Flexible and rigid core molecules in the synthesis of poly(lactic acid) star polymers

Article abstract of DOI:10.1139/V11-002

This study presents the synthesis and physical characterization of a series of structurally well-defined starshaped poly(lactic acids). Polymer stars are prepared from a series of multifunctional alcohol cores including flexible polyols pentaerythritol and dipentaerythritol (four-armed and six-armed cores, respectively) and rigid substituted arenes tri(hydroxymethyl) benzene and hexa(hydroxymethyl)benzene. Utilizing a tin(II) octanoate catalyst, arms of 10 monomer units long are built from rac-lactide and L-lactide to form atactic and isotactic star polymers. Polymers were subsequently characterized by means of NMR spectroscopy, gel permeation chromatography, differential scanning calorimetry, and thermogravimetric analysis. Our results support previous work that suggests that the length of the individual arms, not the total molecular weight, correlates to physical characteristics including glass, melt, crystallization, and decomposition temperatures. In addition, differences between core molecules suggest that the chemical nature of the core can significantly alter the physical properties of the star polymer. Trends in crystallization and glass transition temperatures relative to the core used merit further study and correlate closest to the molecular weight and the number of arms emanating from the star core. It is also clear that the rigidity provided by aromatic cores has a significant effect on the melting temperatures of these macromolecules.

Full text of DOI:10.1139/V11-002

499
Flexible and rigid core molecules in the synthesis
of poly(lactic acid) star polymers
Mitchell R. Perry and Michael P. Shaver
Abstract: This study presents the synthesis and physical characterization of a series of structurally well-defined star-
shaped poly(lactic acids). Polymer stars are prepared from a series of multifunctional alcohol cores including flexible poly-
ols pentaerythritol and dipentaerythritol (four-armed and six-armed cores, respectively) and rigid substituted arenes tri(hy-
droxymethyl)benzene and hexa(hydroxymethyl)benzene. Utilizing a tin(II) octanoate catalyst, arms of 10 monomer units
long are built from rac-lactide and L-lactide to form atactic and isotactic star polymers. Polymers were subsequently char-
acterized by means of NMR spectroscopy, gel permeation chromatography, differential scanning calorimetry, and thermog-
ravimetric analysis. Our results support previous work that suggests that the length of the individual arms, not the total
molecular weight, correlates to physical characteristics including glass, melt, crystallization, and decomposition tempera-
tures. In addition, differences between core molecules suggest that the chemical nature of the core can significantly alter
the physical properties of the star polymer. Trends in crystallization and glass transition temperatures relative to the core
used merit further study and correlate closest to the molecular weight and the number of arms emanating from the star
core. It is also clear that the rigidity provided by aromatic cores has a significant effect on the melting temperatures of
these macromolecules.
Key words: Polymer star, lactide, poly(lactic acid), ring opening polymerization.
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Resume : On a realise la synthese et effectue la caracterisation physique d’une serie d’acides poly(lactique) bien definie
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et en forme etoilee. Les polymeres etoiles ont ete prepares a partir d’une serie l’alcools de base multifonctionnels, dont les
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polyols flexibles pentaerythritol et dipentaerythritol (respectivement les alcools de base a quatre et a six branches) et des
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arenes substitues rigides, tels le tri(hydroxymethyl)benzene et l’hexa(hydroxymethyl)benzene. Faisant appel a l’octanoate
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de zinc comme catalyseur et les rac-lactide et L-lactide comme monomeres, on prepare des bras comportant 10 unites mo-
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nomeres de longueur dans les formes atactique et isotactique des polymeres etoiles. On a subsequemment caracterise ces
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polymeres par spectroscopie RMN, par chromatographie par permeation de gel, par calorimetrie a balayage differentiel et
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par analyse thermogravimetrique. Les resultats obtenus sont en accord avec ceux obtenus anterieurement d’apres lesquels
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c’est avec le nombre total de bras individuels, et non le poids moleculaire total, qu’il existe une correlation avec les carac-
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teristiques physiques, dont les temperatures de transition vitreuse, de fonte, de cristallisation et de decomposition. De plus,
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les differences entre les molecules de base suggerent que la nature chimique de la base peut alterer les proprietes physi-
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ques d’un polymere etoile d’une fac¸on significative. Les tendances dans les temperatures de cristallisation et de transition
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vitreuse par rapport a celles des molecules de base meritent des etudes ulterieures et donne la meilleure correlation avec le
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poids moleculaire et le nombre de bras emanant de la base etoilee. Il est aussi clair que la rigidite fournit par les molecules
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de base aromatiques provoque un effet significatif sur les temperatures de fusion de ces macromolecules.
