Polym Sci Part A: Polym Chem 2005,

43, 4196-4205.

42. Breyfogle, L. E.; Williams, C. K.; Young, V. G., Jr.; Hillmyer, M. A.; Tolman, W. B.

J Chem Soc, Dalton Trans 2006, 928-936.

43. Chisholm, M. H.; Eilerts, N. W.; Huffman, J. C.; Iyer, S. S.; Pacold, M.; Phomphrai,

K. J Am Chem Soc 2000, 122, 11845-11854.

44. Chisholm, M. H.; Gallucci, J.; Phomphrai, K. Inorg Chem 2002, 41, 2785-2794.

45. Chisholm, M. H.; Huffman, J. C.; Phomphrai, K. J Chem Soc, Dalton Trans 2001,

222-224.

46. Huang, B.-H.; Lin, C.-N.; Hsueh, M.-L.; Athar, T.; Lin, C.-C. Polymer 2006, 47,

6622-6629.

47. Wu, J.-C.; Huang, B.-H.; Hsueh, M.-L.; Lai, S.-L.; Lin, C.-C. Polymer 2005, 46,

9784-9792.

48. Chen, H.-Y.; Huang, B.-H.; Lin, C.-C. Macromolecules 2005, 38, 5400-5405.

49. Chisholm, M. H.; Gallucci, J. C.; Zhen, H.; Huffman, J. C. Inorg Chem 2001, 40,

5051-5054.

50. Chisholm, M. H.; Lin, C.-C.; Gallucci, J. C.; Ko, B.-T. J Chem Soc, Dalton Trans

2003, 406-412.

51. Cheng, M.; Attygalle, A. B.; Lobkovsky, E. B.; Coates, G. W. J Am Chem Soc 1999,

121, 11583-11584.

52. Hung, W.-C.; Huang, Y.; Lin, C.-C. J Polym Sci Part A: Polym Chem 2008, 46,

6466-6476.

25

53. Wu, J.-C.; Huang, B.-H.; Hsueh, M.-L.; Lai, S.-L.; Lin, C.-C. Polymer 2005, 46,

9784-9792.

54. Kricheldorf, H. R.; Damrau, D.-O. J Macromol Sci Part A Pure Appl Chem 1998, 35,

1875-1887.

55. Wei, Z.; Liu, L.; Yu, F.; Wang, P.; Qu, C.; Qi, M. Polym Bull 2008, 61, 407-413.

56. Yu, T.-L.; Wu, C.-C.; Chen, C.-C.; Huang, B.-H.; Wu, J.; Lin, C.-C. Polymer 2005,

46, 5909-5917.

57. Shueh, M.-L.; Wang, Y.-S.; Huang, B.-H.; Kuo, C.-Y.; Lin, C.-C. Macromolecules

2004, 37, 5155-5162.

58. Wang, X.; Liao, K.; Quan, D.; Wu, Q. Macromolecules 2005, 38, 4611-4617.

59. Gibson, V. C.; Marshall, E. L.; Navarro-Llobet, D.; White, A. J. P.; Williams, D. J. J

Chem Soc, Dalton Trans 2002, 4321-4322.

60. O'Keefe, B. J.; Monnier, S. M.; Hillmyer, M. A.; Tolman, W. B. J Am Chem Soc

2001, 123, 339-340.

61. O'Keefe, B. J.; Breyfogle, L. E.; Hillmyer, M. A.; Tolman, W. B. J Am Chem Soc

2002, 124, 4384-4393.

62. Stolt, M.; Sodergård, A. Macromolecules 1999, 32, 6412-6417.

63. Zhong, Z.; Schneiderbauer, S.; Dijkstra, P. J.; Westerhausen, M.; Feijen, J. Polym

Bull 2003, 51, 175-182.

64. Zhong, Z.; Schneiderbauer, S.; Dijkstra, P. J.; Westerhausen, M.; Feijen, J. J Polym

Enviro 2001, 9, 31-38.

65. Zhong, Z.; Dijkstra, P. J.; Birg, C.; Westerhausen, M.; Feijen, J. Macromolecules

2001, 34, 3863-3868.

26

66. Piao, L.; Sun, J.; Zhong, Z.; Liang, Q.; Chen, X.; Kim, J.-H.; Jing, X. J Appl Polym

Sci 2006, 102, 2654-2660.

67. Piao, L.; Deng, M.; Chen, X.; Jiang, L.; Jing, X. Polymer 2003, 44, 2331-2336.

68. Darensbourg, D. J.; Choi, W.; Karroonnirun, O.; Bhuvanesh, N. Macromolecules

2008, 41, 3493-3502.

69. Chen, H.-Y.; Tang, H.-Y.; Lin, C.-C. Polymer 2007, 48, 2257-2262.

70. Chisholm, M. H.; Gallucci, J.; Phomphrai, K. J Chem Soc, Chem Commun 2003, 48-

49.

71. Chisholm, M. H.; Gallucci, J. C.; Phomphrai, K. Inorg Chem 2004, 43, 6717-6725.

72. Colwell, J. M. Synthesis of Polycaprolactone Polymers for Bone Tissue Repair. Ph.

D. Thesis, Queensland University of Technology, Brisbane, Australia, November, 2007.

73. Khan, J. Synthesis and Characterization of Poly e-Caprolactone on functionalized

silica substrate. Ph.D. Thesis, Queensland University of Technology, Brisbane, Australia,

December, 2008.

