Visible Light Photoinitiated Metal-Free Living Cationic Polymerization of 4-Methoxystyrene

Article abstract of DOI:10.1021/jacs.5b03733

Metal-free, visible light-initiated, living cationic polymerization of 4-methoxystyrene using 2,4,6-tri(p-tolyl)pyrylium tetrafluoroborate and methanol is demonstrated. Molecular weight and dispersity are controlled by the concentration of methanol. Initial mechanistic analysis suggests that methanol likely serves to regulate propagation of the cation chain end via reversible chain transfer in a manner analogous to reversible addition-fragmentation chain transfer polymerization.

Full text of DOI:10.1021/jacs.5b03733

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Communication
Visible Light Photoinitiated Metal-Free Living
Cationic Polymerization of 4-Methoxystyrene
Andrew J. Perkowski, Wei You, and David A. Nicewicz
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 08 Jun 2015
Downloaded from http://pubs.acs.org on June 8, 2015
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Visible Light Photoinitiated Metal-Free Living Cationic Polymer-
ization of 4-Methoxystyrene
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Andrew J. Perkowski, Wei You,* and David A. Nicewicz*
Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, 27599ꢀ3290, United States
Supporting Information Placeholder
ABSTRACT: Metalꢀfree, visible lightꢀinitiated, living cationic
polymerization of 4ꢀmethoxystyrene using 2,4,6ꢀtri(pꢀ
tolyl)pyrylium tetrafluoroborate and methanol is demonstrated.
Molecular weight and dispersity are controlled by the concentraꢀ
tion of methanol. Initial mechanistic analysis suggests that
methanol likely serves to regulate propagation of the cation
chain end via reversible chain transfer in a manner analogous to
RAFT polymerization.
The ongoing development of new methods for the synthesis
of polymers of complex architecture has led to a continuing
interest in controlled/living polymerization.1,2 Inspired by rapid
progress in photoredox catalysis,3 a few research groups have
recently begun to explore the application of these systems for
controlled polymerization.4–7 Pioneering work by the Hawker
group has seen the development of photoredoxꢀpromoted Atom
Transfer Radical Polymerization (ATRP) for the controlled
polymerization of acrylates making use of Ir(ppy)3 as the single
electron photoreductant.4 Using similar visible lightꢀactivated
catalysts, Boyer and coworkers have developed a Photoinduced
Electron Transfer Reversible AdditionꢀFragmentation chain
Transfer (PETꢀRAFT) polymerization system.5 Hawker and
coworkers have recently extended their photopromoted ATRP
methodology to an organic system, making use of 10ꢀ
phenylphenothiazine (PTH) in the place of Ir(ppy)3 as the phoꢀ
toreductant (Figure 1).7 While only very low loadings of
Ir(ppy)3 were necessary to carry out efficient ATRP, the elimiꢀ
nation of transition metals entirely eases purification of the reꢀ
sulting materials, a necessary step for a wide variety of applicaꢀ
tions.
A recent report by the Boydston group furthers the nascent
field of metalꢀfree controlled photopolymerization by demonꢀ
Figure 1. Recent advances in metalꢀfree controlled photopolymerizaꢀ
tion. PMP = 4ꢀ(OMe)C6H4; pꢀtol = 4ꢀ(Me)C6H4
strating the use of pyrylium salts in a visible light promoted
ringꢀopening metathesis polymerization (ROMP).6 Using substiꢀ
photochemical generation of organohalide coꢀinitiators for subꢀ
sequent Lewis acid mediated living polymerization of vinyl
ethers.12–14 Given our experience in the field of visible light
photoredox catalysis, and particularly our research into the conꢀ
trolled dimerization of electron rich styrenes, we were intrigued
by the possibility of harnessing this reactivity for controlled
photopolymerization.15 In particular, we set out to develop a
procedurally simple method for a metalꢀfree controlled/living
photopolymerization using visible light. Reported herein is a
system for the controlled cationic photopolymerization of 4ꢀ
methoxystyrene (2) using low concentrations of methanol in
combination with 2,4,6ꢀtri(pꢀtolyl)pyrylium tetrafluoroborate (3)
tuted enol ethers as coꢀinitiators and 2,4,6ꢀtri(4ꢀ
methoxyphenyl)pyrylium tetrafluoroborate (1) under visible
light irradiation, an efficient and controlled ROMP of norꢀ
bornene was demonstrated (Figure 1). The metathesis mechaꢀ
nism differs from traditional transition metalꢀalkylidene cataꢀ
lyzed ROMP as it involves the intermediacy of radical cationic
species generated by the initial oneꢀelectron oxidation of the
enol ether coꢀinitiator by the excited state of 1.
