Proton-Transfer Polymerization by N-Heterocyclic Carbenes: Monomer and Catalyst Scopes and Mechanism for Converting Dimethacrylates into Unsaturated Polyesters

Article abstract of DOI:10.1021/jacs.5b13019

This contribution presents a full account of experimental and theoretical/computational investigations into the N-heterocyclic carbene (NHC)-catalyzed proton-transfer polymerization (HTP) that converts common dimethacrylates (DMAs) containing no protic gr

Full text of DOI:10.1021/jacs.5b13019

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Article
Proton-Transfer Polymerization by N-Heterocyclic Carbenes:
Monomer and Catalyst Scopes and Mechanism for
Converting Dimethacrylates into Unsaturated Polyesters
Miao Hong, Xiaoyan Tang, Laura Falivene, Lucia Caporaso, Luigi Cavallo, and Eugene Y.-X. Chen
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b13019 • Publication Date (Web): 18 Jan 2016
Downloaded from http://pubs.acs.org on January 20, 2016
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Proton-Transfer Polymerization by N-Heterocyclic Carbenes: Monomer and Catalyst Scopes and
Mechanism for Converting Dimethacrylates into Unsaturated Polyesters
†
†
‡,§
§
*,‡,§
Miao Hong, Xiaoyan Tang, Laura Falivene, Lucia Caporaso, Luigi Cavallo,
and Eugene Y.ꢀX.
*,†
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Chen
†
Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523ꢀ1872, USA
‡
King Abdullah University of Science and Technology (KAUST), Physical Sciences and Engineering
Division, Kaust Catalysis Center, Thuwal 23955ꢀ6900, Saudi Arabia
§
Dipartimento di Chimica e Biologia, Università di Salerno, Via Papa Paolo Giovanni II, Iꢀ84084,
Fisciano, Italy
ABSTRACT: This contribution presents a full account of experimental and theoretical/computational
investigations into the Nꢀheterocyclic carbene (NHC)ꢀcatalyzed protonꢀtransfer polymerization (HTP)
that converts common dimethacrylates (DMAs) containing no protic groups into unsaturated polyesters.
This new HTP proceeds through the stepꢀgrowth propagation cycles via enamine intermediates,
consisting of the proposed conjugate addition–proton transfer–NHC release fundamental steps. This study
examines the monomer and catalyst scopes as well as the fundamental steps involved in the overall HTP
mechanism. DMAs having six different types of linkages connecting the two methacrylates have been
polymerized into the corresponding unsaturated polyesters. The most intriguing unsaturated polyester of
the series is that based on the biomassꢀderived furfuryl dimethacrylate, which showed a unique selfꢀ
curing ability Four MeO– and Cl–substituted TPT (1,3,4ꢀtriphenylꢀ4,5ꢀdihydroꢀ1Hꢀ1,2,4ꢀtriazolꢀ5ꢀ
Rx
ylidene) derivatives as methanol insertion products, TPT(MeO/H) (R = MeO, Cl; x = 2, 3), and two free
OMe2
OMe3
OMe2
OMe2
carbenes (catalysts),
TPT and
TPT, have been synthesized, while
TPT(MeO/H) and
TPT
have also been structurally characterized. The structure/reactivity relationship study revealed that
OMe2
TPT, being both a strong nucleophile and a good leaving group, exhibits the highest HTP activity and
also produced the polyester with the highest M , while the Cl–substituted TPT derivatives are least active
n
and efficient. Computational studies have provided mechanistic insights into the tailꢀtoꢀtail dimerization
*
Corresponding author. Eꢀmails: luigi.cavallo@kaust.edu.sa; eugene.chen@colostate.edu
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coupling step as a suitable model for the propagation cycle of the HTP. The extensive energy profile was
mapped out and the experimentally observed unicity of the TPTꢀbased catalysts was satisfactorily
explained with the thermodynamic formation of key spirocyclic species.
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Introduction
1
The sustained rise of organopolymerization, which uses smallꢀmolecule organic compounds as
catalysts or initiators in polymer synthesis and becomes a preferred method especially when metalꢀfree
products or processes are of primary concern, has profited from the emergence of powerful organic
2
catalysts and unique mechanistic pathways developed in the rapidly growing field of organocatalysis. A
prominent class of organic catalysts that attracted increasing attention is Nꢀheterocyclic carbenes (NHCs),
due to their inherently high Brønsted basicity and nucleophilicity that brought about unique reactivity and
3
selectivity often observed in many different types of organic reactions. Thanks to the pioneering work of
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Hedrick, Waymouth, and their coꢀworkers, the utility of the NHCꢀmediated reactions has been expanded
5
to polymer synthesis through NHCꢀmediated polymerizations, via predominantly the ringꢀopening
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polymerization (ROP) of heterocyclic monomers, such as lactides, lactones, epoxides, cyclic
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carbonates, cyclic (carbo)siloxanes,
and Nꢀcarboxylꢀanhydrides.
NHCꢀmediated stepꢀgrowth
1
2
polymerization has been reported as well. Polymerization of acrylic monomers such as methyl
methacrylate (MMA) has also been realized through the groupꢀtransfer polymerization initiated by a silyl
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ketene acetal (SKA), using NHCs as alternative nucleophilic catalysts for activating the SKA initiator.
1
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Such acrylic monomers can be rapidly polymerized by frustrated Lewis pairs (FLPs ) consisting of bulky
t
NHC bases, such as the Arduengo carbenes 1,3ꢀdiꢀtertꢀbutylimidazolinꢀ2ꢀylidene (I Bu) and 1,3ꢀdiꢀ
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mesitylimidazolinꢀ2ꢀylidene (IMes), and the strongly acidic, sterically encumbered alane Al(C F ) , via
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5 3
1
7
the proposed zwitterionic imidazolium enolaluminate intermediates.
For α,βꢀunsaturated esters or acrylic monomers, an important class of Michael acceptors, there
exhibits an exquisite selectivity of the substrate or monomer structure to the NHC structure, as shown by
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the following five types of reactions observed between such substrates and NHCs. Fu et al. reported the
first intramolecular umpolung of α,βꢀunsaturated esters carrying a ωꢀpendant leaving group to form βꢀ
alkylation/cyclization products, using triazolylidene carbenes such as the Enders TPT (1,3,4ꢀtriphenylꢀ
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4,5ꢀdihydroꢀ1Hꢀ1,2,4ꢀtriazolꢀ5ꢀylidene). With common methacrylates such as MMA, we found that the
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imidazolylidene carbene IMes, which was estimated to be 10 times more nucleophilic than TPT, reacts
with the substrate to form a stable singleꢀaddition product, an enamine; the formation of the enamine, or
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the deoxyꢀBreslow intermediate, analogous to the Breslow intermediate involved in the benzoin
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condensation reaction, was proposed to proceed through the initial Michael (conjugate) addition of IMes
to MMA to form the corresponding zwitterionic enolate intermediate followed by proton transfer.
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Glorius and Matsuoka discovered that TPT catalyzes tailꢀtoꢀtail umpolung dimerization of MMA and
other methacrylates, carried out typically at 80 °C, while the common imidazolylidene carbenes such as
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IMes are ineffective. Subsequently, analogous dimerization of methacrylonitrile, styrenes, other vinyl
compounds (including 2ꢀvinylpyridine, acrylonitrile, dimethyl acrylamide, and various functionalized
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i
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acrylates), and crotonates (by I PrMe , 1,3ꢀdiꢀisopropylꢀ4,5ꢀdimethylimidazolꢀ2ꢀylidene) has also been
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i
recently realized. Switching the NHC to I Pr (1,3ꢀdiꢀisopropylimidazolꢀ2ꢀylidene), Taton et al. recently
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found this NHC promotes cyclodimerization of MMA to form an imidazoliumꢀenolate cyclodimer. On
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the other hand, Matsuoka et al. revealed that IMes catalyzes cyclotetramerization of acrylates.
t
Intriguingly, I Bu promotes polymerization of MMA in dimethylformamide (DMF) at room temperature
(
RT) to produce PMMA with M (numberꢀaverage molecular weight) = 33.2 kg/mol and Đ (M /M ) =
n w n
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t
1
.99. We also revealed that I Bu catalyzes rapid polymerization of γꢀmethylꢀαꢀmethyleneꢀγꢀ
butyrolactone, a biorenewable cyclic analogue of MMA, converting quantitatively 3000 or 10000 equiv.