74. Braun, B.; Dorgan, J. R.; Dee, S. F. Macromolecules 2006, 39, 9302-9310.

75. Messman, J. M.; Storey, R. F. J Polym Sci Part A: Polym Chem 2004, 42, 6238-6247.

76. Degée, P.; Dubois, P.; Jacobsen, S.; Fritz, H.-G.; Jérôme, R. J Polym Sci Part A:

Polym Chem 1999, 37, 2413-2420.

77. Tam, C. N.; Bour, P.; Keiderling, T. A. J Am Chem Soc 1996, 118, 10285-10293.

78. Rahn, L. A.; Rosasco, G. J. Phys Rev A 1990, 41, 3698-3706.

79. Tanaka, M.; Young, R. J. Macromolecules 2006, 39, 3312-3321.

80. Ramamoorthy, P.; Krishnamurthy, N. Spectrochim Acta Part A 1997, 53, 655-663.

81. Szymanski, R. Macromolecules 2002, 35, 8239-8242.

27

FIGURES:

Figure 1. 1 H NMR spectrum and peak assignment for the 4-arm star PLLA oligomer

(sample B in Table 1).

28

Figure 2. Proposed reaction scheme for the polymerization of L-lactide using calcium

hydride and pentaerythritol.

29

Figure 3. The 4050 – 4250 cm -1 region in the FT-Raman spectrum of the head-space of a

sealed tube containing stoichiometric amounts of pentaerythritol and calcium hydride

after heating for 1 hour at 100 C.

Figure 4. FT-Raman spectra of (from the top to bottom) L-lactide, pentaerythritol, star

PLLA (sample C in Table 1), and calcium hydride. All reagents were sealed in glass

under vacuum. The spectrum of the star PLLA polymer was obtained from a sample in an

open tube.

30

Figure 5. Plot of conversion versus time by in situ FT Raman (lines) and 1 H NMR

(points) with LL / PE ratios of 13 (bold line, triangles); 40 (solid line, circles); and 66.7

(dotted line, diamonds). FT-Raman spectra were collected at 10 minutes intervals.

Figure 6. First-order kinetic test plot for the synthesis of star PLLA polymers. Initial LL /

PE ratios of: 13.0 (triangles); 40.0 (circles); and 66.7 (diamonds). The dotted line

represents the value where 80 % of L-lactide has been consumed.

31

Figure 7. nM versus conversion for the synthesis of star PLLA polymers. Initial LL / PE

ratios of 13 (triangles); 40 (circles); and 66.7 (diamonds). Solid data points represent 1 H

NMR data and open data points represent MALLS-GPC data. The solid lines represent

the theoretical nM versus conversion plot based on a living polymerization scenario and

the dotted line represents the line of best fit for the polymerization with an initial LL / PE

ratio of 13. The PDI values of each MALLS-GPC measurement are given in brackets

along with the number of peaks identified in the GPC trace.

32

Figure 8. Molar ratio of lactide to pentaerythritol versus time calculated from 1 H NMR

data from crude polymer solutions with feed LL / PE ratios of: 13 (triangles); 40 (circles);

and 66.7 (diamonds). Solid lines represent the molar feed ratio of L-lactide to

pentaerythritol.

33

TABLES

Table 1

Core

Reaction Time

(min)

Mole ratio [LL]: [core]a

Theoretical

nM b

(g / mol)

nM c

(g / mol)

nM ; wM d

(g / mol)

Conversionc

(%)

No. of polymeric

arms c

PDId

Sample/ Ref.

Pentaerythritol

Pentaerythritol

Pentaerythritol

500

600

960

13

40

66.7

2000

5900

9700

2000

5900

10 100

2500; 2700

6000; 6200

9400; 9700

100

99

98

3.8

3.9

3.7

1.07

1.03

1.03

A

B

C

Erythritol

Erythritol

3000

e

40

59

6000

8600

5500

33 500

e

e

95.6

96.3

e

e

1.16

1.12

6f

Pentaerythritol

Pentaerythritol

3000

3000

16

32

2400

4900

e

e

e; 2340

e; 4600

e

e

3.8

4.0

e

e 13g

Pentaerythritol

Pentaerythritol

Pentaerythritol

180

180

180

8

12.5

20

1300

1900

3000

1700

2800

4000

e

e

e

98.1

97.8

97.9

3.5 – 3.8

3.5 – 3.9

3.4 – 3.8

1.3

1.2

1.4

11h

Table 2

Mole Ratio

[LL]:[CaH 2] a

Conversion (%) b

nM c

(g / mol)

No. of peaks

observed in GPC curve

PDI of main peak c

1.00 : 0.077 41 26,000 2 1.3

1.00 : 0.025d 21 - - -

1.00 : 0.015 16 7000 2 1.1

a Mole ratio of L-lactide to calcium hydride in the feed mixture

b Determined from 1 H NMR spectra of crude polymers

c Determined from MALLS-GPC

34

d Values could not be obtained for this sample due to its insolubility in THF, implying that the molecular weight of the polymer is greater than that observed for the other samples in this study

35

GRAPHICAL ABSTRACT:

The bulk polymerization of poly(L-lactide) stars where both the core molecule and

initiator are insoluble in molten L-lactide relies on the surface reaction of the insoluble

particles to create an alkoxide which reacts with L-lactide to produce a polymerizable

soluble species. However, the polymerization reaction can result in a situation, observed

only for small star polymers (theoretical molecular weight of 2000 g / mol), where the

maximum conversion of L-lactide occurs before complete reaction of the pentaerythritol

molecules. This results in greater molecular weights than predicted. In contrast, the

polymerization of larger stars proceeds via a pseudo-living mechanism.