The combined synthesis and coloration of poly(lactic acid)

Article abstract of DOI:10.1002/anie.201004920

True colors: A catalytic process that allows color to be integrated directly into poly(lactic acid) eliminates the need for subsequent wet-processing operations. The technical performance of the new polymers is superior to that of conventionally dyed substrates and the polymers are significantly more sustainable for applications in the textiles industry (see picture) in terms of environmental impact and economics of the coloration process. Copyright

Full text of DOI:10.1002/anie.201004920

DOI: 10.1002/anie.201004920
Dyes/Pigments
The Combined Synthesis and Coloration of Poly(lactic acid)**
Robert O. MacRae, Christopher M. Pask, Lucy K. Burdsall, Richard S. Blackburn,
Christopher M. Rayner, and Patrick C. McGowan*
With the continuing depletion of petrochemical feedstocks, it
has become necessary to produce new, useful, and environ-
mentally friendly polymers for a sustainable future.[1] Syn-
thesis of a polyester from lactic acid was pioneered by
Carothers in 1932 and developed by DuPont.[2] Poly(lactic
acid) (PLA) is a linear aliphatic thermoplastic polyester
derived from 100% renewable sources,[3–5] and the polymer is
compostable.[4] The production of PLA uses 20–50% less
fossil fuel resources than comparable petroleum-based
fibers.[6] PLA is a particularly “green” polymer in terms of
sustainability and degradation, two vital components of the
ꢀcradle-to-graveꢁ life cycle. Plants process atmospheric CO2
and water through photosynthesis to make the raw materials
from which the building blocks of PLA can be obtained; and
composting converts PLA into CO2, water, biomass, humus,
and other natural substances (Figure 1). PLA is formed either
by direct condensation of lactic acid, or, most effectively, via
the cyclic intermediate dimer (lactide) through a catalyzed
ring-opening polymerization (ROP) process.[3] Metal alkox-
ides are the most common catalysts employed in ROP of
cyclic esters. Aluminum alkoxides have been shown to give a
controlled and living polymerization of lactides through a so-
called coordination/insertion mechanism.[7]
For a typical textile dyeing process, poly(ethylene tereph-
Figure 1. Life cycle of PLA for textile applications (Figure adapted from
thalate)[8] and PLA[9] (Figure 1) are initially scoured with
detergent and alkali to remove hydrophobic auxiliaries (aids
knitting and weaving), and then dyed with disperse dyes in an
aqueous dyebath buffered to pH 4.5 using sodium acetate or
acetic acid at 1308C or 1158C, respectively. To achieve
acceptable wash fastness properties (ability of dye to adhere
to material), “after-clearing” with reducing agents is
employed to remove the excess dye. Reduction after-clearing
has a significant detrimental environmental impact because of
Gross and Kalra).[4]
the strong alkaline conditions, large amounts of water used,
and the discharge of high levels of sulfur with the waste-
water.[10,11] There is also concern that effluent from disperse
dyeing operations of the reduction after-clearing process
contains by-products, particularly aromatic amines from the
reduction of azo dyes, that have mutagenic and carcinogenic
activity.[12,13]
Herein we report the use of a catalyst containing a
chromophore, which will simultaneously carry out the poly-
merization, and in addition, incorporate the dye, needed to
color the material, into the polymer backbone itself—termed
the DyeCat process. Thus, the coloration process can achieve
high color strength without exposing the fiber to potentially
damaging conditions; these are two essential prerequisites for
future commercial applications of PLA.
As shown in Figure 1, the wet processing stages (prepa-
ration, dyeing, finishing) are not required for DyeCat PLA
because coloration is achieved through the polymerization
process. Hence the consumption of energy, water, chemicals,
and time, as well as effluent production are all avoided.
Residual catalyst in the polymers can lead to undesirable
polymer properties, including discoloration, and removal of
the spent catalyst from a polymer is often difficult and
