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<record><TEI><teiHeader><fileDesc><titleStmt><title xml:lang="en">Consequences of Dispersity on the Self-Assembly of
ABA-Type Amphiphilic Block Co-Oligomers</title>
<author><name sortKey="Das, Anindita" sort="Das, Anindita" uniqKey="Das A" first="Anindita" last="Das">Anindita Das</name>
</author>
<author><name sortKey="Petkau Milroy, Katja" sort="Petkau Milroy, Katja" uniqKey="Petkau Milroy K" first="Katja" last="Petkau-Milroy">Katja Petkau-Milroy</name>
</author>
<author><name sortKey="Klerks, Gilian" sort="Klerks, Gilian" uniqKey="Klerks G" first="Gilian" last="Klerks">Gilian Klerks</name>
</author>
<author><name sortKey="Van Genabeek, Bas" sort="Van Genabeek, Bas" uniqKey="Van Genabeek B" first="Bas" last="Van Genabeek">Bas Van Genabeek</name>
</author>
<author><name sortKey="Lafleur, Rene P X0a M" sort="Lafleur, Rene P X0a M" uniqKey="Lafleur R" first="René P. X0a M." last="Lafleur">René P. X0a M. Lafleur</name>
</author>
<author><name sortKey="Palmans, Anja R A" sort="Palmans, Anja R A" uniqKey="Palmans A" first="Anja R. A." last="Palmans">Anja R. A. Palmans</name>
</author>
<author><name sortKey="Meijer, E W" sort="Meijer, E W" uniqKey="Meijer E" first="E. W." last="Meijer">E. W. Meijer</name>
</author>
</titleStmt>
<publicationStmt><idno type="wicri:source">PMC</idno>
<idno type="pmid">29862138</idno>
<idno type="pmc">5973780</idno>
<idno type="url">http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5973780</idno>
<idno type="RBID">PMC:5973780</idno>
<idno type="doi">10.1021/acsmacrolett.8b00168</idno>
<date when="2018">2018</date>
<idno type="wicri:Area/Pmc/Corpus">000121</idno>
<idno type="wicri:explorRef" wicri:stream="Pmc" wicri:step="Corpus" wicri:corpus="PMC">000121</idno>
</publicationStmt>
<sourceDesc><biblStruct><analytic><title xml:lang="en" level="a" type="main">Consequences of Dispersity on the Self-Assembly of
ABA-Type Amphiphilic Block Co-Oligomers</title>
<author><name sortKey="Das, Anindita" sort="Das, Anindita" uniqKey="Das A" first="Anindita" last="Das">Anindita Das</name>
</author>
<author><name sortKey="Petkau Milroy, Katja" sort="Petkau Milroy, Katja" uniqKey="Petkau Milroy K" first="Katja" last="Petkau-Milroy">Katja Petkau-Milroy</name>
</author>
<author><name sortKey="Klerks, Gilian" sort="Klerks, Gilian" uniqKey="Klerks G" first="Gilian" last="Klerks">Gilian Klerks</name>
</author>
<author><name sortKey="Van Genabeek, Bas" sort="Van Genabeek, Bas" uniqKey="Van Genabeek B" first="Bas" last="Van Genabeek">Bas Van Genabeek</name>
</author>
<author><name sortKey="Lafleur, Rene P X0a M" sort="Lafleur, Rene P X0a M" uniqKey="Lafleur R" first="René P. X0a M." last="Lafleur">René P. X0a M. Lafleur</name>
</author>
<author><name sortKey="Palmans, Anja R A" sort="Palmans, Anja R A" uniqKey="Palmans A" first="Anja R. A." last="Palmans">Anja R. A. Palmans</name>
</author>
<author><name sortKey="Meijer, E W" sort="Meijer, E W" uniqKey="Meijer E" first="E. W." last="Meijer">E. W. Meijer</name>
</author>
</analytic>
<series><title level="j">ACS Macro Letters</title>
<idno type="eISSN">2161-1653</idno>
<imprint><date when="2018">2018</date>
</imprint>
</series>
</biblStruct>
</sourceDesc>
</fileDesc>
<profileDesc><textClass></textClass>
</profileDesc>
</teiHeader>
<front><div type="abstract" xml:lang="en"><p content-type="toc-graphic"><graphic xlink:href="mz-2018-00168t_0006" id="ab-tgr1"></graphic>
</p>
<p>Intriguingly, little
is known about the impact of dispersity on
the crystallization driven self-assembly (CDSA) of amphiphilic block
copolymers in aqueous media. Here, we investigate the influence of
dispersity on the CDSA of ABA-type amphiphilic block co-oligomers
(ABCOs). Two pairs of ABCOs are synthesized comprising discrete (<italic>Đ</italic>
= 1.00) or disperse (<italic>Đ</italic>
=
1.20) isotactic <sc>l</sc>
-lactic acid 16-mers as the semicrystalline
hydrophobic block and either oligo(ethylene glycol) methyl ether (MeOoEG)
or oligo(tetraethylene glycol succinate) (oTEGSuc) as the discrete
hydrophilic block. Self-assembly studies in water with 10% THF reveal
uniform nanofibers/2D sheets for the discrete oligomers, but such
structural regularity is largely compromised in the disperse oligomers.