On the other hand, while photoinitiated cationic polymerizaꢀ
tion has been known for decades,8–11 there has been little attenꢀ
tion to the development of variants which facilitate controlled,
or even living, polymerization beyond procedures for in situ
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as a photoinitiator.16 To our knowledge, this is the first report of
a living cationic polymerization controlled by methanol.
b
c
e
HH
b
H
d
CH3
1
O
H
n
a
2
3
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c
Specifically, we commenced our investigations with
irradiation of 2 and substoichiometric loadings of 3 in
dichloromethane (DCM) using blue lightꢀemitting diodes
(LEDs). Poly(4ꢀmethoxystyrene) of moderately high molecular
weight was produced with greater than 95% conversion after
only a few minutes (1H NMR). Initial studies via additive probes
intended to elucidate the predominant mode of polymerization
revealed an interesting effect. Small amounts of methanol
resulted in markedly lower number average (Mn) and weight
average (Mw) molecular weights, unexpectedly accompanied by
much lower than anticipated dispersity (Ð, Mw/Mn) (Table 1,
entry 2). Proton NMR analysis of the resulting poly(4ꢀ
methoxystyrene) indicated the presence of methoxy end groups
at the chain termini. Resonances centered around δ 2.95 ppm
were identified and found to be in agreement with methoxyꢀ
capped poly(4ꢀmethoxystyrene) synthesized by a different living
cationic polymerization (Figure 2, A).17 Substitution of methaꢀ
nolꢀd3 in the standard conditions verified that the methoxy endꢀ
group was coming from the methanol added to the reaction mixꢀ
ture, and not from subsequent precipitation and purification
steps (Figure 2,B). Further, resonances centered around δ 1.00
ppm are indicative of a methyl end group, likely resulting from
initiation via protonation. This provided strong evidence that the
polymerization propagation mode is cationic in nature, and is in
agreement with previous literature reports on pyrylium initiated
polymerization.18,19
e
O
A
CH3
a
d
e’
b’
c’
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B
a’
1
Figure 2. H NMR spectra of poly(4ꢀmethoxystyrene). Conditions: 2.5
mol % 3, 6.5 mol % methanol (A) or methanolꢀd3 (B), [0.60 M] 2 in
DCM, blue LED irradiation. A: Mn = 5.35 kg/mol, Ð = 1.18 or methaꢀ
nolꢀd3, B: Mn = 6.26 kg/mol, Ð = 1.15.
3) to trifluoroethanol (entry 6); the latter is less nucleophilic
than ethanol due to the potent electronꢀwithdrawing nature of
the trifluoromethyl group thereby leading to the observed loss of
control over the polymerization (entry 6).
Furthermore, methanol loading exhibited a strong influence
over Mn and Ð (Figure 3). Increased methanol loading is assocꢀ
iated with decreased Ð up to loadings of approximately 6.5 mol
%. At this concentration of methanol and above, Ð remains
relatively constant at approximately 1.2. Increased methanol
loadings were also associated with decreased Mn, which
suggests that methanol likely plays a key role in the number of
chains initiated. On the other hand, the loading of the
photooxidant 3 had a relatively small influence on the resulting
poly(4ꢀmethoxystyrene), similarly exhibiting low Ð around 1.2
and only a modest increase in Mn at lower loadings of 3 (Table 1
entry 7, see Figure S1 and Table S1 in SI).
Table 1. Alcohol Additive Study
1
The kinetics of the polymerization were studied using H
Entry
Alcohol
–
Mn (kg/mol)a
72.5
Ða
Yield (%)b
NMR and gel permeation chromatography via quenching after
varying irradiation times (Figures 4 and 5). The polymerization
exhibits an induction period of several minutes during which
monomer conversion is very low and no polymer is produced.