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of the monomer in 1 or 5 min at RT to the corresponding bioplastics. Buchmeiser et al. showed that
t
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MMA can also be polymerized by the CO ꢀprotected NHC latent precatalyst, I BuꢀCO . Taton et al.
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2
t
recently reported the polymerization of MMA by I Bu in the presence of alcohol to produce PMMA and
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α
ꢀalkoxy PMMA (M up to 8,000 g/mol), via both Michael and oxaꢀMichael addition pathways.
n
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Protonꢀtransfer polymerization (HTP) describes a polymerization in which each propagation step
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involves a proton transfer, a process to (re)generate the active propagating species or activate the
monomer or the nucleophile. For acrylic monomers, in the 1950’s scientists at Hercules showed that the
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anionic polymerization of acrylamide (CH =CHCONH ) afforded poly(βꢀalanine), instead of the
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anticipated polyacrylamide, which was proposed to proceed through the intermolecular hydrogen
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transfer, first to monomer, then to dimer, trimer, etc. Subsequently, baseꢀcatalyzed HTP processes of
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acrylic acid to poly(βꢀpropiolactone), hydroxyalkyl acrylates to poly(esterꢀether)s, acrylates containing
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two hydroxyl groups to hyperbranched poly(esterꢀether)s,
2ꢀhydroxyethyl (meth)acrylates to
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oligo(etherꢀester)s, 2ꢀhydroxyethyl methacrylate to a hyperbranched polymethacrylate, and N,Nꢀbis(2ꢀ
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hydroxyalkyl (meth)acrylamides to hyperbranched poly(etherꢀamide)s have been reported. Most
recently, Matsuoka et al. reported the polymerization of hydroxyalkyl acrylates by an NHC catalyst (TPT),
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producing low MW poly(esterꢀether)s with M up to 2,400 g/mol (Đ = 3.8). This NHCꢀmediated
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polymerization was proposed to proceed via an initial zwitterionic intermediate, derived from Michael
addition of TPT to the monomer, followed by a proton transfer from the HOꢀcarrying moiety to generate
the alkoxide which undergoes oxaꢀMichael addition to the monomer, followed by another proton transfer
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to complete a propagation cycle. For epoxide monomers, Fréchet et al. developed a basedꢀcatalyzed
HTP of HꢀAB type monomers to produce hyperbranched epoxy or hydroxyl functionalized polymers
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(
aromatic epoxides) through repeated nucleophilic ringꢀopening and proton transfer processes, the latter
Ө
of which is to activate the monomer to generate the stronger nucleophile (A ꢀB ) for the epoxide ringꢀ
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opening.
This strategy has also been employed for the synthesis of hyperbranched polyesters and
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porphyrins. More recently, Khan et al. reported the HTP of HꢀAB monomer bearing two expoxide and
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a thiol groups to produce a polythioetherꢀbased hyperbranched polymer.
It can be seen from the above overview, the monomers applicable to HTP are acrylic and epoxide
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monomers bearing acidic protons, typically OH group(s). Departing from this conventional HTP
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process, we recently disclosed a new type of HTP by an NHC catalyst that enables polymerization of
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common acyclic monomers containing no such protic groups, like typical dimethacrylates, uniquely into
unsaturated polyesters, instead of forming typical poly(methacrylate)s via vinylꢀaddition pathways by
any other polymerization methods (Figure 1). Such unsaturated polyesters, which are of scientific and
technological interest for producing tailorꢀmade polyester materials through postꢀfunctionalization and
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crossꢀlinking, exhibit high thermal stability and can be subsequently crossꢀlinked to robust polyester
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materials. This unconventional HTP is catalyzed by a selective NHC [TPT or methoxytrizaoline,
TPT(MeO/H)] capable of promoting intermolecular umpolung condensation coupling via proton transfer
and proceeds through the stepꢀgrowth propagation cycles via enamine intermediates, consisting of the
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proposed Michael (conjugate) addition–proton transfer–NHC release fundamental steps. It is revealed
that the added suitable phenol plays a critical role in achieving an effective HTP: shutting down the
radically induced chainꢀgrowth addition polymerization under HTP conditions (typically at 80–120 ºC)
and facilitating proton transfer following each monomer enchainment. As typical acrylic monomers are
currently converted exclusively through polyadditions across C=C double bonds into nonꢀbiodegradable
polyacrylates on large industrial scales, this HTP stepꢀgrowth process offers new polymer products from
readily available acrylic monomers in the form of degradable polyesters, is a metalꢀfree process, splits off
no small molecules, and therefore is 100% atomꢀeconomical, thus bearing hallmarks of the green
chemistry. However, two important fundamental aspects of this new HTP are largely unaddressed or
currently unknown: mechanistic details as well as monomer and catalyst scopes. Accordingly, the central
objective of this study was to address these questions through combined experimental and
theoretical/computational investigations, the results of which are presented herein.
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O
O
O
O
IM-NHC:
R
NHC
R
N
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N
n
O
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n
HTP
ItBu
IiPr
unsaturated polyesters
IMes
dimethacrylates (DMA)
DMA:
TPT-NHC:
O
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Si
O
N
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N
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N
BDMA
SiDMA
O
O
O
O
O
Cl
Cl
Cl
TPT
OMe2TPT
O
TGDMA
O
O
O
Cl2TPT
O
O
O
O
O
FDMA
H
N
N
N
N
O
O
O
O
N
N
O
O
O
O
H
O
O
O
Cl
Cl
OMe3TPT
Cl3TPT
IDMA
BPADMA
Figure 1. Structures of dimethacrylate (DMA) monomers, NHC catalysts, and unsaturated polyesters
investigated in this study.
Results and Discussion
Synthesis and Characterization of New TPT Derivatives. As imidazolylidene (IM) carbenes
t
such as I Bu and IMes are ineffective in promoting the HTP of DMAs while the triazolylidene carbene
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TPT is highly effective for this HTP, we used TPT as a guide in our computational screening to identify
other potentially effective NHC catalysts. In this context, we found MeO– and Cl–substituted TPT
derivatives are promising, whose energetics is similar to TPT (vide infra). Accordingly, we set out to
synthesize such TPT derivatives.
Scheme 1 outlines the procedures used for the synthesis of four MeO– and Cl–substituted TPT
OMe2
OMe3
derivatives as precatalysts (methanol insertion products),
TPT(MeO/H),
TPT(MeO/H),
Cl2
Cl3
OMe2
TPT(MeO/H), and TPT(MeO/H), as well as catalysts in the form of free carbenes,
TPT and
OMe3
TPT, starting from commercially available MeO– and Cl–substituted acyl chloride and aniline.
OMe3
OMe2
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Deprotonation of the MeO–substituted triazolium salts [
TPT]ClO and [
TPT]ClO4 with
4
OMe2
OMe3
NaOMe/MeOH readily afforded the pure methoxytrizaolines
TPT(MeO/H) and
TPT(MeO/H) in
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good yields (85–87%). The characteristic resonances for the NCHOCH and NCHOCH protons appear
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OMe2
OMe3
at
δ
6.65 and 3.13 ppm for
TPT(MeO/H) and
δ
6.68 and 3.18 ppm for
TPT(MeO/H). Major ion
peaks at m/e 358.10 and 388.16 were observed in highꢀresolution mass spectroscopy (HRMS) spectra of
OMe2
OMe3
TPT(MeO/H) and
TPT(MeO/H), respectively, which are 31 mass units lower than the theoretical
OMe2
OMe3
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values [389.17 for
TPT(MeO/H) and 420.47 for
TPT(MeO/H)], due to the loss of the methoxyl
OMe2
group during the analysis. Single crystals of
TPT(MeO/H) suitable for Xꢀray crystallographic studies
were grown from MeOH at –20 ºC and its molecular structure was confirmed by Xꢀray diffraction
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analysis (Figure 2). The metric parameters are comparable with those of the parent TPT(MeO/H).