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Mots-cles : polymerer etoile, lactide, acide poly(lactique), polymerisation par ouverture de cycle.
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[Traduit par la Redaction]
complex macrostructures like polymer stars4 and brushes.5
In particular, PLA polymer stars have found utility as matri-
ces for controlled-release drug delivery and other biomedical
applications.6
The core-first approach dominates star polymer synthesis
for PLA, where a multifunctional initiator promotes the
ring-opening polymerization of the lactide monomer from
alcohol or amine functionalities. When catalyzed by tin(II)
octanoate (Sn(Oct)2), the reaction transforms rac- or L-lactide
into atactic PLA (a-PLA) and isotactic PLA (L-PLA) stars,
Introduction
Poly(lactic acid) (PLA) has become an essential green
material as it is derived from renewable feedstocks, is biode-
gradable and bioassimilable, and has found many niche ap-
plications across a broad sweep of commercial fields.1 Of
particular interest is the role these and other aliphatic poly-
esters have to play in the biomedical industry.2 Whereas lin-
ear PLA homo- and co-polymers have been used in sutures
and stents since 1960,3 the physical properties of the macro-
molecules can be refined by incorporating PLA into more
Received 19 October 2010. Accepted 6 December 2010. Published on the NRC Research Press Web site at canjchem.nrc.ca on 19 March
2011.
M.R. Perry and M.P. Shaver.1 Department of Chemistry, University of Prince Edward Island, 550 University Avenue, Charlottetown,
PE C1A 4P3, Canada.
1Corresponding author (e-mail: mshaver@upei.ca).
Can. J. Chem. 89: 499–505 (2011)
doi:10.1139/V11-002
Published by NRC Research Press

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Can. J. Chem. Vol. 89, 2011
Fig. 1. Core molecules for poly(lactic acid) star polymers.
respectively. Previous studies have focused on the role of
molecular weight7 or polymer tacticity8 in shaping polymer
properties, whereas others have examined the incorporation
of PLA stars into miktoarm9–11 and dendritic frame-
works12–14 or examined the application of these materials in
self assembly and drug delivery.15,16 The initiator is also a
key differentiator between polymer stars. A diverse array of
simple multifunctional polyols have been used as initiators
for PLA polymer stars4 including three-armed stars prepared
from trimethylolpropane and glycerol; four-armed stars from
pentaerythritol, ditrimethylolpropane, and erythritol; five-
armed stars from xylitol; six-armed stars from dipentaery-
thritol, inositol, and sorbitol; and eight-armed stars from tri-
pentaerythritol. Less clear is the effect of these cores on
polymer star synthesis or properties, but some key trends
have been observed. The macromolecular structure depend-
ence of thermal properties were investigated for three- to
six-armed L-PLA stars.17 Increasing molecular weights of
six-armed sorbitol polymers correlated to concomitant in-
creases in thermal decomposition (Td) and melting (Tm) tem-
peratures, a trend exacerbated by the different reactivities
between primary and secondary alcohol functionalities in
the sorbitol core. For polymer stars with similar theoretical
absolute molecular weights (Mn,th), structures with fewer
arms exhibited higher Tm values, suggesting that physical
properties correlate with the length of the polymer arm ema-
nating from the star core rather than the absolute molecular
weight of the star.17
chased from Sigma-Aldrich Chemical Co. and distilled prior
to use. Dipentaerythritol and pentaerythritol were purchased
from Sigma-Aldrich Chemical Co. and purified by recrystal-
lization from methanol and vacuum sublimation three times,
respectively, before being dried in vacuo for 48 h prior to
use. 1,4-Dioxane was dried over CaH2 and distilled prior to
use. Benzene-1,3,5-tricarboxylic acid (98%) was purchased
from Sigma-Aldrich Chemical Co. and used as received.
The monomers rac- and L-lactide were purchased from
PURAC Biomaterials and purified by three successive vac-
uum sublimations of the material prior to use.