The results are corroborated by sharp melting transitions in both
solution and bulk for the discrete ABCOs, unlike their disperse analogues
that show a lack of crystallization. Interestingly, the discrete MeOoEG-LLA
oligomer reveals crystallization driven gelation, illustrating the
contrasting differences between the discrete oligomers and their disperse
counterparts.</p>
</div>
</front>
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<author><name sortKey="Kim, S W" uniqKey="Kim S">S. W. Kim</name>
</author>
</analytic>
</biblStruct>
<biblStruct><analytic><author><name sortKey="Sackett, D L" uniqKey="Sackett D">D. L. Sackett</name>
</author>
<author><name sortKey="Wolff, J" uniqKey="Wolff J">J. Wolff</name>
</author>
</analytic>
</biblStruct>
</listBibl>
</div1>
</back>
</TEI>
<pmc article-type="rapid-communication" xml:lang="EN"><pmc-dir>properties open_access</pmc-dir>
<front><journal-meta><journal-id journal-id-type="nlm-ta">ACS Macro Lett</journal-id>
<journal-id journal-id-type="iso-abbrev">ACS Macro Lett</journal-id>
<journal-id journal-id-type="publisher-id">mz</journal-id>
<journal-id journal-id-type="coden">amlccd</journal-id>
<journal-title-group><journal-title>ACS Macro Letters</journal-title>
</journal-title-group>
<issn pub-type="epub">2161-1653</issn>
<publisher><publisher-name>American Chemical Society</publisher-name>
</publisher>
</journal-meta>
<article-meta><article-id pub-id-type="pmid">29862138</article-id>
<article-id pub-id-type="pmc">5973780</article-id>
<article-id pub-id-type="doi">10.1021/acsmacrolett.8b00168</article-id>
<article-categories><subj-group><subject>Letter</subject>
</subj-group>
</article-categories>
<title-group><article-title>Consequences of Dispersity on the Self-Assembly of
ABA-Type Amphiphilic Block Co-Oligomers</article-title>
</title-group>
<contrib-group><contrib contrib-type="author" id="ath1"><name><surname>Das</surname>
<given-names>Anindita</given-names>
</name>
</contrib>
<contrib contrib-type="author" corresp="yes" id="ath2"><name><surname>Petkau-Milroy</surname>
<given-names>Katja</given-names>
</name>
<xref rid="cor3" ref-type="other">*</xref>
</contrib>
<contrib contrib-type="author" id="ath3"><name><surname>Klerks</surname>
<given-names>Gilian</given-names>
</name>
</contrib>
<contrib contrib-type="author" id="ath4"><name><surname>van Genabeek</surname>
<given-names>Bas</given-names>
</name>
</contrib>
<contrib contrib-type="author" id="ath5"><name><surname>Lafleur</surname>
<given-names>René P.
M.</given-names>
</name>
</contrib>
<contrib contrib-type="author" corresp="yes" id="ath6"><name><surname>Palmans</surname>
<given-names>Anja R. A.</given-names>
</name>
<xref rid="cor2" ref-type="other">*</xref>
</contrib>
<contrib contrib-type="author" corresp="yes" id="ath7"><name><surname>Meijer</surname>
<given-names>E. W.</given-names>
</name>
<xref rid="cor1" ref-type="other">*</xref>
</contrib>
<aff id="aff1">Institute for Complex Molecular Systems and Laboratory of Macromolecular and Organic Chemistry,<institution>Eindhoven University of Technology</institution>
, P.O. Box 513, 5600 MB Eindhoven,<country>The Netherlands</country>
</aff>
</contrib-group>
<author-notes><corresp id="cor1"><label>*</label>
E-mail: <email>e.w.meijer@tue.nl</email>
.</corresp>
<corresp id="cor2"><label>*</label>
E-mail: <email>a.palmans@tue.nl</email>
.</corresp>
<corresp id="cor3"><label>*</label>
E-mail: <email>k.petkau-milroy@tue.nl</email>
.</corresp>
</author-notes>
<pub-date pub-type="epub"><day>16</day>
<month>04</month>
<year>2018</year>
</pub-date>
<pub-date pub-type="ppub"><day>15</day>
<month>05</month>
<year>2018</year>
</pub-date>
<volume>7</volume>
<issue>5</issue>
<fpage>546</fpage>
<lpage>550</lpage>
<history><date date-type="received"><day>28</day>
<month>02</month>
<year>2018</year>
</date>
<date date-type="accepted"><day>04</day>
<month>04</month>
<year>2018</year>
</date>
</history>
<permissions><copyright-statement>Copyright © 2018 American Chemical Society</copyright-statement>
<copyright-year>2018</copyright-year>
<copyright-holder>American Chemical Society</copyright-holder>
<license><license-p>This is an open access article published under a Creative Commons Non-Commercial No Derivative Works (CC-BY-NC-ND) Attribution <ext-link ext-link-type="uri" xlink:href="http://pubs.acs.org/page/policy/authorchoice_ccbyncnd_termsofuse.html">License</ext-link>
, which permits copying and redistribution of the article, and creation of adaptations, all for non-commercial purposes.</license-p>
</license>
</permissions>
<abstract><p content-type="toc-graphic"><graphic xlink:href="mz-2018-00168t_0006" id="ab-tgr1"></graphic>
</p>
<p>Intriguingly, little
is known about the impact of dispersity on
the crystallization driven self-assembly (CDSA) of amphiphilic block
copolymers in aqueous media. Here, we investigate the influence of
dispersity on the CDSA of ABA-type amphiphilic block co-oligomers
(ABCOs). Two pairs of ABCOs are synthesized comprising discrete (<italic>Đ</italic>
= 1.00) or disperse (<italic>Đ</italic>
=
1.20) isotactic <sc>l</sc>
-lactic acid 16-mers as the semicrystalline
hydrophobic block and either oligo(ethylene glycol) methyl ether (MeOoEG)
or oligo(tetraethylene glycol succinate) (oTEGSuc) as the discrete
hydrophilic block. Self-assembly studies in water with 10% THF reveal
uniform nanofibers/2D sheets for the discrete oligomers, but such
structural regularity is largely compromised in the disperse oligomers.