O
O
N
R
1
PCl5
0oC
R2
+
R1
NH
2
N
R1
R2
9
Cl
Cl
R2
R2
N
R1
R1
1
. R1
NHNH HCl
N
2
1. Ac O/ HCOOH
2
N
N
N
2. HClO4
2
. CH COOH
N
3
R2
ClO4
R1
R1
R2
R2
OMe2TPT(MeO/H) & OMe2TPT
= OMe, R = H
0oC/0.05 mbar
R
1
2
9
NaOMe/MeOH
N
N
N
-
MeOH
OMe3TPT(MeO/H) & OMe3TPT
R = R = OMe
N
N
N
1
2
OMe
R1
R1
R1
R = OMe, R = H, > 99%
R1
Cl2
Cl3
TPT(MeO/H): R = Cl, R = H
1
2
R = OMe, R = H, 87%; R = R = OMe, 85%
1
2
1
2
1
2
TPT(MeO/H): R = R = Cl
1
2
R = Cl, R = H, 80%; R = R = Cl, 78%
R = R = OMe, > 99%
1 2
1
2
1
2
Scheme 1. Outlined synthetic route to MeO– and Cl–substituted TPT derivatives as methanol insertion
products and also as free carbenes.
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OMe2
Figure 2. Xꢀray crystal structure of
TPT(MeO/H). Hydrogen atoms (except H1) omitted for clarity
and ellipsoids drawn at 50% probability. Selected bond lengths (Å) and angles (º): C(1)–N(1), 1.458(6);
C(1)–H(1), 1.000(4); C(1)–O(1), 1.413(5); C(1)–N(2), 1.451(6); H(1)–C(1)–N(1), 109.8(4); H(1)–C(1)–
N(2), 109.8(4); H(1)–C(1)–O(1), 109.8(4); N(1)–C(1)–N(2), 100.2(3); H(1)–C(1)–O(1), 113.5(4).
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7
8
9
0
1
2
3
4
5
6
7
8
9
0
OMe2
OMe3
Free carbenes
TPT and
TPT were obtained in quantitative yields via thermal αꢀelimination
of methanol from their corresponding methoxytrizaolines at 90 ºC/0.05 mbar. Compared with
OMe3
TPT(MeO/H), which required 32 h of thermal treatment to completely remove the methanol to yield
OMe2
the pure free carbene, the methanol elimination from
TPT(MeO/H) was faster, producing the pure
OMe2
1
free
TPT in 24 h. This elimination process can be readily monitored by H NMR, which showed
gradually disappearance of the NCHOCH protons in the methanol adduct upon formation of the free
3
13
carbene (Figure 3). C NMR further confirmed the successful formation of the free carbenes, as
1
9
evidenced by the characteristic chemical shifts corresponding to the carbene carbon at
δ
213.2 and 212.6
OMe2
OMe3
OMe2
ppm for
TPT and
TPT, respectively. Lastly, the molecular structure of the free carbene
TPT
OMe2
was confirmed by singleꢀcrystal Xꢀdiffraction analysis (Figure 4). Compared with
TPT(MeO/H), the
most significant change in metric parameters are the shorten the C(1)–N bonds by ~0.1 Å upon formation
OMe2
OMe2
of the carbene, from 1.458(6) and 1.451(6) Å in
TPT(MeO/H) to 1.349(2) and 1.376(2) Å in
TPT,
while the N–C(1)–N angle remained essentially the same (100.2 º), which is typical of a singlet carbene.
MeO
N
MeO
N
N
OMe
N
MeO
N
N
OMe
OMe
OMe
C
(H)OMe
C(H)OMe
MeO
N
N
N
N
MeO
N
OMe
MeO
N
OMe
9
8
7
6
ppm
5
4
3
9
8
7
6
ppm
5
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Figure 3. Comparisons of H NMR (C D ) spectra of the methanol adducts (top) and the free carbenes
6
6
(
bottom).
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11
12
13
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60
OMe2
Figure 4. Xꢀray crystal structure of the free carbene
TPT. Hydrogen atoms omitted for clarity and
ellipsoids drawn at 50% probability. Selected bond lengths (Å) and angles (º): C(1)ꢀN(1), 1.349(2); C(1)ꢀ
N(3), 1.376(2); N(1)ꢀC(1)ꢀN(3), 100.16(3).
Procedures for the preparation of MeO–substituted [TPT]ClO and TPT(MeO/H) were modified to
4
synthesize Cl–substituted derivatives. It is imperative that, in the deprotonation step of converting the Cl–
Cl3
Cl2
substituted triazolium salts [ TPT]ClO and [ TPT]ClO with NaOMe/MeOH, the reaction be carried
4
4
out in a short time (< 15 min) as prolonged reaction times induced decomposition to form side products,
1
as shown by monitoring the reaction with H NMR. After purification by recrystallization,
Cl2
Cl3
TPT(MeO/H) and
TPT(MeO/H) were obtained in good yields (78~80%). The characteristic
Cl2
resonances for the NCHOCH and NCHOCH protons appear at
δ 6.29 and 2.88 ppm for TPT(MeO/H)
3
3
Cl3
and 6.24 and 2.84 ppm for
TPT(MeO/H). However, different from the MeO–substituted TPT
Cl2
Cl3
derivatives, generation of the free carbene from TPT(MeO/H) or TPT(MeO/H) was unsuccessful via
αꢀelimination of methanol; upon heating at 90 ºC/0.05 mbar for up to 36 h, the NMR signals due to the
precursors gradually reduced, but the resulting (unidentifiable) species gave a messy NMR spectrum and
had no polymerization activity when tested for the HTP of DMAs. These results indicated that the
methanol elimination from the Cl–substituted TPT derivatives is a slow process and, more importantly,
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2
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2
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the resulting free carbene is not thermally stable (in the absence of monomer) under such conditions.
Cl2
Cl3
Hence, the precatalysts
TPT(MeO/H) and
TPT(MeO/H) were used directly for the HTP
polymerization studies carried out in the presence of monomer at relatively high temperature of 110 °C so
0
1
2
3
4
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0
1
2
3
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1
2
3
4
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1
2
3
4
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8
9
0
1
2
3
4
5
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7
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9
0
that the catalyst, once generated in situ, is immediately reacted with the monomer.
5
1
Fundamental Reactions of Dimethacrylates with TPT. HTP promoted by TPT was proposed to
proceed through the stepꢀgrowth propagation cycles via enamine intermediates [TPT=P], consisting of
repeated Michael (conjugate) addition–proton transfer–NHC (TPT) release fundamental steps (Scheme
2). In this work, DMAs with six different types of linkages connecting the two methacrylate moieties
were utilized for the HTP studies, including 1,4ꢀbutanediol dimethacrylate (BDMA) with an alkyl linker,
triethylene glycol dimethcrylate (TGDMA) with an ether linker, 2,2'ꢀ[(dimethylsilylene)dioxy]
dimethacrylate (SiDMA) with a silyl ether linker, 2,5ꢀbis(hydroxymethyl)furan dimethacrylate (FDMA)
with a furan linker, isosorbide dimethacrylate (IDMA) with a bicyclic linker, and bisphenol A
dimethacrylate (BPADMA) with an aromatic linker. Among these dimethacrylates, BDMA, TGDMA
and BPADMA are commercially available, SiDMA was prepared by the reaction of chlorodimethylsilane
and 2ꢀhydroxyethyl methacrylate, while FDMA and IDMA are biomassꢀderived and were prepared by the
5
3
reaction of the corresponding diol and methacryloyl chloride.
