A bifunctional monomer derived from lactide for toughening polylactide

Article abstract of DOI:10.1021/ja804357u

(6S)-3-Methylene-6-methyl-1,4-dioxane-2,5-dione was synthesized from l-lactide and used as the dienophile to prepare spiro[6-methyl-1,4-dioxane-2,5-dione-3,2′-bicyclo[2.2.1]hept[5]ene] via an exoselective and diastereofacial-selective Diels-Alder reaction. Polymerizations of this bifunctional lactide derivative were successfully carried out under ring-opening and ring-opening metathesis polymerization conditions to yield high molecular weight and high T_g polymers. We further demonstrated that by incorporating a small percentage of spiro[6-methyl-1,4-dioxane-2,5-dione-3,2-bicyclo[2.2.1]hept[5]ene] into poly(1,5-cyclooctadiene) and copolymerizing it with dl-lactide, novel polymeric alloys of PLA can be created that have tremendous improvements in toughness over PLA and the corresponding binary blend of PLA and poly(1,5-cyclooctadiene). Copyright

Full text of DOI:10.1021/ja804357u

Published on Web 09/27/2008
A Bifunctional Monomer Derived from Lactide for Toughening Polylactide
Feng Jing and Marc A. Hillmyer*
Department of Chemistry, UniVersity of Minnesota, Minneapolis, Minnesota 55455-0431
Received June 9, 2008; E-mail: hillmyer@umn.edu
Scheme 1. Synthesis of 1 and 2 from L-Lactide
Polylactide (PLA) is a biorenewable, biocompatible, and bio-
degradable polyester derived from the cyclic ester lactide. While
the properties of PLA are attractive for many applications,1 there
has been much effort on rendering its properties to be more
competitive with those of traditional, petroleum-based polymers.
Among those approaches, the preparation and polymerization of
lactide derivatives have been explored to enhance thermal, barrier,
and mechanical properties in these related polyesters. For example,
functionalized lactides have been prepared, in general, from the
corresponding R-hydroxyacids and subsequently polymerized with
varying degrees of success.2,3 Given that lactide is a well-established
renewable monomer, we explored its derivatization to give new
monomers that could be utilized to improve the properties of the
PLA. In this communication, we report the synthesis and poly-
merization of a bifunctional cyclic ester derived from lactide. We
utilize the resultant polymers to enhance the toughness of PLA
through a reactive grafting approach to create novel polymeric alloys
of PLA that exhibit much improved toughness compared to PLA
homopolymer.
Scheme 2. Polymerization of 2 by Two Different Mechanisms
Table 1. ROMP of 2 by Third Generation Grubbs’ Catalyst (Ru)a
[2]0/[Ru]0
yield (%)b
Mn (kg mol-1
)
Mw/Mn
Tg (°C)
50
83
86
88
89
93
96
14.0
27.2
48.5
62.8
91.6
154
1.04
1.09
1.10
1.16
1.12
1.22
168
191
191
191
189
192
100
200
300
500
1000c
Modification of lactide without concomitant ring opening is rare.4
In 1969, Scheibelhoffer et al. demonstrated that 3-methylene-6-
methyl-1,4-dioxane-2,5-dione could be prepared from lactide using
a bromination/elimination approach.4a Shown in Scheme 1 is an
approach starting from L-lactide, with (3R,6S)-3-bromo-3,6-dimeth-
yl-1,4-dioxane-2,5-dione and (6S)-3-methylene-6-methyl-1,4-diox-
ane-2,5-dione (1, 95% ee) as the bromination and elimination
products, respectively (see Supporting Information). The captodative
alkene (i.e., an alkene substituted with both electron-withdrawing
and electron-donating groups)5 in 1 can be used as a dienophile in
a Diels-Alder reaction to construct a new tricyclic compound (2).
We prepared cycloadduct 2 on a multigram scale in good yield
from L-lactide and explored its ring-opening polymerization (ROP)
behavior.
The reaction of 1 with cyclopentadiene gives 2 as four diaster-
eomers (Scheme S1) due to the endo and exo forms of the tricycle.
Using 1D NOE, 1H-1H COSY, and 1H NMR spectroscopy of the
mixture of isomers (and their diol derivatives,6 Figures S6-S9),
the stereoisomer depicted in Scheme 1 was identified as the
preferred cycloadduct (74%; all isomers were assigned in Figure
S10). The observed diastereofacial selectivity (85:15) was consistent
with the methyl group on dienophile 1 being positioned opposite
the approaching cyclopentadiene to minimize steric repulsion. In
addition, we observed exo selectivity (85:15) consistent with the
preference for anti-Alder addition.6,7
a Polymerization conditions: [2]0 ) 0.2 M in CH2Cl2, RT, 30 min.
All reactions were quenched with ethyl vinyl ether. b All conversions are
near quantitative (>99%). c Reaction time ) 1 h.
through the lactide ring) produced after 24 h was 4.9 kg mol-1
,
and the PDI was 1.85. Performing the same reaction at -20 °C
gave 94% conversion after 24 h with Mn ) 12.2 kg mol-1 and
PDI ) 1.27.9 By controlling [2]0/[benzyl alcohol]0 we obtained