The results are corroborated by sharp melting transitions in both
solution and bulk for the discrete ABCOs, unlike their disperse analogues
that show a lack of crystallization. Interestingly, the discrete MeOoEG-LLA
oligomer reveals crystallization driven gelation, illustrating the
contrasting differences between the discrete oligomers and their disperse
counterparts.</p>
</abstract>
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<meta-value>mz8b00168</meta-value>
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<custom-meta><meta-name>document-id-new-14</meta-name>
<meta-value>mz-2018-00168t</meta-value>
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</front>
<body><p>Self-assembly of block copolymers
is a topic of considerable interest in polymer science due to the
tremendous potential for applications in both biomedical engineering
and nanolithography.<sup><xref ref-type="bibr" rid="ref1">1</xref>
,<xref ref-type="bibr" rid="ref2">2</xref>
</sup>
Although abiotic polymers synthesized
via controlled polymerization techniques show great diversity in their
structures and functions, they still suffer from significant molar
mass distribution and cannot match up with the architectural purity,
precision, and complexity displayed by biomacromolecules.<sup><xref ref-type="bibr" rid="ref3">3</xref>
,<xref ref-type="bibr" rid="ref4">4</xref>
</sup>
In contrast, most biopolymers such as DNA, RNA, and polypeptides
are monodisperse and sequence-specific, which is critical to their
overall three-dimensional organization and thus their properties and
functions. In the recent past, polymer research started focusing on
both discrete (<italic>Đ</italic>
< 1.000002)<sup><xref ref-type="bibr" rid="ref4">4</xref>
−<xref ref-type="bibr" rid="ref10">10</xref>
</sup>
and sequence-specific polymers<sup><xref ref-type="bibr" rid="ref11">11</xref>
−<xref ref-type="bibr" rid="ref15">15</xref>
</sup>
in an attempt to mimic these aspects of biomacromolecules.</p>
<p>Amphiphilic block copolymers (ABCPs) have been a topic of long-standing
interest in biomedical research for their ability to form nanocarriers
such as micelles, vesicles, nanorods, and other tailored shapes for
drug delivery.<sup><xref ref-type="bibr" rid="ref2">2</xref>
,<xref ref-type="bibr" rid="ref16">16</xref>
,<xref ref-type="bibr" rid="ref17">17</xref>
</sup>
Depending on the hydrophilic/hydrophobic block ratio, molecular
weight of the polymer, and crystallinity of the hydrophobic core,
the morphology and properties of these nanoparticles can be engineered.
Surprisingly, the impact of the molar mass distribution (<italic>Đ</italic>
) on nanoparticle formation, shape, structural uniformity, and efficacy
for uptake and release of guest molecules has hardly been investigated
in amphiphilic block copolymers. Comparative self-assembly studies
between ultradefined discrete ABCPs (<italic>Đ</italic>
= 1.000)
and their disperse counterparts could be extremely important in the
fundamental understanding of the influence of dispersity on their
properties after self-assembly in the aqueous phase. Contrasting differences
between the discrete and disperse BCPs have been recently observed
in the bulk phase. Our group has reported on the self-assembly of
discrete diblock co-oligomers (BCOs) composed of oligolactic acid
(oLA) and oligodimethylsiloxane (oDMS) obtained by iterative coupling-deprotection
based synthetic strategies.<sup><xref ref-type="bibr" rid="ref18">18</xref>
,<xref ref-type="bibr" rid="ref19">19</xref>
</sup>
Whereas the discrete
polymer formed well-organized lamellar structures, its disperse counterpart
revealed a lower extent of ordering with an increase of the domain
spacing and greater stability of the phase-separated structures.<sup><xref ref-type="bibr" rid="ref19">19</xref>
</sup>
In a complementary study, the group of Hawker
observed similar differences in bulk between semidiscrete and disperse
BCOs composed of oligomethyl methacrylate (oMMA) and oDMS.<sup><xref ref-type="bibr" rid="ref20">20</xref>
</sup>
</p>
<p>Intrigued by these results, we here aim
to investigate the effect
of dispersity in the aqueous phase, where the high mobility of the
flexible polymers chains in solution presents an additional challenge.
With this objective, we synthesized two pairs of ABA-type amphiphilic
block co-oligomers (ABCOs) composed of discrete (<italic>Đ</italic>
= 1.000) or disperse (<italic>Đ</italic>
= 1.2) isotactic
oligo(L-lactic acid) (LLA) as the hydrophobic block and either oligoethylene
glycol methyl ether (MeOoEG) or oligo(tetraethylene glycol succinate)
(oTEGSuc) as the discrete hydrophilic block (<xref rid="fig1" ref-type="fig">Figure <xref rid="fig1" ref-type="fig">1</xref>
</xref>
A). The rationale behind choosing these blocks
is as follows: the self-assembly of polylactic acid-<italic>b</italic>
-polyethylene glycol (PLA-<italic>b</italic>
-PEG) has been explicitly
studied<sup><xref ref-type="bibr" rid="ref21">21</xref>
−<xref ref-type="bibr" rid="ref23">23</xref>
</sup>
in the context of drug delivery and regenerative
medicine because these polymers are known to be biocompatible.<sup><xref ref-type="bibr" rid="ref24">24</xref>
,<xref ref-type="bibr" rid="ref25">25</xref>
</sup>
oTEGSuc block was chosen as a biodegradable substitute to PEG, to
generate fully biodegradable ABCOs.<sup><xref ref-type="bibr" rid="ref26">26</xref>
</sup>
Discrete
chains of oTEGSuc can be synthesized following the iterative synthetic
approach presented in <xref rid="sch1" ref-type="scheme">Scheme <xref rid="sch1" ref-type="scheme">1</xref>
</xref>
. The <sc>l</sc>
-lactic acid 16-mer was selected as the hydrophobic
core either as a discrete 16-mer of exact molecular weight (LLA<sub>16</sub>
) or as a disperse one (LLA<sub>∼16</sub>