There is a surprising lack of chemical diversity in the
choice of simple discrete initiators for this ring-opening po-
lymerization because of an expected focus on flexible sugar
alcohols. Cores with more rigidity include b-cyclodextrins,18
modified perylenes,19 hexahydroxytriphenylene,20 and diben-
zoylmethane- and bipyridine-ligated metal complexes.21
Whereas these cores impart photochemical, spectroscopic,
and supramolecular properties to the stars, no correlation be-
tween core structure and properties has been made.
In this vein we endeavoured to develop synthetic method-
ologies to prepare three-, four-, and six-armed star polymers
of rac- and L-lactide with variable core structures with a fo-
cus on simple rigid cores. Specifically, we targeted PLA
stars built from 1,3,5-tri(hydroxymethyl)benzene (THMB,
1), hexa(hydroxymethyl)benzene (HHMB, 2), pentaerythritol
(PE, 3), and dipentaerythritol (DPE, 4) (Fig. 1) and charac-
terized the star polymers with gel permeation chromatogra-
phy (GPC), 1H NMR spectroscopy, thermogravimetric
analysis (TGA), and differential scanning calorimetry
(DSC).
General considerations
All experiments involving moisture- and air-sensitive
compounds and catalysts were performed under a nitrogen
atmosphere using an MBraun LABmaster sp. glovebox sys-
tem equipped with a –35 8C freezer and [H2O] and [O2] an-
alyzers. GPC was carried out in THF (flow rate:
1 mL min–1) at 50 8C with a Polymer Labs PL–GPC 50
Plus integrated GPC system using three 300 mm ꢀ 7.5 mm
Resipore columns. Polystyrene standards were used for cali-
bration and corrected for PLA.22 1H NMR spectra were re-
corded with a Bruker Avance spectrometer (300 MHz) in
CDCl3 or D2O. TGA was performed on a TA instruments
TGA Q500 under an inert nitrogen atmosphere with a flow
rate of 60 mL min–1 and a heating rate of 10 8C min–1. DSC
was performed on a TA instruments DSC Q100 using a
heating rate of 5 8C min–1 and a flow rate of 50 mL min–1.
Guelph Analytical Laboratories conducted the CHN elemen-
tal analyses.
Core syntheses
Experimental
1,3,5-Tri(hydroxymethyl)benzene
Materials
This procedure was modified from an existing literature
procedure.23 In a glovebox, lithium aluminum hydride
(2.505 g, 6.60 mmol) was added to a 250 mL round bottom
Schlenk flask and suspended in 10 mL of anhydrous THF.
The vessel was sealed and removed from the glovebox. Tri-
methyl 1,3,5-benzene-tricarboxylate (5.002 g, 1.98 mmol)
was dissolved in 50 mL of anhydrous THF in a 250 mL
Schlenk tube. The carboxylate solution was added via can-
nula dropwise to the LiAlH4, and the dark grey lithium
HPLC-grade toluene, THF, methanol, and dichloromethane
were purchased from Fisher Scientific and deuterated sol-
vents were purchased from Cambridge Isotopes. Anhydrous
toluene was obtained by passing the solvent through an In-
novative Technologies solvent purification system consisting
of columns of alumina and a copper catalyst. Anhydrous
solvents were tested with addition of a toluene solution of
sodium benzophenone ketyl prior to use. Sn(Oct)2 was pur-
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Perry and Shaver
501
Table 1. Polymer data for atactic and isotactic poly(lactic acid)
suspension turned to a deep orange colour that quickly dissi-
pated. Upon completion of the carboxylate addition, the
mixture was a deep green colour. The mixture was allowed
to reflux for 5 h after which the vessel was cooled in an ice–
water bath and stirred. Three solutions (5 mL H2O, 5 mL
15% (w/w) NaOH in H2O, and 15 mL H2O) were sequen-
tially added in a dropwise fashion to quench any remaining
LiAlH4 and hydrolyze the Al salts. Filtration and washing
with three 20 mL portions of THF provided a colourless fil-
trate. Organics were removed under reduced pressure, yield-
ing a colourless crude oil in 92% yield. Purification of this
material by recrystallization from ethyl acetate and column
chromatography gave an analytically pure white powder
(0.934 g, 0.56 mol, 28.2%). Improved yields were obtained
by forgoing recrystallization and purifiying the material by
silica gel column chromatography. 1H NMR data
(300 MHz, MeOD, 298 K) d: 7.25 (s, 3H, ArH), 4.60 (s,
6H, ArCH2). 13C NMR data (75 MHz, MeOD, 298 K) d:
141.41 (s, Ar-CH2), 126.00 (s, Ar-H), 64.04 (s, CH2). Ele-
mental anal. calcd (%): C 65.27, H 7.19, N 0.00; found
(%): C 65.41, H 7.22, N 0.00.