controlled molecular weights of poly 2-L as high as Mn ) 33.6 kg
mol-1 (see Table S1). The structure of poly 2-L shown in Scheme
2 was corroborated by NMR spectroscopy and MALDI mass
spectrometry (Figures S12, S14). The increased conversion at low
temperature is consistent with the bulky nature of the norbornene
side group limiting the exothermicity of the polymerization reac-
tion.10 By DSC, high molecular weight poly 2-L exhibited a glass
transition temperature of 113 °C, the highest Tg reported to date
among all polylactide derivatives.3c,d,11
We also polymerized 2 by ring-opening metathesis polymeriza-
tion (ROMP) using the third generation Grubbs’ catalyst (Scheme
2).12 As shown in Table 1, high molecular weight polymers with
narrow PDIs were readily prepared, and the molecular weights were
well-controlled by the [2]0/[Ru]0 ratio. The Tg of the high molecular
weight poly 2-N (where the N denotes ring opening through the
norbornene ring) polymer was 192 °C.13 The structure of poly 2-N
shown in Scheme 2 was supported by NMR spectroscopy and
MALDI mass spectrometry (Figures S13, S15).
Compound 2 has two polymerizable rings, a substituted nor-
bornene and a substituted lactide. To effect the ROP of the lactide
ring (100 equiv) we used 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD)8
as the catalyst (0.5 equiv) and benzyl alcohol (1 equiv) as the
initiator. We achieved 81% conversion of 2 ([2]0 ) 1.0 M in
CH2Cl2) after 3 h at RT and 84% conversion after 24 h (reactions
were quenched by adding at least 10 equiv of benzoic acid). The
Mn (SEC, PS stnd) of the poly2-L (the L denotes ring opening
With the aim of toughening PLA, we exploited the facile dual
polymerizability of 2 to produce a lactide-substituted polybutadiene
and utilized that reactive polymer for the formation of a PLA
9
13826 J. AM. CHEM. SOC. 2008, 130, 13826–13827
10.1021/ja804357u CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Scheme 3. Preparation of a PLA-polyCOD Composite Using 2
in toughness over PLA and the corresponding binary blend of PLA
and poly(1,5-cyclooctadiene) can be prepared.
Acknowledgment. This work was supported by the USDA and
U.S. DOE through Grant DE-PS36-06GO96002P. Parts of this work
were carried out in the University of Minnesota I.T. Characterization
Facility, which receives partial support from the NSF through the
NNIN program. We thank Liang Chen and Drs. Yang Qin,
Christopher J. Cramer, Letitia Yao, and Victor G. Young, Jr. for
technical assistance.
Supporting Information Available: Experimental procedures,
characterization data for monomers and polymers. This material is
available free of charge via the Internet at http://pubs.acs.org.
composite containing polybutadiene-PLA graft copolymer (Scheme
3).14,15 The ROMP of a mixture of 3 mol% of 2 and 97 mol% of
1,5-cyclooctadiene (COD) yielded a statistical copolymer (SEC,
PS stnd: Mn ) 46.6 kg mol-1, PDI ) 1.69).16 This COD/2
copolymer (20 wt%) was added to a TBD-catalyzed, benzyl alcohol-
initiated polymerization of DL-lactide (80 wt%). The reaction
product,17 C1 (SEC: Mn ) 48.7 kg mol-1, PDI ) 2.89, Figure
S18), was compression-molded into translucent films. In a separate
reaction, polyCOD homopolymer (SEC: Mn ) 44.9 kg mol-1, PDI
) 1.70) was prepared (20 wt%) and added to a TBD-catalyzed,
benzyl alcohol-initiated DL-lactide (80 wt%) polymerization. The
reaction product, C2 (SEC: Mn ) 34.5 kg mol-1, PDI ) 1.88,
Figure S18), formed opaque films consistent with macrophase
separation of polyCOD and PLA.
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Analysis of the composites C1 and C2 by SEM revealed
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addition, small-angle X-ray scattering analysis of C1 (Figure S23)
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.
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to markedly improved toughness.18
In sum, we prepared 1 from L-lactide and used it as the dienophile
to prepare 2 in a selective Diels-Alder reaction. Two distinct
polymerizations of this bifunctional lactide derivative were suc-
cessfully carried out to give high molecular weight and high Tg
polymers. Furthermore, we demonstrated that by incorporating a
small percentage of 2 into polyCOD and copolymerizing it with
DL-lactide, a novel PLA composite with significant improvements
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(16) Chain transfer agent cis-2-butene-1,4-diol diacetate was used to control
the molecular weight and lower the loading ratio of the second generation
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Figure 1. SEM backscattered electron images of composite C1. Samples
were cryo-microtomed, stained by RuO4, and coated with platinum.
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