) with <italic>Đ</italic>
= 1.20. Discrete LLA<sub>16</sub>
is semicrystalline
and forms ordered lamellae in the bulk.<sup><xref ref-type="bibr" rid="ref27">27</xref>
</sup>
In the context of drug delivery, the crystallization driven self-assembly
(CDSA) of amphiphilic block copolymers with a crystallizable hydrophobic
core has been applied to fabricate nonspherical nanostructures in
solution.<sup><xref ref-type="bibr" rid="ref28">28</xref>
,<xref ref-type="bibr" rid="ref29">29</xref>
</sup>
However, as of yet there is no study on
understanding the consequence of dispersity on block crystallinity
in solution, which is investigated in the present work.</p>
<fig id="fig1" position="float"><label>Figure 1</label>
<caption><p>(a) Structures
of discrete and disperse amphiphilic block co-oligomers
and (b) their MALDI-ToF spectra.</p>
</caption>
<graphic xlink:href="mz-2018-00168t_0001" id="gr2" position="float"></graphic>
</fig>
<fig id="sch1" position="float"><label>Scheme 1</label>
<caption><title>Synthesis of (a) Discrete Tetraethylene Glycol Succinate (TEGSuc)<sub><italic>x</italic>
</sub>
Oligomers, (b) Their MALDI-ToF Spectra,
and (c) Synthesis of Discrete ABCOs</title>
</caption>
<graphic xlink:href="mz-2018-00168t_0005" id="gr1" position="float"></graphic>
</fig>
<p>For the synthesis of discrete oTEGSuc, we followed a modified
iterative
coupling-deprotection route (<xref rid="sch1" ref-type="scheme">Scheme <xref rid="sch1" ref-type="scheme">1</xref>
</xref>
a) recently reported by our group for monodisperse
lactic acid oligomers.<sup><xref ref-type="bibr" rid="ref18">18</xref>
</sup>
Succinic acid
monobenzyl ester (<bold>1</bold>
) was coupled with mono <italic>tert</italic>
-butyl dimethylsilyl (TBDMS) ether protected tetraethylene glycol
(TEG; <bold>2</bold>
) to obtain double-protected monomer <bold>(TEGSuc)<sub>1</sub>
</bold>
. Orthogonal deprotection of the TBDMS ether and the
benzyl ester resulted in free hydroxyl and carboxylic acid containing <bold>3</bold>
and <bold>4</bold>
, respectively. Carbodiimide-promoted
coupling between the two afforded double-protected dimer <bold>(TEGSuc)<sub>2</sub>
</bold>
. By repetition of the deprotection and coupling steps,
tetramer <bold>(TEGSuc)<sub>4</sub>
</bold>
was obtained. A stack plot
of the MALDI-ToF spectra of the double-protected <bold>(TEGSuc)<sub><italic>x</italic>
</sub>
</bold>
oligomers from monomer to tetramer
is shown in <xref rid="sch1" ref-type="scheme">Scheme <xref rid="sch1" ref-type="scheme">1</xref>
</xref>
b. Single peaks corresponding to the mass of the desired species
complexed with sodium ion and potassium ion indicate that precisely
defined block lengths were obtained.</p>
<p>The synthesis of discrete
telechelic LLA<sub>16</sub>
with free
carboxylic acid moieties (HOOC-SA-LLA<sub>16</sub>
-COOH, <xref rid="sch1" ref-type="scheme">Scheme <xref rid="sch1" ref-type="scheme">1</xref>
</xref>
c) is based on the
synthetic strategy previously reported by Hawker and co-workers.<sup><xref ref-type="bibr" rid="ref30">30</xref>
</sup>
Disperse telechelic LLA<sub>∼16</sub>
was synthesized by ring-opening polymerization. The dispersity of
LLA<sub>∼16</sub>
(<italic>Đ</italic>
= 1.2) was determined
using size exclusion chromatography. Full synthetic details on the
preparation of the hydrophobic blocks can be found in the <ext-link ext-link-type="uri" xlink:href="http://pubs.acs.org/doi/suppl/10.1021/acsmacrolett.8b00168/suppl_file/mz8b00168_si_001.pdf">Supporting Information</ext-link>
. Subsequent ligation of
the acid functionalized LLA block with two equivalents of hydroxyl
functionalized discrete (TEGSuc)<sub>2</sub>
-OH, or commercially available
discrete MeOoEG<sub>11</sub>
–OH resulted in the target ABA-type
ABCOs <bold>P1</bold>
and <bold>P2</bold>
(<xref rid="sch1" ref-type="scheme">Scheme <xref rid="sch1" ref-type="scheme">1</xref>
</xref>
c). The discrete ABCOs are designated as <bold>P<italic>x</italic>
</bold>
<sup><italic><bold>discrete</bold>
</italic>
</sup>
and their disperse analogues are referred to as <bold>P<italic>x</italic>
</bold>
<sup><italic><bold>disperse</bold>
</italic>
</sup>
. All the compounds were purified by automated column chromatography
and fully analyzed by <sup>1</sup>
H NMR, <sup>13</sup>
C NMR, and matrix-assisted
laser desorption/ionization time-of-flight (MALDI-ToF) mass spectrometry
(<ext-link ext-link-type="uri" xlink:href="http://pubs.acs.org/doi/suppl/10.1021/acsmacrolett.8b00168/suppl_file/mz8b00168_si_001.pdf">Figures S1–S9</ext-link>
). Despite similar
degrees of polymerization based on <sup>1</sup>
H NMR, the MALDI-ToF
spectra of the discrete and the disperse oligomers reveal a wide distribution
in the chain length for disperse samples compared to a single peak
for the discrete ones (<xref rid="fig1" ref-type="fig">Figure <xref rid="fig1" ref-type="fig">1</xref>
</xref>
).</p>
<p>The thermal behavior and degree of ordering in the
bulk of the
discrete and disperse ABCOs was investigated using differential scanning
calorimetry (DSC; <ext-link ext-link-type="uri" xlink:href="http://pubs.acs.org/doi/suppl/10.1021/acsmacrolett.8b00168/suppl_file/mz8b00168_si_001.pdf">Figures S10 and S11</ext-link>
)
and small and wide angle X-ray scattering (SAXS and WAXS; <ext-link ext-link-type="uri" xlink:href="http://pubs.acs.org/doi/suppl/10.1021/acsmacrolett.8b00168/suppl_file/mz8b00168_si_001.pdf">Figure S12</ext-link>
). The introduction of dispersity into
the LLA block has a clear effect on the position and intensity of
the DSC transitions for both the <bold>P1</bold>
and <bold>P2</bold>
systems (see <ext-link ext-link-type="uri" xlink:href="http://pubs.acs.org/doi/suppl/10.1021/acsmacrolett.8b00168/suppl_file/mz8b00168_si_001.pdf">SI</ext-link>
for a detailed discussion).