O
O
O
O
Ph
O
O
O
O
R
O
O
R
N
N
R
O
O
R
O
O
R
O
O
O
O
R
O
O
N
n
Ph
Ph
O
O
n-1
O
O
TPT
H-transfer
Michael Addition
O
O
O
R
O
Ph
N
O
O
O
O
Ph
N
H
O
O
O
O
N
R
O
O
R
O O
N
R
O
O
R
R
O
O
R
O
O
O
O
N
H
O
O
n-1
N
O
O
n-1
Ph
Ph
Ph
Ph
Michael Addition
H-transfer
DMA
[TPT=P]
Ph
N
O
O
O
O
N
R
O
O
R
O
O
R
O
O
N
H
O
O
n-1
Ph
Ph
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Scheme 2. Proposed propagation cycle for the HTP of DMA by TPT to unsaturated polyesters.
To ascertain the influence of monomer structure on the HTP activity, the formation of bis(enamine)
intermediates and the resulting polyesters was investigated through monitoring stoichiometric and
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12
13
14
15
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43
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45
46
47
48
49
50
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52
53
54
55
56
57
58
59
60
1
polymerization reactions by H NMR. First, the formation of bis(enamine) intermediates was studied by
treating 2 equiv. of TPT with 1 equiv. of DMA in tolueneꢀd at RT [note that the 1:1 reaction produced a
8
mixture of monoꢀ and bis(enamine) adducts]. The conversion data of different DMAs are summarized in
1
Table 1 and H NMR spectra of the reaction of FDMA + 2 TPT toward the formation of the bis(enamine)
1
intermediate FDMA(TPT) at different times are provided in Figure 5, while the H NMR spectra for the
2
reaction with other DMAs are included in Figures S5–8 of the SI. The reactivity was found to increase in
the following the order: BDMA < SiDMA < TGDMA < FDMA < IDMA < BPADMA, which apparently
correlates with increasing the electron deficiency of the methacrylate double bond and the rigidity of the
DMA structure. Among these monomers, IDMA exhibited the second highest reactivity, which achieved
6
8.4 % conversion in 10 min (run 5, Table 1), while BDMA displays the lowest reactivity, which reached
a lower conversion of 63.0 % even after 3 h (run 1, Table 1). BPADMA, although its enamine formation
with TPT was the fastest, achieving 76.8% monomer conversion in 10 min, afforded some light yellow
solid which gradually precipitated from the solution after longer reaction time (1 h), due to unknown side
1
reactions (run 6, Table 1). As a result, the peaks of the bis(enamine) intermediate in the H NMR
spectrum (Figure S8) obtained at a late stage of reaction became broad with some amount of TPT
remained unreacted while BPADMA consumed completely at the end of reaction.
Table 1. Conversion Data of DMAs in the Formation of Bis(enamine) Intermediates and Polyesters by
TPT Performed in J. Youngꢀtype NMR Tubes
a
b
DMA/
TPT/MP
1/2/0
Temp.
(°C)
25
25
25
25
25
25
Conv.(%)
3 h
Run
DMA
10 min
7.20
23.4
37.7
57.3
68.4
76.8
60 min
40.2
72.7
76.9
83.8
91.0c
100
Longer time
83.8 (24 h)
96.2 (10 h)
100 (7 h)
100 (6 h)
ꢀ
1
2
3
4
5
6
BDMA
SiDMA
TGDMA
FDMA
IDMA
BPADMA
63.0
86.1
87.6
90.0
100.0
ꢀ
1/2/0
1/2/0
1/2/0
1/2/0
1/2/0
ꢀ
1
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2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
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BDMA
SiDMA
TGDMA
FDMA
IDMA
BPADMA
5/1/0.1
5/1/0.1
5/1/0.1
5/1/0.1
5/1/0.1
5/1/0.1
80
80
80
80
80
80
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
72.0
54.3
86.6
69.0
100c
86.0
98.4
83.8
100
100
ꢀ
100 (6 h)
96.4 (7 h)
ꢀ
ꢀ
ꢀ
ꢀ
1
1
a1
0
1
2
c
100
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Conditions: TPT = 14.9 mg (0.05 mmol), DMA = 0.025 mmol in 0.50 mL tolueneꢀd for runs 1–6,
8
DMA = 0.25 mmol in 0.25 mL tolueneꢀd for runs 7–12, MP (4ꢀmethoxyphenol) = 0.62 mg (0.005 mmol)
in 0.25 mL tolueneꢀd8. Conv.(%) = monomer conversion measured by H NMR spectroscopy.
BPADMA monomer consumed but companied by some side reactions.
8
b
1
c
Ph
Ph
Ph
N
O
O
N
N
7
6
6
7
9
N
9
O
O
O
Ph
N
N
Ph
Ph
8
5
5
8
Ph
6
5
8
7
*
*
*
9
*
(
d)
(
c)
TPT
b)
(
O
O
2
3
2
3
4
O
3
O
O
4
1
2
4
1
1
2
(
a)
8
7
6
5
4
3
2
1
ppm
1
Figure 5. H NMR (tolueneꢀd , 25 °C) spectra of the reaction of FDMA + 2 TPT toward formation of the
8
bis(enamine) intermediate FDMA(TPT) at different times: (a) FDMA monomer; (b) after 10 min (57.3
2
%
conversion); (c) after 60 min (90.0 % conversion); (d) after 6 h (100 % conversion). Peaks marked as *
are characteristic of the minor isomer of the bis(enamine) intermediate.
Next, the HTP in a DMA/TPT/MP ratio of 5/1/0.1 performed in J. Youngꢀtype NMR tubes at 80 °C
1
was followed by H NMR, where a small amount of MP (4ꢀmethoxyphenol) was added to inhibit
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thermally (radically) induced vinyl addition side polymerization. As can be seen from runs 7–12 (Table
1
), all DMA monomers can be readily polymerized, with increasing the polymerization activity in the
order of: SiDMA < BDMA ≈ FDMA < TGDMA ≈ BPADMA < IDMA, which is somewhat different
from the reactivity trend observed for the bis(enamine) intermediate formation (BDMA < SiDMA <
TGDMA < FDMA < IDMA < BPADMA). For FDMA (Figure 6) and TGDMA, a quantitative conversion
was achieved in 3 h (runs 9 and 10), but for IDMA a quantitative conversion was achieved in 1 h (run 11).
As in the case of bis(enamine) intermediate formation, some side reactions were accompanied in the HTP
of BPADMA upon achieving quantitative monomer conversion (Figure S12).
10
11
12
13
14
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16
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O
O
11
5
7
7
O
10
O
O
8
9
n
6
6
10
11
7
6
8
5
9
9
(b)
O
O
3
3
2
O
2
O
O
4
4
1
1
4
3
5
1
2
2
(
a)
8
7
6
4
3
2
1
ppm
1
Figure 6. H NMR spectra (tolueneꢀd ) of the HTP of FDMA (FDMA/TPT/MP = 5/1/0.1) in a JꢀYoung
8
NMR tube: (a) 10 min, 25 °C, 10 min; (b) 80 °C, 3 h (100 % conversion).
Polymerization of Dimethacrylates with TPT. Table 2 summarizes the polymerization results
obtained from the preparative scale polymerization runs. As these polymerizations are only effective
when carried out at temperature ≥ 80 °C, a small amount of the radical inhibitor such as MP is added,
which not only effectively shuts down the radically induced vinylꢀaddition chainꢀgrowth pathway but also
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2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
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3
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enhances the activity of the stepꢀgrowth polymerization by facilitating the proton transfer process.
Hence, in the presence of 0.1 mol% (or 0.02 equiv. relative to TPT) of MP, the polymerization of BDMA
by TPT (5 mol%) in toluene at 80 °C led to exclusive formation of the unsaturated polyester PBDMA
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
with M = 9.42 kg/mol and Đ = 1.90 (run 1, Table 2), without formation of any detectable amount of the
n
vinylꢀaddition polymer. The polymerization with a lower TPT loading of 1 mol% in a BDMA/TPT/MP
ratio of 100/1/0.1 produced PBDMA with a significantly higher M of 14.2 kg/mol and Đ = 1.70 (run 2,
n
Table 2) at 100 °C, or with M = 15.8 kg/mol and Đ = 1.73 at 110 °C (run 3, Table 2). However, the
n
polymerization performed at a further higher temperature of 120 °C became uncontrolled and gelled in 3
5
1
h, producing PBDMA with a low molecular weight, plus the vinylꢀaddition polymer.
a
Table 2. Results of HTP of Dimethacrylates by TPT
c
b
Mn
c
DMA/
TPT/MP
Temp.