[*] R. O. MacRae, L. K. Burdsall, C. M. Rayner, P. C. McGowan
School of Chemistry, University of Leeds
Leeds, LS2 9JT (UK)
E-mail: P.C.McGowan@leeds.ac.uk
R. S. Blackburn
Centre for Technical Textiles, University of Leeds
Leeds, LS2 9 JT (UK)
C. M. Pask, R. S. Blackburn, C. M. Rayner, P. C. McGowan
DyeCat Ltd, 103 Clarendon Road, Leeds, LS2 9DF (UK)
[**] We wish to thank Yorkshire Forward, Techtran Ltd. and the
University of Leeds for financial support. We would like to
acknowledge B. Thomas and E. Gaston (University of Leeds) for
help with Figure 1 and the table of contents graphic, respectively.
We thank A. Hebden, C. Kilner, and J. Mannion for help with the
crystallography.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201004920.
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expensive.[14,15] The paradigm shift for the process described
coordinate aluminum center having a square-pyramidal
herein is that the catalyst can be functionalized and deliber-
ately left in the polymer.
geometry. The compound 2 crystallized in a triclinic cell and
the structural solution was performed in the space group P1
ꢀ
Aluminum catalysts have a significantly lower environ-
mental impact than their heavy-metal counterparts, and have
been shown to be particularly effective at lactide polymeri-
zation.[16] We have identified three main strategies (and
combinations thereof) for the incorporation of a dye into an
aluminum catalyst structure: 1) a dye molecule may be
appended to a ligand framework known to stabilize the Al
center (Figure 2a), 2) a novel catalyst utilizing a known dye
(selected bond lengths and angles are given in Supporting
Information).
Aluminium alkoxide species have been implicated as the
active species in PLA polymerization[16,17] and bimetallic zinc
complexes containing a bridging aloxide have been proven to
show high catalytic activity in lactide ring-opening polymer-
ization.[18,19] Reaction of a p-nitrobenzylalcohol initiator with
compound 1 forms the dimeric aluminium alkoxide species 3
(Figure 3) in the solid state, the crystal structure of which is
shown in Figure 4. The molecular structure of compound 3 is
dimeric, containing two aluminum centers bridged by two
alkoxide ligands. Compound 3 crystallized in a monoclinic cell
and the structural solution was performed in the space group
P21/n (selected bond lengths and angles are given in
Supporting Information). A simple chemical modification of
existing dye structures to incorporate functionality (e.g.,
primary alcohol group) required for initiation have been
devised (e.g., 4; Figure 5 and see the Supporting Informa-
tion), and indeed some suitable dyes are already commer-
cially available (e.g., 5–9). Control reactions indicate that a
polymer with a melting point of 1728C (1648C for in situ
catalyst generation) and degree of crystallinity (xc) of 45.5%
(44.6% for in situ catalyst generation) is obtained using
Figure 2. Three successful approaches for the DyeCat process. a) dye
attached to ligand, b) dye as ligand, c) dye as initiator.
molecule as the ligand may be designed (Figure 2b), or 3) the
dye may be used as the polymerization initiator (Figure 2c).
These three complementary approaches have been investi-
gated and all have been successful in producing colored PLA.
As an exemplar of the DyeCat process, the method shown in
Figure 2c will be discussed in detail. To synthesize the
precatalyst, two equivalents of either C5H4N(CO)NHC6H3F2
or C5H4N(CO)NHC6H4NO2 were added to a solution of
trimethylaluminium in toluene to yield pre-catalysts 1 and 2
(Figure 3). These were characterized using NMR spectros-
copy, mass spectrometry, and elemental analysis, and X-ray
crystallographic analysis for compound 2 (Figure S1 and
Table S1 in the Supporting Information) shows a five-
Figure 3. Precatalyst (1 and 2) used in polymerization reactions and
proposed structure of a typical catalyst (3).
Figure 4. The molecular structure of compound 3. Thermal ellipsoids
are shown at the 50% probability.
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Angew. Chem. Int. Ed. 2011, 50, 291 –294