(PLA) stars.
Initiator
1
1
Monomer
rac-LA
L-LA
rac-LA
L-LA
rac-LA
L-LA
rac-LA
L-LA
% Conv.
68.4
76.9
Mn,GPC
5023
3490
Mn,th
4488
4506
PDI
1.23
1.32
2
2
89.7
98.8
13787
8944
8030
8845
1.22
1.51
3
3
78.2
86.3
4642
5920
5707
5905
1.20
1.16
4
4
86.7
78.7
9899
8452
7723
7011
1.21
1.18
Note: PDI, polydispersity index; Mn,GPC, molecular weight as determined
by size exclusion chromatography; Mn,th, theoretical absolute molecular
weight.
Table 2. Differential scanning calorimetry (DSC) and thermogra-
vimetric analysis (TGA) thermal data for poly(lactic acid) (PLA)
stars.
Star Tg (8C)
Tc (8C)
—
86.99
—
93.33
—
99.63
—
Tm (8C)
—
Td (8C)
228.39
Xc (%)
—
29.1
—
27.0
—
17.5
—
1-r
1-L
2-r
2-L
3-r
3-L
4-r
4-L
42.14
41.16
45.25
49.72
40.48
43.67
45.72
49.21
130.60, 138.43 235.35
234.35
148.14, 159.25 242.35
218.14
121.10, 135.42 232.39
227.75
125.41, 139.35 256.16
Hexa(hydroxymethyl)benzene
—
This synthesis was modified from an existing literature
procedure.24 In a glovebox, 2-butyne-1,4-diol (6.008 g,
6.98 mmol) was dissolved using 50 mL anhydrous dioxane
in a 250 mL round bottom flask. Cyclopentadienylcobalt di-
carbonyl (2.06 g, 1.05 mmol) was added, along with 50 mL
dioxane, to a 500 mL Strauss flask, to which the alkyne sol-
ution was added in a dropwise fashion. The vessel was
sealed and removed from the glovebox where it was left to
react in a 130 8C oil bath for 4 days. The formation of an
orange precipitate was evident after 1 day. After the allotted
reaction time, the mixture was filtered using a fine glass fil-
ter frit, the solid was collected, washed three times using
25 mL THF, and dried in vacuo (3.73 g, 1.44 mmol,
62.0%). 1H NMR data (300 MHz, d6-DMSO, 298 K) d:
4.86 (s, 6H, ArCH2), 4.69 (s, 12H, OH). 13C NMR data
(75 MHz, d6-DMSO, 298 K) d: 138.79 (s, Ar), 56.75 (s,
CH2). Elemental anal. calcd (%): C 55.81, H 7.02, N 0.00;
found (%): C 56.06, H 6.88, N 0.00.
—
—
84.38
27.3
Note: Tg, glass transition temperature; Tc, crystallization transition tem-
perature; Tm, melting transition temperature; Td, onset temperature for
polymer decomposition; and Xc,percent crystallinity of the star samples.
hexa(hydroxymethyl)benzene (0.0162 g, 6.83 mmol) were
added to a dry 25 mL round bottom ampoule equipped with
a magnetic stirring bar. The ampoule was connected to a
Schlenk line, sealed, and evacuated for 10 min before being
filled with N2. This evacuation/refill procedure was repeated
three times. A 7.33 mmol/L Sn(Oct)2 solution in toluene
(2.1 mL, 6.24 mg, 0.0154 mmol) was syringed into the ves-
sel that was then sealed and put under vacuum to remove
the toluene. The ampoule was then placed into a preheated
oil bath at 125 8C for 8 h. After the allotted time, the vessel
was cooled to room temperature, opened to the atmosphere,
and quenched with 10 mL of a 9:1 v/v solution of CH2Cl2
and methanol. The solution was stirred for 30 min and then
added dropwise to 100 mL of cold methanol to precipitate
the polymer. The resultant white polymer was collected via
filtration, washed with 50 mL MeOH and 50 mL H2O, and
dried in vacuo for 12 h. Monomer conversion was deter-
mined gravimetrically to be 89.7% (0.5033 g).