(°C)
Time
(h)
Conv.
(%)
Đ
Run
DMA
(kg/mol)
(M /M )
w
n
1
2
3
4
5
6
7
8
9
BDMA
BDMA
BDMA
SiDMA
TGDMA
TGDMA
FDMA
FDMA
FDMA
IDMA
IDMA
20/1/0.02
100/1/0.1
100/1/0.1
20/1/0.02
20/1/0.02
100/1/0.1
20/1/0.02
20/1/0.10
100/1/0.2
20/1/0.10
20/1/0.20
20/1/0.10
80
100
110
80
80
100
80
80
100
80
80
24
48
24
24
12
24
12
6
24
2
12
24
100
100
100
89.0
100
100
100
100
100
78.0
100
9.42
14.2
15.8
4.42
15.3
11.2
14.4
10.7
9.52
1.90
1.70
1.73
1.35
2.02
1.68
3.02
2.63
2.27
10
11
5.29(176)
10.9
1.81(1.36)
2.30
d
12
BPADMA
80
100
4.11
1.40
a
b
1
Conditions: [monomer] = 1.6 M in toluene. Conv.(%) = monomer conversion measured by H NMR
c
spectroscopy. Numberꢀaverage molecular weight (M ) and molecular weight distribution (Đ = M /M )
n
w
n
d
determined by GPC at 40 °C in DMF relative to PMMA standards. Isolated yield was only 56%, due to
side reactions.
Consistent with the relative activity observed in the polymerizations performed in JꢀYoung NMR
tubes with a high catalyst loading of 20 mol% (vide supra), SiDMA exhibited the lowest activity (run 4,
Table 2) compared with other monomers. For TGDMA, the polymerization at 80 °C with a 5 mol% TPT
loading reached quantitative monomer conversion in 12 h, producing PTGDMA with a relatively high Mn
of 15.3 kg/mol (run 5, Table 2). Decreasing the catalyst loading to 1 mol% and increasing the
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polymerization temperature to 100 °C achieved quantitative conversion in 24 h, and the resulting polymer
still had relatively high molecular weight (run 6 vs. 5, Table 2). In the case of the biomassꢀderived
FDMA, the use of 2 mol% of MP (relative to TPT) can not prevent thermally induced radical
polymerization any more, and the resulting polyester contained some vinyl addition polymer (confirmed
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
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41
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43
44
45
46
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48
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by H NMR) which brought about a broad molecular weight distribution (Đ = 3.02, run 7, Table 2).
However, this problem can be easily overcome by employing 10 mol% MP, which yielded the pure
polyester with M = 10.7 kg/mol and Đ = 2.63 (run 8, Table 2). Decreasing the catalyst loading to 1
n
mol% and increasing the polymerization temperature to 100 °C did not further increase the polymer
molecular weight (run 9 vs. 8, Table 2). For the other biomassꢀderived monomer with a more rigid
structure, the polymerization of IDMA became less controlled. Even in the presence of 10 mol% of MP,
the polymerization still gelled in 2 h with formation of the vinylꢀaddition polymer side product, with the
resulting polymer exhibiting a bimodal molecular weight distribution (run 10, Table 2). To prevent the
vinyl addition polymerization, the amount of MP was increased to 20 mol%, under which conditions the
polymerization went smoothly and achieved quantitative monomer conversion in 12 h, affording the pure
polyester with M = 10.9 kg/mol and Đ = 2.30 (run 11, Table 2). Finally, in the case of BPADMA
n
polymerization, due to side reactions (vide supra), the isolated polymer yield was only 56 %, although the
monomer was consumed completely in 24 h (run 12, Table 2).
The structure of the resulting unsaturated polyesters with the unique umpolung linkage –
5
1
[
C(Me)=CHꢀCH ꢀCH(Me)]–, can be readily identified by resonances appeared at
δ
6.7 (E/Z = 96/4),
2
1
1
2.2–2.7, 1.8, and 1.2 ppm in H NMR spectra (Figures 7 and 8). All resonances in the H NMR spectra
were readily assigned, except for the bicyclic ring protons in the case of PIDMA. Nevertheless, these
1
1
protons on the cyclic ring were assigned with the assistance of Hꢀ H COSY, the correct connectivity of
which was clearly shown by correlation of the peaks marked in Figure S17. The structures of the obtained
1
3
unsaturated polyesters were further confirmed by C NMR spectra (Figure S15–16). The characteristic
chemical shifts of the umpolung linkage –[C(Me)=CHꢀCH ꢀCH(Me)]– appear at
δ 139.4, 129.1, 38.62,
2
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2.36, 16.98, and 12.47 ppm. According to the assignments of H NMR spectrum and correlation of Hꢀ
13
13
C HMQC NMR spectra (Figure S18), all resonances in the C NMR spectrum of PIDMA were finally
assigned.
0
1
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O
O
7
2
2
3
1
3
O
O
O
Si
O
8
4
6
5
8
8
6
3
7
2
1
4 5 5
(a)
1
5
O
13
12
1
0
11 11
O
15
O
O
O
1
1
11
10
9
n
14
22
O
14
11
10
9
1
2
313
1
(b)
O
O
22
1
6
18
18
O
O
O
21
20 19 n
18
21
1
7
17
17
1
6
1920
2
0
(c)
25 30
O
H
29
2
7
O
O
2
8
24
2
4
n
O
23
27
O
31
30
2
31
H
O
2
7
2
6
24
2
5
26
29 29
23
28
(
d)
7
6
5
4
3
1
0
ppm
1
Figure 7. H NMR (CDCl ) spectra of selected unsaturated polyesters: (a) PSiDMA (run 4, Table 2), (b)
3
PTGDMA (run 5, Table 2), (c) PFDMA (run 8, Table 2), (d) PIDMA (run 11, Table 2).
7
7
2
2
3
3
3
O
O
8
1
2
2
O
O
4
5
n
3
6
7
6
8
3
2
4
5
5
1
8
7
6
5
4
3
2
1
ppm
1
Figure 8. H NMR (CDCl ) spectrum of PBPADMA (run 12, Table 2).
3
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Thermal properties of the unsaturated polyesters were examined by differential scanning
calorimetry (DSC) and thermal gravimetric analysis (TGA) analyses. DSC curves obtained from the
10
11
12
13
14
15
16
17
18
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59
60
second scan were overlaid in Figure 9, showing the glass transition temperatures (T ) in a wide range
g
from –63.3 ºC to 60 ºC, depending on the linker. As anticipated, the T increases with an increase in the
g
rigidity of the main chain, following the order: PSiDMA (–63.3 ºC) < PBDMA (–54.7 ºC) < PTGDMA (–
47.7 ºC) < PFDMA (2.76) < PIDMA (40.4 ºC) < PBPADMA (60.0 ºC). Thus, PBPADMA with the most
rigid main chain exhibits the highest T of 60 ºC, while PSiDMA with the most flexible main chain
g
displays the lowest T of –63.3 ºC. No melting transitions were detected under current DSC conditions,
g
contributable to the disruption of the polyester crystallization by the double bond segment on the main
chain.
(f) PBPADMA
g
T = 60.0 °C
(e) PIDMA
T
g
= 40.4 °C
(d) PFDMA
g
T = 2.76 °C
(c) PTGDMA
T
g
= -47.7 °C
= -54.7 °C
= -63.3 °C
(b) PBDMA
T
g
(a) PSiDMA
T
g
-80 -60 -40 -20
0
20
40
60
80
100
Temp. (°C)
Figure 9. DSC curves of the unsaturated polyesters: (a) PSiDMA (run 4, Table 2), (b) PBDMA (run 1,
Table 2), (c) PTGDMA (run 5, Table 2), (d) PFDMA (run 8, Table 2), (e) PIDMA (run 11, Table 2), (f)
PBPADMA (run 12, Table 2).
