Controlled (Co)polymerization of methacrylates using a novel symmetrical trithiocarbonate raft agent bearing diphenylmethyl groups

Article abstract of DOI:10.3390/molecules26154618

Herein, we report a novel type of symmetrical trithiocarbonate chain transfer agent (CTA) based diphenylmethyl as R groups. The utilization of this CTA in the Reversible Addition-Fragmentation chain Transfer (RAFT) process reveals an efficient control in the polymerization of methacrylic monomers and the preparation of block copolymers. The latter are obtained by the (co)polymerization of styrene or butyl acrylate using a functionalized macro-CTA polymethyl methacrylate (PMMA) previously synthesized. Data show low molecular weight dispersity values (D < 1.5) particularly in the polymerization of methacrylic monomers. Considering a typical RAFT mechanism, the leaving groups (R) from the fragmentation of CTA should be able to re-initiate the polymerization (formation of growth chains) allowing an efficient control of the process. Nevertheless, in the case of the polymerization of MMA in the presence of this symmetrical CTA, the polymerization process displays an atypical behavior that requires high [initiator]/[CTA] molar ratios for accessing predictable molecular weights without affecting the D. Some evidence suggests that this does not completely behave as a common RAFT agent as it is not completely consumed during the polymerization reaction, and it needs atypical high molar ratios [initiator]/[CTA] to be closer to the predicted molecular weight without affecting the D. This work demonstrates that MMA and other methacrylic monomers can be polymerized in a controlled way, and with “living” characteristics, using certain symmetrical trithiocarbonates.

Full text of DOI:10.3390/molecules26154618

molecules
Article
Controlled (Co)Polymerization of Methacrylates Using a
Novel Symmetrical Trithiocarbonate RAFT Agent Bearing
Diphenylmethyl Groups
Alvaro Leonel Robles Grana 1, Hortensia Maldonado-Textle 1, José Román Torres-Lubián 1
,
Claude St Thomas 2 , Ramón Díaz de León 1 , José Luis Olivares-Romero 3 , Luis Valencia 4
and Francisco Javier Enríquez-Medrano 1,
*
1
Centro de Investigación en Química Aplicada, Enrique Reyna Hermosillo, No. 140,
Col. San José de los Cerritos, Saltillo 25294, Mexico; robles.leonel.m19@ciqa.edu.mx (A.L.R.G.);
hortensia.maldonado@ciqa.edu.mx (H.M.-T.); roman.torres@ciqa.edu.mx (J.R.T.-L.);
ramon.diazdeleon@ciqa.edu.mx (R.D.d.L.)
2
3
4
CONACyT-Centro de Investigación en Química Aplicada, Enrique Reyna Hermosillo, No. 140,
Col. San José de los Cerritos, Saltillo 25294, Mexico; claude.stthomas@ciqa.edu.mx
Red de Estudios Moleculares Avanzados, Clúster Científico y Tecnológico BioMimic, Campus III,
Instituto de Ecología, Xalapa 91073, Mexico; jose.olivares@inecol.mx
Biofiber Tech Sweden AB, Norrsken Hourse, Birger Jarlsgatan 57 C, SE-113 56 Stockholm, Sweden;
luis.valencia@biofibertech.com
*
Correspondence: javier.enriquez@ciqa.edu.mx
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Citation: Robles Grana, A.L.;
Abstract: Herein, we report a novel type of symmetrical trithiocarbonate chain transfer agent
(CTA) based diphenylmethyl as R groups. The utilization of this CTA in the Reversible Addition-
Fragmentation chain Transfer (RAFT) process reveals an efficient control in the polymerization
of methacrylic monomers and the preparation of block copolymers. The latter are obtained by
the (co)polymerization of styrene or butyl acrylate using a functionalized macro-CTA polymethyl
methacrylate (PMMA) previously synthesized. Data show low molecular weight dispersity values
(Ð < 1.5) particularly in the polymerization of methacrylic monomers. Considering a typical RAFT
mechanism, the leaving groups (R) from the fragmentation of CTA should be able to re-initiate the
polymerization (formation of growth chains) allowing an efficient control of the process. Nevertheless,
in the case of the polymerization of MMA in the presence of this symmetrical CTA, the polymerization
process displays an atypical behavior that requires high [initiator]/[CTA] molar ratios for accessing
predictable molecular weights without affecting the Ð. Some evidence suggests that this does
not completely behave as a common RAFT agent as it is not completely consumed during the
polymerization reaction, and it needs atypical high molar ratios [initiator]/[CTA] to be closer to the
predicted molecular weight without affecting the Ð. This work demonstrates that MMA and other
methacrylic monomers can be polymerized in a controlled way, and with “living” characteristics,
using certain symmetrical trithiocarbonates.
Maldonado-Textle, H.; Torres-Lubián,
J.R.; St Thomas, C.; Díaz de León, R.;
Olivares-Romero, J.L.; Valencia, L.;
Enríquez-Medrano, F.J. Controlled
(Co)Polymerization of Methacrylates
Using a Novel Symmetrical
Trithiocarbonate RAFT Agent Bearing
Diphenylmethyl Groups. Molecules
2021, 26, 4618. https://doi.org/
10.3390/molecules26154618
Academic Editor: Ivan Gitsov
Received: 29 June 2021
Accepted: 26 July 2021
Published: 30 July 2021
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in
published maps and institutional affil-
iations.
Keywords: RAFT polymerization; methacrylates; block copolymers
1. Introduction
Since its early report at the end of the 1990s [1], the reversible addition-fragmentation
Copyright:
© 2021 by the authors.
chain transfer (RAFT) polymerization gained the attention of the scientific community.
This arises as a result of the capacity to polymerize vinyl monomers and allowing the
preparation of polymers with complex architectures, such as block copolymers, star copoly-
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This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
mers and dendrimers [
ity of RAFT has increased impressively throughout the years. Indeed, this technique
counts thousands of reports in the scientific literature [ 11]. This technique is part of the
2–5] adapting to different reactions media [6–8]. The popular-
9–
Molecules 2021, 26, 4618. https://doi.org/10.3390/molecules26154618
https://www.mdpi.com/journal/molecules