Notably, the SAXS data show a primary scattering peak only for the
discrete samples, corresponding to <italic>d</italic>
= 6.5 nm for <bold>P1</bold>
<sup><italic><bold>discrete</bold>
</italic>
</sup>
and <italic>d</italic>
= 10.5 nm for <bold>P2</bold>
<sup><italic><bold>discrete</bold>
</italic>
</sup>
<italic>Bn</italic>
(<ext-link ext-link-type="uri" xlink:href="http://pubs.acs.org/doi/suppl/10.1021/acsmacrolett.8b00168/suppl_file/mz8b00168_si_001.pdf">Figure S12</ext-link>
). A higher order scattering peak at <italic>d</italic>
= 5.2 nm
in case of <bold>P2</bold>
<sup><italic><bold>discrete</bold>
</italic>
</sup>
<italic>Bn</italic>
suggests a lamellar packing of the ABCO
in the bulk. Such clear scattering peaks at low <italic>q</italic>
values are absent for both disperse <bold>P1</bold>
<sup><italic><bold>disperse</bold>
</italic>
</sup>
and <bold>P2</bold>
<sup><italic><bold>disperse</bold>
</italic>
</sup>
<italic>Bn</italic>
, indicating
reduced phase segregation between the two blocks and a less defined
morphology. All in all, the differences in bulk properties of discrete
and disperse ABCOs are in good correspondence to our previous results
published on oLLA-oDMS block-<italic>co</italic>
-oligomers,<sup><xref ref-type="bibr" rid="ref19">19</xref>
</sup>
where we observed a significant loss in long-range
order when dispersity was introduced into the oLLA block.</p>
<p>Subsequently,
the self-assembly of the ABCOs was studied in aqueous
media. Due to the more hydrophobic nature of TEGSuc as compared to
MeOoEG, <bold>P2</bold>
could not be dissolved in pure water. As a
result, all the studies were performed in water/THF mixtures with
10% THF.<sup><xref ref-type="bibr" rid="ref31">31</xref>
</sup>
To prepare the solutions, each
compound was dissolved in THF, and water was added dropwise to reach
a 1:9 THF/water binary mixture at 1–5 mg ABCO per mL. The formation
of the nanoparticles was studied with light and X-ray scattering (LS
and SAXS), micro-DSC, and microscopy (cryoTEM and total internal reflection
fluorescence (TIRF) microscopy). Diffusion coefficients were obtained
from multiangle light scattering by fitting the decay rate (<inline-formula id="d30e534"><inline-graphic xlink:href="mz-2018-00168t_m001.gif"></inline-graphic>
</inline-formula>
) versus the scattering vector (<italic>q</italic>
) plot (<ext-link ext-link-type="uri" xlink:href="http://pubs.acs.org/doi/suppl/10.1021/acsmacrolett.8b00168/suppl_file/mz8b00168_si_001.pdf">Figure S13</ext-link>
). Using the
Stoke–Einstein equation, the hydrodynamic radius (<italic>R</italic>
<sub>h</sub>
) of the particles was calculated. After self-assembly
in water, the <italic>R</italic>
<sub>h</sub>
was found to be larger
for the discrete variants (<italic>R</italic>
<sub>h</sub>
= 74 nm
for <bold>P1</bold>
<sup><italic><bold>discrete</bold>
</italic>
</sup>
and 125 nm for <bold>P2</bold>
<sup><italic><bold>discrete</bold>
</italic>
</sup>
) than for the disperse analogues (<italic>R</italic>
<sub>h</sub>
= 42 nm for <bold>P1</bold>
<sup><italic><bold>disperse</bold>
</italic>
</sup>
and 90 nm for <bold>P2</bold>
<sup><italic><bold>disperse</bold>
</italic>
</sup>
). When comparing the LS data of <bold>P1</bold>
with <bold>P2</bold>
, it appears that larger particles are for the <bold>P2</bold>
pairs. However, fitting the decay rate versus the scattering vector
reveals some anisotropy in the structures, indicating that the particles
are not spherical, and thus, the Stoke–Einstein equation does
not apply. To get an indication of the shape of the particles formed,
the scattering intensity (<italic>I</italic>
) was plotted against <italic>q</italic>
(<ext-link ext-link-type="uri" xlink:href="http://pubs.acs.org/doi/suppl/10.1021/acsmacrolett.8b00168/suppl_file/mz8b00168_si_001.pdf">Figure S14</ext-link>
). The slope of
−1 for both <bold>P1</bold>
<sup><italic><bold>discrete</bold>
</italic>
</sup>
and <bold>P1</bold>
<sup><italic><bold>disperse</bold>
</italic>
</sup>
indicates that this ABCO self-assembles into cylindrical
micelles, whereas <bold>P2</bold>
<sup><italic><bold>discrete</bold>
</italic>
</sup>
and <bold>P2</bold>
<sup><italic><bold>disperse</bold>
</italic>
</sup>
<italic>self-assemble</italic>
into vesicles or flat
bilayers (slope of −2). The morphology of an ABCP is largely
dependent on the hydrophobic/hydrophilic block ratio.<sup><xref ref-type="bibr" rid="ref2">2</xref>
</sup>
For an invariant ratio, MeOoEG was replaced with a TEGSuc
block of comparable molar mass. Possibly, the more hydrophobic nature
of the TEGSuc block<sup><xref ref-type="bibr" rid="ref26">26</xref>
</sup>
changes this balance,
leading to the formation of vesicles or bilayers.</p>
<p>To substantiate
the formation of cylindrical micelles by <bold>P1</bold>
in solution,
SAXS measurements were performed. The SAXS
profile obtained of <bold>P1</bold>
<sup><italic><bold>discrete</bold>
</italic>
</sup>
was best fitted with a flexible cylinder model (<xref rid="fig2" ref-type="fig">Figures <xref rid="fig2" ref-type="fig">2</xref>
</xref>
a and <ext-link ext-link-type="uri" xlink:href="http://pubs.acs.org/doi/suppl/10.1021/acsmacrolett.8b00168/suppl_file/mz8b00168_si_001.pdf">S15a</ext-link>
). The radius of 3.2 nm, the Kuhn length
of 103 nm and the overall length of 1038 nm agrees well with the cryoTEM
observations (<xref rid="fig3" ref-type="fig">Figure <xref rid="fig3" ref-type="fig">3</xref>
</xref>
b), which confirms the formation of elongated thin fibers of consistent
width. In contrast, the cryoTEM image (<xref rid="fig3" ref-type="fig">Figure <xref rid="fig3" ref-type="fig">3</xref>
</xref>
a) of the <bold>P1</bold>
<sup><italic><bold>disperse</bold>
</italic>
</sup>
reveals the coexistence of two populations.