Polymerization experiments
Polymerization procedures were modified from previously
reported protocols.25 All polymerization reactions were per-
formed in sealed 25 mL round bottom ampoules under an
inert atmosphere, at 125 8C, with a molar ratio of
monomer – alcohol functionality – catalyst of 10:1:0.04. The
catalyst employed in each instance was Sn(Oct)2. Monomers
employed included rac-lactide and L-lactide and core initia-
tors 1–4. Experiments were also conducted with a benzyl
alcohol initiator to prepare a standard material for compara-
tive purposes. The reaction proceeded for ~8 h and was then
quenched with 10 mL of a 9:1 v/v solution of CH2Cl2–
MeOH for 30 min at room temperature. The resulting mix-
ture was precipitated from cold methanol, collected via fil-
tration, washed with 50 mL H2O and 50 mL MeOH, and
dried in vacuo prior to analysis.
Results and discussion
Core and polymer synthesis
Whereas PE and DPE are commercially available polyols,
the rigid THMB and HHMB cores had to be synthesized.
The reduction of 1,3,5-benzene-tricarboxylate with excess
lithium aluminum hydride provided the crude desired aro-
matic 1 in high yields. Purification substantially lowered
For example, rac-lactide (0.545 g, 3.86 mmol) and
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502
Can. J. Chem. Vol. 89, 2011
Fig. 2. Differential scanning calorimetry (DSC) profile for hexa(hydroxymethyl)benzene (HHMB) poly(lactic acid) (PLA) star 2-L.
yields, but enough product was isolated from this effort that
optimization of this step was not pursued. For the hexa-
substituted core initiator, the CpCo(CO)2 catalyzed cyclotri-
merization of 2-butyne-1,4-diol afforded 2 in moderate
yields. Attempts to synthesize 1 via a cobalt-catalyzed cy-
½2ꢁ
clotrimerization of 1-propyn-3-ol were unsuccessful, access-
ing a mixture of aromatic isomers with various substitution
patterns arising from head-to-tail additions and an insoluble
polymeric material resulting from thermal polymerization of
the propargyl alcohol. These reactions are shown in eqs. [1]
PLA stars with 10 monomer units per alcohol functional-
and [2], respectively.
ity were synthesized. Oligomeric arm lengths were chosen
because their low overall molecular weights accentuate the
structural differences.4 A molar ratio corresponding to 0.04
Sn(Oct)2 units and 10 lactide monomer units per alcohol
functionality was used (ie. 0.25:1:60 using one DPE mole-
½1ꢁ
cule). Higher catalyst loadings, up to the one catalyst unit
per growing arm expected for living ring-opening polymer-
izations, gave consistent bimodal product distributions.
This suggests self-initiation behavior by the Sn(Oct)2 or a
reaction impurity while also increasing the propensity for
Published by NRC Research Press

Perry and Shaver
503
Fig. 3. Thermogravimetric analysis (TGA) profile for 2-r (gray line) and 2-L (black line).
transesterification reactions leading to reduced molecular
weights for long reaction times. Additionally, the efficacy
of low catalyst loadings suggests some immortal behavior
for the tin catalyst under these reaction conditions.26 As an
example, the synthesis of the resultant star polymer for an
HHMB core is shown in eq. [3].
formity of the spectral resonances is suggestive of arms of
similar lengths. Characterization by GPC is collected in Ta-
ble 1. Percent conversions, measured gravimetrically, are
shown along with the determined polydispersity and molec-
ular weight values calculated relative to polystyrene stand-
ards and corrected for linear PLA, but remain uncorrected
for the differences in star hydrodynamic volumes. Polymer-
ization times of 8 h were chosen as shorter reaction times
and led to inefficient initiation and broadened polydisper-
sities, whereas reaction times of 24 h led to significant trans-
esterification.
Low to moderate polydispersities (PDIs) imply good con-
trol over the polymerization reactions. Slightly higher PDIs
for aromatic initiators 1 and 2 may be due to longer initia-
tion times, as expected from comparative benzyl alcohol
and isopropanol initiations of linear PLA.1 Deviations from
expected molecular weights are likely due to significant dif-
ferences in effective hydrodynamic volume between linear
and star-shaped polymers, affecting size-exclusion chroma-
tographic techniques,4 with six-armed stars built from 2 and
4 showing, as expected, the highest Mn deviations.