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4
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5
5
5
5
5
5
5
5
5
6
TGA curves of the unsaturated polyesters were provided in Figure 10. Except PSiDMA which
showed low thermal stability (T = 134.2 °C, T defined by the temperature of 5 % weight loss in the TGA
d
d
curve) with a twoꢀstep degradation profile, all other polyesters exhibited high thermal stability showing
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
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7
8
9
0
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8
9
0
oneꢀstep degradation profiles with high T ’s ranging from 290 to 308 °C and high maximum degradation
d
temperatures (Tmax, measured by the derivative TGA curves) ranging from 296 to 410 °C (Figure 10).
Judged by the relative T values, the thermal stability of the current six unsaturated polyesters follows this
d
trend: PSiDMA < PFDMA < PBPADMA < PIDMA < PTGDMA < PBDMA.
100
PSiDMA
PFDMA
PTGDMA
PBPADMA
PIDMA
8
6
4
2
0
0
0
0
0
PBDMA
0
100
200
300
400
500
600
Temp. (°C)
Figure 10. TGA curves for PSiDMA (T = 134.2 °C, Tmax1 = 240.1 °C, Tmax2 = 383.0 °C, run 4, Table 2),
d
PBDMA (T = 336.8 °C, Tmax = 407.7 °C, run 1, Table 2), PTGDMA (T = 307.9 °C, T = 365.9 °C, run
d
d
max
5
, Table 2), PFDMA (T = 270.0°C, T = 296.0 °C, run 8, Table 2), PIDMA (T = 293.3 °C, T
=
max
d
max
d
4
09.1 °C, run 11, Table 2), PBPADMA (T = 290.4 °C, T = 410.0 °C, run 12, Table 2).
d
max
Unique Properties of PFDMA. Behaving differently from other unsaturated polyesters that
thermally decomposed completely without leaving any residue at > 500 °C, PFDMA yielded ~10 wt%
residue at temperatures as high as 650 °C, attributable to the formation of stable carbonaceous materials
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from high temperature treatment of the furfuryl or furanꢀcontaining polymers. Another interesting
property of PFDMA is the ability to undergo selfꢀcuring via various crossꢀlinking approaches to produce
more robust polyester materials.
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11
12
13
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16
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PFDMA is stable as long as being kept in solution. However, selfꢀcrosslinking of PFDMA occurred
readily as soon as the solvent was removed. Even during drying under vacuum, selfꢀcrosslinking was
observed. In this context, three different approaches to form crossꢀlinked PFDMAs were examined:
thermally induced, microwaveꢀinduced, and Lewis acidꢀcatalyzed crossꢀlinking. Generally, the T value
g
enhances with increasing the degree of crossꢀlinking and, therefore, the observed change of the T of
g
crossꢀlinked PFDMAs was used as an indicator to measure the degree of crosslinking. The PFDMA via
thermally induced crossꢀlinking after heating at 150 °C for 1 h exhibited a T of 10.6 °C, which is higher
g
than that via microwaveꢀinduced crossꢀlinking (4.83 and 8.00 °C) with microwave irradiation (MI) at 25
ºC (1500W, 1 h and 3 h) and that via vacuum drying (2.72 °C). TGA measurements of the resulting
products (Figure 11) showed that the crossꢀlinked PFDMAs prepared by thermally (T = 233.0 °C) and
d
microwave (T = 231.0–234.5 °C) induced crossꢀlinking approaches exhibited lower thermal stability
d
compared with the crossꢀlinked polymer prepared by vacuum drying (T = 270.0 °C). Noteworthy here is
d
that the amount of the stable carbonaceous residue formed at 650 °C increased with increasing the degree
of crossꢀlinking. Hence, the crossꢀlinked PFDMA after heating at 150 °C for 1 h yielded a high residue
amount of 22.2 wt% formed at 650 °C.
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8
9
0
Figure 11. TGA curves of crossꢀlinked PFDMAs prepared by different approaches: by vacuum drying, Td
=
270 °C, residue = 9.3 wt%; by microwave irradiation (MI) at 25 °C for 1 h (MIꢀ1 h), T = 231 °C,
d
residue = 15.9 wt%; by MI at 25 °C for 3 h (MIꢀ3 h), T = 235 °C, residue = 20.5 wt%; by heating at 150
d
°C for 1 h, T = 233 °C, residue = 22.2 wt%; by AlCl ꢀcatalyzed, T = 213 °C, residue = 31.4 wt%.
d
3
d
Scheme 3 shows the proposed mechanism for the selfꢀcrosslinking of PFDMA that proceeds
through the Diels–Alder reaction in which the furan rings act as the diene source and the unsaturated
double bonds act as the dienophile. To provide evidence supporting this pathway, the hydrogenation
reaction of PFDMA over Pd/C was carried out to convert the uncrossꢀlinked, unsaturated PFDMA into
30 1
the corresponding saturated polyester. H NMR spectra of the unsaturated PFDMA and saturated
polyester before and after the selective hydrogenation are shown in Figure 12. The most significant
change in chemical shifts after the hydrogenation reaction is the complete disappearance of the signal at
δ
6.7 ppm for the double bond (marked as H6). Behaving differently from the unsaturated PFDMA, the
obtained saturated polyester had good dissolvability after removing the solvent or heating and showed no
sign of crossꢀlinking. Hence, the inertness of the furanꢀcontaining saturated polyester towards
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crosslinking under identical conditions was used to confirm the selfꢀcrosslinking that proceeds through
the Diels–Alder reaction as depicted in Scheme 3.
O
O
O
O
O
n
O
O
O
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
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40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
O
O
Diels-Alder Reaction
O
O
O
O
O
n
H
2
Cross-linked Polyester
O
O
O
O
O
n
Scheme 3. Proposed mechanism for selfꢀcrosslinking of PFDMA through the Diels–Alder reaction and
the inertness of the saturated polyester toward such crosslinking.
4
O
O
2
2
5
3
O
O
O
n
5
1
1
2
4
1
3
5 5
(d)
(c)
(
b)
O
O
12
6
8
8
O
8
O
O
12
10 9
n
11
7
1
1
7
7
6
9
10
10
(a)
7
6
5
4
ppm
3
2
1
1
Figure 12. H NMR spectra (CDCl ) monitoring the hydrogenation reaction: (a) PFDMA (run 8, Table 2),
3
(
b) hydrogenation for 6 h (conv. = 15.3 %), (c) hydrogenation for 24 h (conv. = 41.2 %), and (d)
hydrogenation for 48 h (conv. = 100 %).
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5
5
5
6
The third approach we investigated is the Lewis acid (AlCl ) catalyzed Diels–Alder reaction in
3
solution, which was employed to further enhance the degree of PFDMA selfꢀcrosslinking. FTꢀIR
spectrum (Figure S21) of the crossꢀlinked PFDMA by this method exhibited considerably broader
absorption bands compared with PFDMA. In addition, the intensity of the absorption bands attributed to
0
1
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6
7
8
9
0
ꢀ1
the bending vibration modes of the furan rings at 930 and 736 cm noticeably decreased. Furthermore,
there was no apparent T observed for the crossꢀlinked PFDMA by this route, and both endothermic and
g
exothermic peaks due to the thermally reversible Diels–Alder reaction were also not observed in DSC
curves. Lastly, the crossꢀlinked PFDMA prepared using the AlCl ꢀcatalyzed Diels–Alder reaction
3
exhibited a T of 213 °C and a Tmax of 315 °C, but it produced the highest amount of the stable
d
carbonaceous residue (31.4 wt%) formed at 650 °C (Figure 11). All together, the above results indicated
the higher degree of the crosslinking between the furan rings and unsaturated double bonds of PFDMA
achieved via the AlCl ꢀcatalyzed Diels–Alder reaction.