Next to the elongated thin fibers, bundles of shorter but much wider
fibers are present. This bundling effect might be due to the coassembly
of LLA blocks of varying lengths. The SAXS profile of <bold>P1</bold>
<sup><italic><bold>disperse</bold>
</italic>
</sup>
could not be fit
well, which is likely due to an overlay of the scattering of multiple
species present in solution (<xref rid="fig2" ref-type="fig">Figures <xref rid="fig2" ref-type="fig">2</xref>
</xref>
a and <ext-link ext-link-type="uri" xlink:href="http://pubs.acs.org/doi/suppl/10.1021/acsmacrolett.8b00168/suppl_file/mz8b00168_si_001.pdf">S15b</ext-link>
). The results
above clearly exemplify the pronounced impact of LLA block dispersity
on the homogeneity of the self-assembled structures. In addition,
the morphologies formed were highly stable over time. CryoTEM images
of aged samples for both <bold>P1</bold>
<sup><italic><bold>discrete</bold>
</italic>
</sup>
and <bold>P1</bold>
<sup><italic><bold>disperse</bold>
</italic>
</sup>
(<ext-link ext-link-type="uri" xlink:href="http://pubs.acs.org/doi/suppl/10.1021/acsmacrolett.8b00168/suppl_file/mz8b00168_si_001.pdf">Figure S16</ext-link>
) retained
the same morphologies even after keeping the solutions at room temperature
for around 90 days.</p>
<fig id="fig2" position="float"><label>Figure 2</label>
<caption><p>Solution SAXS traces of (a) <bold>P1</bold>
<sup><italic><bold>discrete</bold>
</italic>
</sup>
and <bold>P1</bold>
<sup><italic><bold>disperse</bold>
</italic>
</sup>
(0.5 mg mL<sup>–1</sup>
) and of (b) <bold>P2</bold>
<sup><italic><bold>discrete</bold>
</italic>
</sup>
and <bold>P2</bold>
<sup><italic><bold>disperse</bold>
</italic>
</sup>
(0.6 mg
mL<sup>–1</sup>
) after self-assembly in water with 10% THF;
(a, inset) self-assembly of <bold>P1</bold>
<sup><italic><bold>discrete</bold>
</italic>
</sup>
(gel) and <bold>P1</bold>
<sup><italic><bold>disperse</bold>
</italic>
</sup>
(sol) at 5 mg mL<sup>–1</sup>
in water with
10% THF. The lines represent the best fit to the data using either
a flexible cylinder (a) or the lamellar (b) model.</p>
</caption>
<graphic xlink:href="mz-2018-00168t_0002" id="gr3" position="float"></graphic>
</fig>
<fig id="fig3" position="float"><label>Figure 3</label>
<caption><p>CryoTEM images at 25000 magnification of (a) <bold>P1</bold>
<sup><italic><bold>disperse</bold>
</italic>
</sup>
and (b) <bold>P1</bold>
<sup><italic><bold>discrete</bold>
</italic>
</sup>
, both at 5 mg mL<sup>–1</sup>
in water with 10% THF, and (c) <bold>P2</bold>
<sup><italic><bold>disperse</bold>
</italic>
</sup>
and (d) <bold>P2</bold>
<sup><italic><bold>discrete</bold>
</italic>
</sup>
both at 2 mg mL<sup>–1</sup>
in water with 10% THF. Large dark round particles
in “a” are crystalline ice particles and not part of
the sample. The images were recorded at 10 μm (a, b, d) and
5 μm defocus (c). For corresponding low-magnification images
see <ext-link ext-link-type="uri" xlink:href="http://pubs.acs.org/doi/suppl/10.1021/acsmacrolett.8b00168/suppl_file/mz8b00168_si_001.pdf">Figure S17</ext-link>
.</p>
</caption>
<graphic xlink:href="mz-2018-00168t_0003" id="gr4" position="float"></graphic>
</fig>
<p>Interestingly, the discrete oligomer <bold>P1</bold>
<sup><italic><bold>discrete</bold>
</italic>
</sup>
formed a transparent
gel at
5 mg mL<sup>–1</sup>
when the solution was heated and cooled
back to room temperature (<xref rid="fig2" ref-type="fig">Figure <xref rid="fig2" ref-type="fig">2</xref>
</xref>
a, inset). The gelation process was found to be reversible
and repeatable. The gel–sol transition (<italic>T</italic>
<sub>gel</sub>
), as determined visually for multiple cycles, varied between
42 and 48 °C upon heating the sample. This transition is very
close to the melting temperature (<italic>T</italic>
<sub>m</sub>
=
41 °C) of discrete LLA<sub>16</sub>
block measured in the bulk
(<ext-link ext-link-type="uri" xlink:href="http://pubs.acs.org/doi/suppl/10.1021/acsmacrolett.8b00168/suppl_file/mz8b00168_si_001.pdf">Figure S10a</ext-link>
), as well as the transition
temperature of 43 °C measured in solution by micro-DSC (<xref rid="fig4" ref-type="fig">Figure <xref rid="fig4" ref-type="fig">4</xref>
</xref>
, vide infra). Notably,
no gelation was observed for <bold>P1</bold>
<sup><italic><bold>disperse</bold>
</italic>
</sup>
under identical conditions, although gelation in
disperse PEG-PLLA-PEG based triblock copolymer is well reported via
interdigitation of micelles through PEG chains.<sup><xref ref-type="bibr" rid="ref32">32</xref>
</sup>
Such discrepancy in the gelation behavior of <bold>P1</bold>
<sup><italic><bold>discrete</bold>
</italic>
</sup>
and <bold>P1</bold>
<sup><italic><bold>disperse</bold>
</italic>
</sup>
nicely corroborates
with their distinctly different morphologies as observed from cryoTEM.</p>
<fig id="fig4" position="float"><label>Figure 4</label>
<caption><p>Micro-DSC
traces of <bold>P1</bold>
<sup><italic><bold>discrete</bold>
</italic>
</sup>
and <bold>P1</bold>
<sup><italic><bold>disperse</bold>
</italic>
</sup>
at 5 mg mL<sup>–1</sup>
after self-assembly
in water with 10% THF.</p>