After this induction period, polymerization is rapid and follows
first order kinetics (Figure 4). Furthermore, the molecular
weight increases steadily with monomer conversion, exhibiting
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2
3.04
1.18
1.22
1.31
1.86
2.05
1.21
92
92
96
96
97
97
86
MeOH
EtOH
5.41
3
6.12
4
iꢀPrOH
tꢀBuOH
CF3CH2OH
MeOH
10.5
5
26.0
6
7c
33.7
6.18
aDetermined by GPC, relative to polystyrene standards, avꢀ
erage of three experiments. bYield after precipitation, average of
three experiments. c0.25 mol % 3.
The marked effect of added methanol implicated a nucleoꢀ
philic interaction with the propagating chain end. If this were
the case, Mn and Ð should be sensitive to changes in the nucleoꢀ
philicity of the alcohol additive. A series of polymerizations
were then carried out under identical conditions varying only the
added alcohol (Table 1). Alcohols of decreasing nucleophilicity
were chosen: methanol, ethanol, isoꢀpropanol, tertꢀbutanol and
trifluoroethanol. As the steric environment of the additive beꢀ
comes more demanding, concomitant with decreasing nucleoꢀ
philicity (entries 2 through 5), both Mn and Ð of the resulting
poly(4ꢀmethoxystyrene) increases. This trend is indicative of a
nucleophilic interaction between the propagating chain end and
the alcohol additive. If this effect were indeed nucleophilic in
origin (rather than simply steric), then alcohols of similar steric
environments around the hydroxyl group, but with differing
nucleophilicity due to electronic factors, should give dissimilar
results. This is indeed the case when comparing ethanol (entry
Figure 3. Influence of methanol loading on Mn (■) and Ð (▲) of
poly(4ꢀmethoxystyrene). Mn and Ɖ were determined by GPC relative to
polystyrene standards, average of three experiments. Conditions: 2.5
mol % 3, [0.60 M] 2 in DCM, blue LED irradiation.
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(see Figure S8 in SI). The resulting methanol captured cationic
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radical could protonate an additional monomer, generating an
active cationic chain end, followed by rapid oxidation of the
resulting benzylic radical by 3 to the corresponding cation
(Scheme 1, II).22,23 Once a cation has been formed, nucleophilic
capture by methanol generates an additional proton that goes on
to protonate an additional monomer, which is itself intercepted
by methanol. This proceeds until all methanol in the reaction has
been converted into methoxy groups and a relatively small
number of active cationic chain ends remain. The observation of
deuterium enrichment on the methyl end groups of poly(4ꢀ
methoxystyrene) prepared using methanolꢀd4 provides additionꢀ
al support for this mechanism (see Figure S9 in SI). In this
manner, the concentration of methanol determines the number
of chains rather than the loading of 3 (for an elaborated mechaꢀ
nistic proposal see Scheme S1 in SI). Control of the polymerizaꢀ
tion then occurs via a mechanism analogous to Reversible Addiꢀ
tionꢀFragmentation chain Transfer (RAFT) polymerization
wherein the methoxy group serves as the chain transfer agent by
transient formation of an oxonium ion intermediate through
nucleophilic capture of a cationic chain end (A, Scheme 1, III).2
This oxonium ion can fragment via carbonꢀoxygen bond
heterolysis to produce a new active chain end (B) and a new
dormant methoxyꢀcapped chain (C). The new cationic chain end
then propagates until it recombines with the methoxy end group
of another chain, again generating an oxonium ion intermediate,
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Figure 4. A plot of ln([2]0/[2]t) versus reaction time (●). Conversions
were determined by 1H NMR analysis of reaction aliquots, after quenchꢀ
ing with trimethylamine, using an internal standard. Conditions: 2.5 mol
% 3, 3.3 mol % methanol, [0.60 M] 2 in DCM, blue LED irradiation,
quenched with triethylamine after varying irradiation times.