3
Effects of TPT Catalysts. To examine the effects of catalyst structure on the HTP activity, the
formation of the bis(enamine) intermediates and resulting polyesters was initially carried out in J. Youngꢀ
1
type NMR tubes and monitored by H NMR. The conversion data for the bis(enamine) formation reaction
of BDMA and NHC (2 equiv.) in tolueneꢀd at 25 °C are summarized in Table S1. The reactivity trend
8
OMe3
OMe2
follows the order of TPT <
TPT <
TPT, indicating that more nucleophilic NHCs with electronꢀ
donating substituents enhance the rate of the enamine intermediate formation. In the case of the NMR
scale polymerization in a ratio of BDMA/NHC/MP = 5/1/0.1 performed at 80 °C, the reactivity order was
OMe3
OMe2
somewhat different:
TPT < TPT <
TPT, suggesting that the more nucleophilic NHC is not
necessary a better catalyst for HTP due to its higher resistance toward its release at the end of the
propagation cycle (i.e., the poorer leaving group). Hence, a fine balance must be struck between the
OMe2
ability to attack the substrate and the propensity to release from the product, and it appears that
both a good nucleophile and leaving group.
TPT is
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As we showed previously, the direct use of TPT(MeO/H) (1 mol%) for the polymerization of
BDMA in the presence of MP (0.1 mol%) in toluene at 100 °C, under which conditions the precatalyst
releases the free carbene as the catalyst, led to a similar polymerization performance to that by the free
51
Cl3
Cl2
10
11
12
13
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17
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23
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carbene TPT. Using this strategy, precatalysts TPT(MeO/H) and TPT(MeO/H) were used for their
HTP studies as their free carbene forms are unstable under the current reaction conditions in the absence
of monomer. In comparison, these Cl–substituted TPT derivatives are considerably less active than the
parent TPT (Table S1), indicating that electronꢀwithdrawing groups on TPT decrease the HTP activity.
The polymerization results obtained from the preparative scale polymerization runs are summarized
in Table 3. Relative to TPT (1 mol%), which achieved quantitative monomer conversion in the BDMA
polymerization (BDMA/TPT/MP = 100/1/0.1) at 100 °C after 48 h and produced PBDMA with M = 14.2
n
OMe2
kg/mol and Đ = 1.70 (run 1, Table 3),
TPT exhibited higher activity under identical conditions, thus
shortening the time to reach the quantitative monomer conversion by more than a half (20 h) and also
producing PBDMA with a higher M of 16.7 kg/mol and a lower Đ value of 1.64 (run 2, Table 3). On the
n
OMe3
other hand, the TPT derivative with three MeO substituents,
TPT, is less active and effective as
compared with TPT and can achieve quantitative monomer conversion only with a relatively high catalyst
OMe3
loading of 5 mol% in 10 h (run 3, Table 3); with a lower catalyst loading of 1 mol% (BDMA/
ꢀTPT
/MP = 100/1/0.1), the polymerization achieved only 86% monomer conversion after 24 h even when the
temperature was raised to 110 °C and produced the PBDMA with a lower molecular weight (run 4, Table
3). Consistent with the reactivity results obtained by the NMR monitoring of the polymerization, the
Cl3
preparative scale polymerization results showed that the Cl–substituted TPT derivatives, TPT(MeO/H)
Cl2
and TPT(MeO/H), are the least active and effective NHC catalysts within the series examined herein;
they can achieve high monomer conversions only with a relatively high catalyst loading of 5 mol% (runs
5 and 6, Table 3), but when the catalyst loading was lowered to 1 mol% the activity of the polymerization
Cl3
by TPT (MeO/H) decreased drastically, with a conversion of only 75.0 % even after 48 h and also
produced the PBDMA product with a relatively low M of 7.15 kg/mol (run 7, Table 3).
n
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1
1
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1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
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4
4
4
4
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5
5
5
5
5
5
5
5
6
a
Table 3. Results of HTP of BDMA by TPT, MeO- and Cl-Substituted TPT Derivatives
c
c
Mn
BDMA/
NHC/MP
Temp.
(°C)
Time
(h)
Conv.
(%)
Đ
w
Run
(pre)catalyst
TPT
b
(
kg/mol)
(M /M )
n
1
2
3
4
5
6
7
100/1/0.1
100/1/0.1
20/1/0.02
100/1/0.1
20/1/0.1
100
100
80
110
110
110
110
48
20
12
24
10
10
48
100
100
100
86.0
99.1
99.2
75.0
14.2
16.7
14.0
8.36
11.5
16.6
7.15
1.70
1.64
1.60
1.15
1.40
1.52
1.10
OMe2
TPT
TPT
TPT
OMe3
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
OMe3
Cl2
TPT(MeO/H)
TPT(MeO/H)
TPT(MeO/H)
Cl3
Cl3
20/1/0.1
100/1/0.1
a
b
1
Conditions: [BDMA] = 1.6 M in toluene. Conv.% = monomer conversion measured by H NMR
spectroscopy. M and Đ determined by GPC at 40 °C in DMF relative to PMMA standards.
c
n
Theoretical Investigation of the HTP Mechanism. In this section we describe the mechanism of
the condensation coupling step in the TPTꢀcatalyzed DMA polymerization. As a suitable model for the
propagation cycle outlined in Scheme 2, we investigated the tailꢀtoꢀtail dimerization of MMA in detail.
The entire dimerization pathway mechanism is reported in Scheme 4. It should be noted that steps 1 to 3’,
corresponding to the initial attack of TPT to a free methacrylate unit toward formation of the enamine
3
3
intermediate 3, and the following attack of 3 to a second methacrylate unit, were discussed previously.
As we have improved the level of theory on which these calculations are performed from internal energies
at the BP86 level to free energies at the M06 level, for the sake of consistency we briefly reꢀdiscuss steps
1
to 3’ here.
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Ph
Ph
N
N
N
N
Ph
O
N
N
O
Ph
Ph
Ph
+ TPT
O
O
1 - 2
4.6(15.9)
O
1
O
1
0.0
2zwitt 6.4(6.4)
2spiro 2.3(3.9)
H-transfer 2 - 2' 24.0(23.1)
Ph
N
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
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60
Ph
Ph
Ph
N
N
N
Ph
O
N
N
Ph
N
N
Ph
O
N
N
Ph
O
O
N
Ph
N
Ph
Ph
Ph
H
O
3'spiro
7.9(-5.9)
4spiro
-10.0(-8.0)
+
O
MeO
-
O
O
O
O
O
2
' -11.1(-10.2)
H-transfer 2' - 3 2.8(4.7)
Ph
Ph
N
N
N
N
Ph
O
Ph
Ph
N
Ph
N
N
H
O
H-transfer
(1,4-shift)
Ph
H
+
MMA
- 3'
10.0(9.5)
H
O
O
γ'
β'
- TPT
N
N
Ph
O
O
O
O
Ph
3
α
O
γ'
α
β
3'- 4
O
4 - 4'
1.8(2.5)
O
β'
β
O
13.9(12.8)
O
4'
O
-
15.6(-14.8)
3
'zwitt
4zwitt
-6.4(-7.4)
3
-
15.2(-14.8)
Ph
-4.2(-5.3)
Ph
N
N
N
N
Ph
N
Ph
N
Ph
O
Ph
H-transfer
3'-5
6.6(17.2)
α
O
α
β
β
1
O
O
3
3
O
O
Ph
Ph
O
O
Ph
Ph
β
N
β
N
N
N
N
N
H
N
N
N
N
Ph
N
α
Ph
O
H-transfer
α
Ph
Ph
β'
H
N
β'
H
α
Ph
α
Ph
5-5'
H
β
β
Ph
Ph
γ'
11.0(13.4)
γ'
O
O
O
O
O
O
O
4
3
5a
5b
-2.7(-1.7)
5
5' -2.9(-1.9)
Scheme 4. Schematic representation of the HTP chain growth step. Free energies (kcal/mol) reported in
both toluene and DMF (in parenthesis).