</caption>
<graphic xlink:href="mz-2018-00168t_0004" id="gr5" position="float"></graphic>
</fig>
<p>The self-assembly of the <bold>P2</bold>
pair was further
investigated
using SAXS in solution. The SAXS profile for both <bold>P2</bold>
<sup><italic><bold>discrete</bold>
</italic>
</sup>
and <bold>P2</bold>
<sup><italic><bold>disperse</bold>
</italic>
</sup>
shows a slope of −2,
indicating the formation of vesicles or flat bilayers (<xref rid="fig2" ref-type="fig">Figures <xref rid="fig2" ref-type="fig">2</xref>
</xref>
b and <ext-link ext-link-type="uri" xlink:href="http://pubs.acs.org/doi/suppl/10.1021/acsmacrolett.8b00168/suppl_file/mz8b00168_si_001.pdf">S15c,d</ext-link>
). In contrast to the <bold>P1</bold>
pair, both profiles
for <bold>P1</bold>
<sup><italic><bold>discrete</bold>
</italic>
</sup>
and <bold>P1</bold>
<sup><italic><bold>disperse</bold>
</italic>
</sup>
are very similar. The SAXS profiles were fitted best using the lamellae
model, indicating a lamellae thickness of 7 nm (<bold>P2</bold>
<sup><italic><bold>discrete</bold>
</italic>
</sup>
) or 8 nm (<bold>P2</bold>
<sup><italic><bold>disperse</bold>
</italic>
</sup>
). Since the <bold>P2</bold>
<sup><italic><bold>disperse</bold>
</italic>
</sup>
sample
was not transparent, indicative of the formation of large aggregates,
cryoTEM imaging of <bold>P2</bold>
<sup><italic><bold>disperse</bold>
</italic>
</sup>
was first performed of a filtered sample, revealing
the presence of uniform sheets rolled into U-shape (<xref rid="fig3" ref-type="fig">Figures <xref rid="fig3" ref-type="fig">3</xref>
</xref>
c and <ext-link ext-link-type="uri" xlink:href="http://pubs.acs.org/doi/suppl/10.1021/acsmacrolett.8b00168/suppl_file/mz8b00168_si_001.pdf">S18</ext-link>
). Flat sheets were observed in a sample that was not filtered
prior to imaging (<ext-link ext-link-type="uri" xlink:href="http://pubs.acs.org/doi/suppl/10.1021/acsmacrolett.8b00168/suppl_file/mz8b00168_si_001.pdf">Figure S18</ext-link>
). To visualize
the aggregates of <bold>P2</bold>
<sup><italic><bold>disperse</bold>
</italic>
</sup>
at the μm length scale, TIRF microscopy was
performed after the addition of the dye Nile Red. This hydrophobic
dye is weakly fluorescent in water but becomes highly fluorescent
after incorporation into hydrophobic domains.<sup><xref ref-type="bibr" rid="ref33">33</xref>
</sup>
Besides the sheets rolled into U-shape, other spherical morphologies
and large aggregates were present (<ext-link ext-link-type="uri" xlink:href="http://pubs.acs.org/doi/suppl/10.1021/acsmacrolett.8b00168/suppl_file/mz8b00168_si_001.pdf">Figure S18</ext-link>
). In contrast, for <bold>P2</bold>
<sup><italic><bold>discrete</bold>
</italic>
</sup>
only the formation of long sheets was observed by
cryoTEM imaging (<xref rid="fig3" ref-type="fig">Figure <xref rid="fig3" ref-type="fig">3</xref>
</xref>
d), as well as by TIRF microscopy (<ext-link ext-link-type="uri" xlink:href="http://pubs.acs.org/doi/suppl/10.1021/acsmacrolett.8b00168/suppl_file/mz8b00168_si_001.pdf">Figure S19</ext-link>
). Also for the <bold>P2</bold>
set, the results clearly exemplify
that the dispersity of the LLA<sub>∼16</sub>
block inhibits
the uniformity of the self-assembled structures in solution.</p>
<p>To study whether the self-assembly in solution is driven by the
crystallization of the LLA block, microdifferential scanning calorimetry
(microDSC) in 1:9 THF/water was performed. The solutions were heated
from 5 to 70 °C at a rate of 0.5 K min<sup>–1</sup>
(<xref rid="fig4" ref-type="fig">Figure <xref rid="fig4" ref-type="fig">4</xref>
</xref>
). <bold>P1</bold>
<sup><italic><bold>discrete</bold>
</italic>
</sup>
showed clear endothermic
and exothermic transitions in the heating and the cooling runs corresponding
to melting (<italic>T</italic>
<sub>m</sub>
= 43 °C) and crystallization
(<italic>T</italic>
<sub>c</sub>
= 33 °C), respectively. The <italic>T</italic>
<sub>m</sub>
in solution (<xref rid="fig4" ref-type="fig">Figure <xref rid="fig4" ref-type="fig">4</xref>
</xref>
) corresponds well with the melting (<italic>T</italic>
<sub>m</sub>
= 41 °C) in bulk (<ext-link ext-link-type="uri" xlink:href="http://pubs.acs.org/doi/suppl/10.1021/acsmacrolett.8b00168/suppl_file/mz8b00168_si_001.pdf">Figure S10a</ext-link>
) for <bold>P1</bold>
<sup><italic><bold>discrete</bold>
</italic>
</sup>
indicating that this transition is indeed connected
to the LLA<sub>16</sub>
crystallization. Further, close matching of
the <italic>T</italic>
<sub>m</sub>
with the <italic>T</italic>
<sub>gel–sol</sub>
of <bold>P1</bold>
<sup><italic><bold>discrete</bold>
</italic>
</sup>
substantiates crystallization driven gelation. Interestingly,
no phase transitions were observed for the discrete <bold>P2</bold>
<sup><italic><bold>discrete</bold>
</italic>
</sup>
(<ext-link ext-link-type="uri" xlink:href="http://pubs.acs.org/doi/suppl/10.1021/acsmacrolett.8b00168/suppl_file/mz8b00168_si_001.pdf">Figure S20</ext-link>
), suggesting that the change in the hydrophilic
block from MeOoEG to TEGSuc might influence the crystallization of
the LLA<sub>16</sub>