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a linear relationship to monomer conversion after a short initial
period of nonꢀlinear growth (Figure 5). Similarly, Ð increases
up to this same conversion and then remains nearly constant.
Altogether, these data suggest that after an induction period, this
Scheme 1. Mechanistic Proposal
photopolymerization becomes
a
wellꢀcontrolled cationic
polymerization exhibiting the characteristics of living polymeriꢀ
zation.
Figure 5. Mn (■) and Ð (▲) of poly(4ꢀmethoxystyrene) as a function of
monomer conversion. Mn and Ɖ were determined by GPC relative to
polystyrene standards. Conversions were determined by 1H NMR analyꢀ
sis of aliquots after quenching with triethyamine using an internal standꢀ
ard. Conditions: 2.5 mol % 3, 3.3 mol % methanol, [0.60 M] 2 in DCM,
blue LED irradiation, quenched with triethylamine after varying irradiaꢀ
tion times.
Given these data and observations, we propose the following
mechanism (Scheme 1): photoredox catalyst 3 serves as the
initiator upon visible light illumination, likely via initial single
electron oxidation of 2 generating a styrenyl cation radical
(Scheme 1, I).20 The subsequent initiation steps are still under
study, but likely involve antiꢀMarkovnikov nucleophilic addiꢀ
tion of methanol to the styrenyl cation radical.21 End group
analysis of poly(4ꢀmethoxystrene) using 2D NMR techniques
indicates correlations which support some proportion of methꢀ
oxy end groups resulting from initial antiꢀMarkovnikov addition
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which can itself undergo further fragmentation (e.g to B’ and
C’).
If our mechanistic proposal is accurate, then the poly(4ꢀ
ASSOCIATED CONTENT
Supporting Information
Experimental procedures and spectral data. This material is available
free of charge via the Internet at http://pubs.acs.org.
1
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3
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8
methoxystyrene) chain ends should still be active once all monꢀ
omer has been consumed. To test this, monomer addition experꢀ
iments were carried out. After polymerization was complete,
irradiation was ceased and an additional equivalent of 2 was
added directly to the solution (Figure 6). A nearly monomodal
molecular weight shift was observed via GPC which retained a
low Ð, a strong indication that the poly(4ꢀmethoxystyrene)
chain ends are still active at the end of polymerization. While
the increase in Mn was less than the theoretically expected douꢀ
bling, an isolated yield of 90% was observed after precipitation
of the poly(4ꢀmethoxystyrene). The GPC trace did indicate a
small higher molecular weight shoulder, suggesting some loss of
control. Given the apparent rapid rate of the polymerization, this
may be a result of initial small differences in monomer concenꢀ
tration at the moment of addition.
AUTHOR INFORMATION
Corresponding Author
wyou@unc.edu
nicewicz@unc.edu
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Funding Sources
The authors declare no competing financial interests.
ACKNOWLEDGMENTS
The authors acknowledge Eastman Chemical for financial support of
this project.
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In conclusion, we have carried out initial studies into the
methanolꢀcontrolled visible light photoinitiated polymerization
of 4ꢀmethoxystyrene using pyrylium salts. First order kinetic
behavior, linear Mn growth with respect to monomer consumpꢀ
tion, narrow Ð, and continued polymerization upon monomer
addition all indicate the controlled/living nature of the polymerꢀ
ization. The accumulated data suggest methanol plays a role as a
reversible chain transfer agent, analogous to that of dithiocarꢀ
bamates in RAFT polymerization. The polymerization does not
require the use of strong Lewis or Brønsted acids or the prior
synthesis or formation of small molecule coꢀinitiators. Poly(4ꢀ
methoxystyrene) of low Ð can be prepared, with Mn controlled
by the concentration of methanol. Further development of this
methodology will focus on reducing the induction period made
apparent via kinetic study, and elucidation of the precise steps
involved in initiation will likely be of key importance in this
endeavor. Expansion of the scope of this methodology to further
monomers and towards the preparation of block copolymers and
polymers of more complex architecture is also planned. Further
tuning of the protic additive should allow for reduced loadings
and greater control over the polymerization. In principle, a
matching of protic additive and monomer should be possible
such that a wide range of monomers could be amenable to conꢀ
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