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reversible-deactivation radical polymerization (RDRP) processes [12
,
13], which allows poly-
mer scientists to prepare advanced materials with specific properties. RAFT is essentially
a radical polymerization technique that provides “living” characteristics to conventional
or free radical polymerization, and thus, allows the control of kinetics and molecular
weight characteristics [14
on the selection of an appropriate chain transfer agent (CTA, Z-(C=S)-S-R), which in turn,
depends on the type of monomer to polymerize, as well as the reaction conditions [ ]. The
,15]. A successful control in the RAFT polymerization depends
9
CTAs contain both Z and R groups in their structures and both play a critical role in the
development (well-control and “living” behavior) of the polymerization reactions. Many
investigations have reported the effect of the Z and/or R groups [11], leading to different
families of CTAs, such as: dithioesters [16,17], trithiocarbonates [18], dithiocarbamates [19]
and xanthates [20]. Furthermore, some switchable RAFT agents based on dithiocarbamate
have been prepared and demonstrated the ability to synthesize well-defined polymers from
the use of several vinyl monomers. Despite these efforts, no CTA could be considered as a
universal RAFT agent, as the polymerization reaction depends on the kind of monomers
and conditions processes [11]. In this sense, vinyl monomers can be classified in those
in which the polymerizable double bond is conjugated to another double bond, includ-
ing those of aromatic rings (i.e., styrene), carbonyl (i.e., methyl methacrylate), or nitrile
groups (i.e., acrylonitrile) and that are typically controlled by dithioesthers or trithiocar-
bonates. On the other hand, there are those in which oxygen (i.e., vinyl acetate), nitrogen
(i.e., N-vinylpyrrolidone) or halogen (i.e., vinyl chloride) are adjacent to the polymerizable
double bond and that are typically controlled by dithiocarbamates or xanthates.
Among the CTAs used in the RAFT polymerization, and excluding the dithioben-
zoates, the trithiocarbonates (Z = S-alkyl) are classified amongst the most reactive and
mainly used to polymerize monomers like styrene (St) [21], methyl methacrylate (MMA),
and/or butyl acrylate (BuA) [5], among many others [22–24]. While the Z group of the
CTA mainly governs the stability of the intermediate radical (formed during the addition-
fragmentation), the R group plays an important role in the control of the polymerization
process. The efficiency of the radical leaving groups (R) to re-initiate the polymerization
depends on different parameters, such as steric hindrance, radical stability, polar factor,
and their chemical nature [25]. Generally, secondary, or tertiary radicals from β-scission
of asymmetrical CTA have demonstrated an excellent control on the polymerization of
styrenes, acrylates, acrylamides, and methacrylates monomers. On the other hand, sym-
metrical CTAs (trithiocarbonates) have typically shown poor control in the polymerization
of methacrylate monomers, even when the chemical nature of the leaving radical group
is secondary or tertiary. Therefore, we postulate that symmetrical trithiocarbonates with
adequate R groups have not been designed or synthesized for the controlled RAFT polymer-
ization of methacrylic monomers. Therefore, a controlled polymerization of methacrylates
demands to this date the use of asymmetrical RAFT agents with R groups able to eject
a tertiary or very specific secondary radicals [26
cyanoisopropyl dithiobenzoate [1].
,27], such as cumyl dithiobenzoate or
Despite the extensive effort in this field, the polymerization of methacrylic monomers
using symmetrical trithiocarbonates (R-S-(C=S)-S-R) as RAFT agents remains unfruitful
with a weak control in the polymeric chains. For example, S,S’-bis(
acid) trithiocarbonate or the corresponding methyl ester were reported for bulk, solution,
or microemulsion MMA polymerizations with a modest control [28 31], such as the values
reported by Lai et al. which are Mn,SEC = 5.7 Kg/mol, Mn 3.3 Kg/mol and Ð = 1.72 [30].
Moreover, the use of symmetrical trithiocarbonates with typical primary or secondary R
groups, like benzyl or ( -methyl)benzyl groups, cannot be found in the literature for
α,α’-dimethylacetic
–
≈
,Th
α
polymerizations of MMA and other common methacrylic monomers, as these reactions
result in uncontrolled polymerizations with very broad molecular weight distributions and
unpredictable molecular weights.
In this work, we introduce the di(diphenylmethyl) trithiocarbonate (see Figure 1a) as
a novel RAFT agent that can specifically carry out, in a controlled manner, the polymer-