core. This corroborates well with the amorphous
nature of TEGSuc block as observed from its DSC profile (<ext-link ext-link-type="uri" xlink:href="http://pubs.acs.org/doi/suppl/10.1021/acsmacrolett.8b00168/suppl_file/mz8b00168_si_001.pdf">Figure S11a</ext-link>
) in contrast to the semicrystalline
PEG chain (<ext-link ext-link-type="uri" xlink:href="http://pubs.acs.org/doi/suppl/10.1021/acsmacrolett.8b00168/suppl_file/mz8b00168_si_001.pdf">Figure S11b</ext-link>
). The very slow
rate of crystallization of <bold>P2</bold>
<sup><italic><bold>discrete</bold>
</italic>
</sup>
<italic>Bn</italic>
in the bulk (<ext-link ext-link-type="uri" xlink:href="http://pubs.acs.org/doi/suppl/10.1021/acsmacrolett.8b00168/suppl_file/mz8b00168_si_001.pdf">Figure S10b,c</ext-link>
) further supports our interpretation that the
PEG chains aid the ordering of the LLA<sub>16</sub>
block unlike TEGSuc
chains for discrete pairs.</p>
<p>No clear transitions were observed
for <bold>P1</bold>
<sup><italic><bold>disperse</bold>
</italic>
</sup>
(<xref rid="fig4" ref-type="fig">Figure <xref rid="fig4" ref-type="fig">4</xref>
</xref>
) or <bold>P2</bold>
<sup><italic><bold>disperse</bold>
</italic>
</sup>
(<ext-link ext-link-type="uri" xlink:href="http://pubs.acs.org/doi/suppl/10.1021/acsmacrolett.8b00168/suppl_file/mz8b00168_si_001.pdf">Figure S20</ext-link>
). Such distinct
differences between <bold>P1</bold>
<sup><italic><bold>discrete</bold>
</italic>
</sup>
and <bold>P1</bold>
<sup><italic><bold>disperse</bold>
</italic>
</sup>
reveal the negative impact of dispersity on the core
crystallinity of the amphiphiles in the solution phase, leading to
their varying self-assembly behavior. Possibly, the dispersity in
the hydrophobic block does not allow effective packing of the LLA
chains of varying length within the core of the nanoparticles in <bold>P1</bold>
<sup><italic><bold>disperse</bold>
</italic>
</sup>
, unlike
in <bold>P1</bold>
<sup><italic><bold>discrete</bold>
</italic>
</sup>
. Although there are multiple reports on crystallization driven self-assembly
of block copolymer amphiphiles, this is the first demonstration of
the consequence of dispersity on the crystallization mediated self-assembly
of oligomeric amphiphiles in aqueous solution.</p>
<p>In summary, we
have methodically manifested the effect of dispersity
on the assembly behavior of two sets of discrete amphiphilic block
co-oligomers by comparing their solution self-assembly behaviors with
their disperse counterparts. The finding of this work reveals remarkable
differences between the discrete and the disperse ABCOs not just in
the bulk but also in the solution phase in terms of crystallinity,
gelation, morphology, and homogeneity of the self-assembled structures.
We anticipate that further fundamental studies on pharmaceutically
relevant PEG–PLLA based block co-oligomers will pave the way
for synthesis of tailor-made nanocarriers with more control over their
structures, dynamics, and functions as delivery vehicles.</p>
</body>
<back><notes id="notes-1" notes-type="si"><title>Supporting Information Available</title>
<p>The Supporting Information
is available free of charge on the <ext-link ext-link-type="uri" xlink:href="http://pubs.acs.org">ACS Publications website</ext-link>
at DOI: <ext-link ext-link-type="uri" xlink:href="http://pubs.acs.org/doi/abs/10.1021/acsmacrolett.8b00168">10.1021/acsmacrolett.8b00168</ext-link>
.<list id="silist" list-type="simple"><list-item><p>Experimental procedures,
synthesis, and characterization
data for all compounds, bulk characterization, and Figures S1–S20
(<ext-link ext-link-type="uri" xlink:href="http://pubs.acs.org/doi/suppl/10.1021/acsmacrolett.8b00168/suppl_file/mz8b00168_si_001.pdf">PDF</ext-link>
).</p>
</list-item>
</list>
</p>
</notes>
<sec sec-type="supplementary-material"><title>Supplementary Material</title>
<supplementary-material content-type="local-data" id="sifile1"><media xlink:href="mz8b00168_si_001.pdf"><caption><p>mz8b00168_si_001.pdf</p>
</caption>
</media>
</supplementary-material>
</sec>
<notes notes-type="" id="notes-2"><title>Author Contributions</title>
<p>All authors
have given approval to the final version of the manuscript.</p>
</notes>
<notes notes-type="COI-statement" id="NOTES-d440e1223-autogenerated"><p>The authors declare no
competing financial interest.</p>
</notes>
<ack><title>Acknowledgments</title>
<p>The authors acknowledge financial support from the Dutch Ministry
of Education, Culture and Science (Gravity Program 024.001.035). The
BM29 beamline at the European Synchrotron Radiation Facilities (Grenoble,
France) is acknowledged for access to the synchrotron facilities and
for help with acquiring SAXS data. We gratefully thank G.M. ter Huurne
for his help of acquiring the SAXS data and S. P. W. Wijnands for
his help with TIRF microscopy.</p>
</ack>
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<ref id="ref31"><note><p><bold>P1</bold>
<sup><italic><bold>discrete</bold>
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) and are soluble in
pure water as well.</p>
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