Figure 6. Examples of DyeCat PLA. The intiators used were 4 (a), 5
(b), 6 (c), 7 (d). e) Black melt-spun DyeCat filament.
Figure 5. Initiators (4–9) used in the polymerization reactions.
and a red polymer (Figure 6c) was synthesized using C.I.
Disperse Red 13 6. Figure 6d shows a DyeCat PLA contain-
ing the fluorescent dye 1-pyrenebutanol[21] 7 that was photo-
graphed under UV light. The fibers and plastics showed little
deterioration in molecular weight or tensile strength during
processing. A particularly challenging color to obtain for PLA
is black. This is the most widely used color in textile
applications, but also the hardest to achieve because of the
high dye loadings required, and the tendency for leaching of
the dyes from the final product. Figure 6e shows black melt-
spun DyeCat PLA filament, prepared by mixing the yellow
and purple polymers (Figures 6a and b, respectively) in a
1.0:1.8 ratio in the melt-spinning process (2208C), and
subsequently drawing over heated rollers to align the polymer
crystallites. The black filament produced was 35.0 tex, had a
compound 3 as the active catalyst; this is in comparison to the
pre-catalyst 2 being treated with initiator and lactide.
To determine inclusion of the initiator within the polymer
chain, polymerizations were conducted with a high catalyst
loading to generate short-chain oligomers. These were probed
by mass spectrometry, the results of which display a Boltz-
mann-like distribution of molecular weights. In the case of the
benzyl alcohol initiator (see the Supporting Information) the
molecular ion possesses the formula: [NaH(OCOCH-
(CH3))nOCH2Ph]+, with n ranging between five and twenty.
The colored initiator 9 yields an ion with the formula:
[H2(OCOCH(CH3))nOCH2CH2N(CH2CH2OH)C6H4N2C6H4-
NH2]+, with n ranging from one to fourteen. Therefore both
mass spectra show incorporation of the initiator at the end of
the oligomer chain. Overall, the characterization data provide
compelling experimental evidence for the formation of the
pre-catalyst, catalyst, and the initial insertions of the mono-
mer to produce the polymeric entity containing the catalyst
incorporating a dye.
When used with initiator 4, compound 1 efficiently
polymerizes l-lactide to yield yellow PLA. The polymer was
characterized by differential scanning calorimetry and gel
permeation chromatography, and exhibited a molecular
weight range of Mw = 170000–340000 with a polydispersity
index (PDI) of 1.2–1.6 (Mn = 140000–300000), and a melting
point range of 171–1798C. These values compare favorably to
those obtained for an undyed commercial PLA, wherein the
melting temperature is between 160–1708C for PLA contain-
ing only the l- or d-isomeric form.[20]
Compound 1 has been combined with colored initiators to
give a range of active catalysts, which in turn have produced a
range of different colored polymers, films, and fibers.
Examples of DyeCat PLA can be seen in Figure 6. A range
of colors has been achieved, and is best demonstrated by the
solvent cast films. Ayellow DyeCat PLA polymer (Figure 6a)
was synthesized using as the initiator the simple synthetic azo
derivative 4 of benzyl alcohol, often used as the initiator in
conventional PLA polymerization reactions.[16,17] Commercial
dyes are also applicable as initiators; a purple polymer
(Figure 6b) was synthesized using C.I. Disperse Blue 106 5
tensile stress of 52.2 kPa, a Youngꢁs modulus of 416.0 cNtexÀ1
,
a tenacity of 11.9 cNtexÀ1 (see the Supporting Information for
a definition of tex), as well as Tm = 178.18C, and xc = 44.4%,
which compare favorably with typical commercial PLA
filaments.[20] This black filament has been woven into a
black dress which is currently being exhibited in the Science
Museum in London (see the Supporting Information).
Samples of the colored PLA filaments were washed
according to the ISO (International Organization of Stand-
ardization) approved 105:C06/C2 (608C) wash test using
undyed multifiber as an indicator.[22] There was no reduction
in color strength of the PLA and no staining of the multifiber
indicator was observed; similarly there was no evidence of
any color in the post-wash liquor. From a performance
perspective, these colored polymers have greater wash fast-
ness properties with respect to their aqueous-dyed counter-
parts, presumably because of the covalent bond between the
dye and the polymer. As PLA is an aliphatic polymer it has
very limited absorbance in the UV range, thereby preventing
its application where such absorbance is required. By analogy
with the use of visible dyes, a UV absorbing polymer was
synthesized using 9-anthracenemethanol 8 as an initiator
(absorbance spectrum shown in Supporting Information).
A catalyst typically contains one dye chromophore moiety
which yields colorant by mass of 0.5–0.7% with respect to
mass of the polymer; this figure is significantly less than that
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Communications
of similar shades in the conventionally dyed textiles wherein
shortcomings of aqueous dyed PLA (and other polymers),
reduces the cost of PLA processing, and fulfills all the
technical requirements for apparel and related uses to afford
an economic, sustainable, and feasible replacement for
standard polyesters.
medium and dark shades would typically require 2.0–10.0%
dye with respect to the mass of the fiber. Additionally, by
incorporating the dye molecule at the polymer synthesis stage
the colorant is homogenous throughout the cross-section of
the fiber, and results in higher color strength compared to
those of fibers dyed using the aqueous exhaustion method,
wherein adsorption and diffusion mechanisms do not neces-
sarily yield complete dye homogeneity through the fiber
cross-section. This difference is shown in Figure 7 where
distribution of color through DyeCat PLA (Figure 7a) is
Received: August 6, 2010
Published online: October 28, 2010
Keywords: aluminum · dyes/pigments · polymers ·
.
synthetic methods · textiles
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