The first step starting from 1, which is the reaction of free TPT + MMA, to 2 corresponds to the
nucleophilic attack of TPT to the C=C double bond of a MMA molecule mimicking one of the
methacrylate moieties of DMA. Further evolution of 2 occurs via a double Hꢀtransfer between two
molecules of 2 via ionꢀpair intermediate 2’, leading to the formation of two molecules of enamine 3 with
free energy barriers around 15 kcal/mol. Formation of 3, roughly 15 kcal/mol below the starting species 1,
both in toluene and in DMF, is clearly exergonic. Nucleophilic attack of 3 to the C=C double bond of a
free MMA molecule, via transition state 3-3’ with an energy barrier around 25 kcal/mol, leads to
intermediate 3’, roughly 10 kcal/mol above 3. The novel feature of the current investigation is the
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5
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inclusion of spirocyclic structures for 2 and 3’; in both cases, the spirocyclic structure is more stable than
the openꢀchain zwitterionic structure by less than 5 kcal/mol, indicating an overall equilibrium between
them. Even with the addition of the spirocyclic structures, the overall chemical scenario for the
conversion of 1 to 3’ depicted here, using M06 free energies, confirms the conclusions drawn on the basis
0
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33
of BP86 internal energies.
Evolution of intermediate 3’ toward the dimerization product 4’ has been studied via two different
pathways (Scheme 4). The first is the proposed 1,4ꢀshift of the H atom from the Cβ to the Cγ’ through
fiveꢀmembered transition state 3’-4 as depicted in Figure 13, with a barrier of about 18ꢀ20 kcal/mol in
both solvents (and an overall energy cost of about 25 kcal/mol from the most stable intermediate 3). The
resulting intermediate 4, roughly 2 kcal/mol lower in energy than 3’, undergoes NHC dissociation
through transition state 4-4’, around 10ꢀ12 kcal/mol lower in energy than transition state 3’-4, leading to
the dimerization product 4’ and release of TPT. The overall transformation from 3 to 4’ is basically
thermoneutral in both toluene and DMF. The second pathway considered corresponds to a twoꢀstep
bimolecular Hꢀtransfer reaction involving 3’ and one molecule of enamine 3. In the first step, one H atom
(colored in blue in Scheme 4) is transferred from the Cβ of 3 to the Cγ' of 3’, through transition state 3’-5
and an energy barrier of roughly 21 kcal/mol in both solvents, leading to the formation of the ion pair
intermediate 5. To complete the tailꢀtoꢀtail condensation step, intermediate 5 undergoes a further Hꢀ
transfer reaction through transition state 5-5’, where one H atom is transferred from the Cβ of the 5a
moiety to the Cβ of the 5b moiety, leading to 4 plus 3. Transition state 5-5’ is around 4ꢀ5 kcal/mol lower
in energy than transition state 3’-5, and the overall step 5 to 5’ is thermoneutral. As in the intramolecular
mechanism depicted above, 4 undergoes TPT dissociation leading to the condensation product 4’.
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46
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50
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52
53
54
55
56
57
58
59
60
Figure 13. Geometries of transition states 3’-4 and 3’-5. Marked distances in Å.
Comparing the two mechanisms depicted in Scheme 4 for the conversion of 3’ to 4, evolution of 3’
through transition state 3’-4 is favored over evolution through transition state 3’-5, since transition state
5
5
3’-4 is roughly 3ꢀ5 kcal/mol lower in energy relative to transition state 3’-5. The geometries of the
pathway determining transition states 3’-4 and 3’-5 are compared in Figure 13. The unimolecular
transition state 3’-4 shows the H atom moving from the Cβ to the Cγ’ within an unstrained fiveꢀ
membered ring transition state. In contrast, the bimolecular transition state 3’-5 shows a much more
crowded geometry, with substituents on both TPT units clashing with the MMA skeleton of the other
adduct, as indicated by the short C–C distances in Figure 13. The final favored energy profile involving
both spiro and zwitterionic species depicted in Figure 14.
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Ph
Ph
N
Ph
N
N
N
N
Ph
O
N
N
Ph
O
N
N
Ph
O
Ph
Ph
Ph
H
H
H-Transfer
O
O
O
O
O
O
O
TPT addition
H-Transfer
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0
M addition
H-Transfer
TPT release
Ph
N
N
N
Ph
Ph
O
O
O
O
Ph
H
O
Ph
Ph
N
Ph
O
N
N
N
N
Ph
N
O
Ph
N
N
Ph
H
N
N
N
Ph
O
N
Ph
O
O
N
Ph
O
Ph
Ph
Ph
N
N
Ph
O
O
Ph
+
O
MeO
O
O
O
O
O
O
O
Figure 14. Energy profile of the HTP chain growth step. Free energies (kcal/mol) are reported in toluene.
Mechanistic scenario with spirocycle structures: TPT vs IMes. In our efforts to find the possible
theoretical basis to explain why so far only the triazolylidene carbene TPT promotes the above HTP
reaction while the imidazolylidene carbenes such as IMes fail to catalyze the same reaction, we focused
on the intermediates along the dimerization pathway (i.e. species 2, 3’ and 4, Scheme 4). As anticipated
7
e
above, we found they can exist as both open zwitterionic and closed spirocyclic structures. The
zwitterionic species are expected to have the positive charge mainly localized on the NHC and the
negative charge delocalized on the chain, while the spirocyclic species have no formal charge separation.
Given this chief difference, the relative stability of the zwitterionic and spirocyclic structures depends on
the solvent polarity, with more polar solvents favoring the zwitterionic form. In line with the above
reasoning, we found that the spirocyclic forms of 2, 3’ and 4 are around 4 kcal/mol more stable than the
corresponding zwitterionic forms in toluene, but this difference is reduced to 1ꢀ2 kcal/mol in DMF. As the
key intermediate along the dimerization pathway, from which the coupling with another polyꢀDMA chain
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end starts, 3’ is remarkably unstable in its open zwitterionic form over the proceeding enamine species 3,
which prevents formation of a sizeable amount of 3’. Hence, we speculated that formation of a more
stable spirocyclic structure for 3’ could drive thermodynamically the reaction towards the polymerization
products.
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To test this hypothesis, we endeavored to locate analogous intermediates derived from IMes, an
NHC unable to promote HTP. Interestingly, we were not able to locate a spirocyclic geometry for 3’ with
IMes as the NHC. Consistently with an open zwitterionic geometry, in all the cases the NHC…O distance
converges to the high value of 2.7 Å, versus a value of 1.5 Å in the spirocyclic form of 3’ with TPT.
Comparing the relative energy (in toluene) of the most stable geometry of 3 and 3’ derived from IMes and
TPT, ꢀ19.1 and 1.9 kcal/mol vs ꢀ15.2 and ꢀ7.9 kcal/mol, it emerges that 3 is roughly 21 kcal/mol more
stable than 3’-zwitt for IMes, whereas this difference is reduced to about 7 kcal/mol only for TPT, due to
the formation of the stable 3’-spiro species. These results account for the experimental formation of the
enamine 3 with IMes, while with TPT the reaction evolves to the dimerization product 4 under the same
experimental conditions. Overall, these results suggest that the different behavior between IMes and TPT
is not due to kinetics (the rate determining step, with a barrier of 24ꢀ25 kcal/mol, is similar for the two
NHCꢀbased systems). Rather, it is due to thermodynamic accessibility of the key intermediate 3’, which is
highly disfavored with IMes, while it can be formed with TPT via a stable spirocycle.
Based on the above knowledge, we explored computationally a series of NHC structures to search
for NHCs other than TPT, which are capable of promoting HTP. The two main guidelines in this
screening were the following. First, sterically demanding and rigid NHCs were excluded, since they were
33
shown to exhibit too high energy barriers to promote the bimolecular 2-2’ and 2’-3 H transfer steps.
Second, we focused on the relative stabilities of 2, 3, 3’, 4 and 4’ and, among the different NHCs tested,
we initially selected the systems showing high stability of 3’ and 4 relative to the enamine 3. Finally, the
key transition state 3ꢀ3’ was located for the most promising NHCs. The NHCs we examined are reported
in Chart 1, while the energies of the key intermediates are included in Table S5 of the SI. This screening
Cl3
OMe3
OMe2
indicated TPT,
TPT,
TPT and NHCꢀ(h) as promising catalysts, since they showed a behavior
2
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