½3ꢁ
1H NMR analysis in CDCl3 of the resultant PLA macro-
molecules shows that all alcohol functionalities associated
with the parent core molecules disappear, suggesting activa-
tion of all arms in the formation of the star polymers. In ad-
dition, each sample exhibits a shift in the methylene (CH2)
resonances towards higher ppm owing to the deshielded en-
vironment of the polymer chain. For example, HHMB-
initiated PLA displays the loss of the OH peak present at
4.86 ppm and shifts the CH2 peak from 4.69 to 5.16 ppm.
Because of peak overlap, end-group analysis of the terminal
alcohol functionalities after polymer isolation could not as-
sist in further polymer characterization, although the uni-
Physical properties of polymer stars
The six polymer stars in question were then characterized
by TGA and DSC to assess the effect of core initiator on
polymer properties. These data are collected in Table 2. Tg,
Tc, and Tm values refer to glass, crystallization, and melting
transitions, respectively, whereas Td refers to the onset tem-
perature for polymer decomposition and Xc refers to the per-
cent crystallinity of the star samples.
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Can. J. Chem. Vol. 89, 2011
The variation of core initiator was shown to influence the
Conclusions
chemical properties of the resulting polymeric material in
both atactic and isotactic PLA. Differences in Tg depended
predominantly on the number of polymer arms, not the core
molecule. Additionally, differences in Tg may also be due to
changes in molecular weight between the different sam-
ples.25 In the case of initiators 2, 3, and 4, the isotactic sam-
ples exhibited higher Tg values than their atactic partners.
Owing to the lower molecular weights (Mn,th = 4.5 vs 5.8–
8.8 kDa), the stars formed from initiator 1 do not follow
this trend. For samples of the same or similar molecular
weight and number of arms little difference is observed in
comparing rigid to flexible cores. While we expected the ri-
gidity present in the aromatic initiator to restrict the move-
ment of the polymer arms and potentially prevent energy
distribution throughout the macromolecule, the low energy
process associated with the glass transition is unaffected.
Similarly, crystallization characteristics observed for stereo-
regular stars derived from L-lactide (Tc and Xc) are unaf-
fected by core rigidity. Crystallization temperatures appear
to have an inverse relationship to percent crystallinity, with
THMB-derived samples exhibiting a low Tc and the highest
Xc, whereas PE-derived samples had the highest Tc and the
lowest Xc.
As expected, melting signals were only observed for iso-
tactic polymer stars.4 In agreement with previous work, the
presence of two melting points is characteristic of semicrys-
talline PLA.27 Here, clear differences exist between rigid
and flexible core initiators: 1 and 2 possess the two highest
Tm values, suggesting increased order and rigidity through-
out the star. The thermal stability imparted by the rigid core
is especially apparent when comparing two six-armed poly-
mer stars of similar molecular weights, with an increase of
~20 8C in the Tm for the hexa-substituted benzene versus
the flexible dipentaerythritol. Comparing 1 with 2 and 3
with 4 also shows that a greater number of arms in PLA
star polymers may induce higher melting temperatures, but
this trend may be an artifact of molecular weight differences
between the samples. A representative DSC profile for sam-
ple 2-L is shown in Fig. 2.
Eight PLA polymer stars were prepared from four multi-
functional core initiators. Whereas good control over the re-
action, in general, is offered by the Sn(Oct)2 catalyst, the
benzyl functionalities in hexa- and tri-(hydroxymethyl)ben-
zene initiators are more challenging to initiate and lead to
minor deviations from ideal living characteristics. Physical
properties of the stars correlate well with established
markers including molecular weight and the number of pol-
ymer arms. A striking effect of the rigidity of the core was
noted in the melting temperatures of the star polymers, with
increased core rigidity correlating with higher Tm values. Ef-
forts to examine whether the core initiator has an effect on
polymer degradation and to expand the range of core initia-
tors accessible for star polymer synthesis continue.
Acknowledgement
We thank the Natural Sciences and Engineering Research
Council of Canada (NSERC) and the University of Prince
Edward Island (UPEI), Charlottetown, for financial support,
the Canada Foundation for Innovation and the Atlantic Can-
ada Opportunities Agency for infrastructure, and Dr. R.
Bissessur, UPEI, for access to TGA and DSC instrumenta-
tion.
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Finally, thermal stability was measured by TGA and re-
ported as the onset temperature (Td) of rapid thermal decom-
position. In each case, the isotactic samples showed higher
thermal stability, as high as a 25 8C difference for DPE
stars. Six-armed stars also possessed higher decomposition
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representative TGA profiles are shown in Fig. 3 for
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