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ization of the MMA and other methacrylic monomers. Based on our results, we observed
that the presence of diphenylmethyl groups (R) in this symmetrical trithiocarbonate are
efficient for controlled polymerization of MMA. The evidence found suggests that this
compound does not behave like a common RAFT agent since it requires atypical high
molar ratios [initiator]/[CTA] for accessing to predictable molecular weights. Due to its
structural characteristics, the diphenylmethyl is a good leaving group that appears to be
ejected, according to RAFT mechanism, as a very stable (by resonance towards two phenyl
groups) secondary radical that would generally cause inhibition or retardation in a RAFT
polymerization, but in this case, it can both re-initiate and control efficiently the MMA
polymerization. Furthermore, we demonstrated the potential of the functionalized PMMA
as macro-CTA to prepare block copolymers by chain extension through the reaction with
St or BuA. These results provide key insights with respect to the synthesis of a new family
of trithiocarbonate based CTAs (including asymmetrical ones) which could have great
potential and diverse applications.
Figure 1. (a) Equation reaction to synthesize the di(diphenylmethyl) trithiocarbonate (CTA-1), (b) its
corresponding 1H NMR and (c) 13C NMR spectra.
2. Results and Discussion
2.1. Chain Transfer Agents
RAFT polymerization is a powerful technique used for tailoring the architecture,
chain length distribution, and composition of polymers. The efficiency of the RAFT
process mainly depends on the chemical nature of the CTA (or RAFT agent) and the
monomer(s). Therefore, the synthesis of novel CTA represents an opportunity to achieve

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functionalized RAFT polymers with a specific structure. A new symmetrical RAFT agent
di(diphenylmethyl) trithiocarbonate here named CTA-1 was prepared. The synthesis of
CTA-1 was carried out in a one-pot reaction using the resin amberlyst-A26(OH), carbon
disulfide (as reagent and solvent), and the corresponding alkyl halide (Figure 1a). The
reaction yield was 92% and the chemical structure of CTA-1 was validated by 1H and 13
C
NMR (Figure 1b,c). The 1H NMR spectrum shows the characteristic signals of the aromatic
protons at 7.3 ppm as a multiplet. The proton attached to the tertiary carbon that bears
two phenyl substituents and neighboring a sulfur atom is observed as a signal (singlet)
at 6.5 ppm. The integration of both signals match with the number of protons expected
in the CTA-1 structure, that is, an integral value of 1 for the CH signal and an integral
value of approximately 10 for the aromatic protons. On the other hand, the 13C NMR
spectrum shows a signal at 218 ppm that is characteristic of the C=S group. Moreover,
the presence of aromatic carbons (125 to 140 ppm), and the carbon signal of the CH with
phenyl substituents and neighboring sulfur at 58 ppm are confirmed.
Furthermore, CTA-1 was used in the bulk◦polymerizations of methacrylic monomers
(MMA, EMA, and GMA), St, and BuA at 60 C for a predetermined time. In addition,
one of the most cited RAFT agents the di(benzyl) trithiocarbonate, known as CTA-2,
was prepared, and characterized in a similar way to CTA-1. Further, it was used as a
reference for comparing the polymerization results obtained from both CTA.
2.2. Methyl Methacrylate
It is well-known that symmetrical CTAs lead to poor control in the polymerization of
MMA. Table 1 shows the results derived from the polymerization of MMA in the presence of
both CTAs. Here, a different molar ratio [CTA]/[initiator] was used for the polymerization
of MMA with CTA-1 and as a consequence the conversion of monomer consumption varies
from 54 to 91%. To our delight, these result are in accordance with the increase of [AIBN].
Indeed, the Ð of PMMA is 1.3, no matter the ratio [CTA]/[initiator] used.
Table 1. Reaction conditions and results of methacrylic monomers polymerizations.
Molar Ratio
[Monomer]/[CTA]/
[AIBN]
A
B
C
Conversion
(%)
Mn
Mn
,SEC
,Th
Entry
CTA
Monomer
Ð
(Kg/mol)
(Kg/mol)
1
2
3
4
5
6
7
8
CTA-1
CTA-2
CTA-1
CTA-1
CTA-1
CTA-2
CTA-1
CTA-2
MMA
MMA
MMA
MMA
GMA
GMA
EMA
EMA
300:2:1
300:2:1
300:2:0.66
300:2:0.33
300:2:1
300:2:1
300:2:1
91
82
70
54
96
>95
79
>95
13.9
12.4
10.7
8.3
26.6
189.7
25.8
21.7
45.1
n.d.
30.5
1.3
2.4
1.3
1.3
1.9
n.d.
1.3
1.9
14.6
D
D
D
n.d.
13.5
15.4
300:2:1
189.3
T = 60 ◦C, t = 15 h, A determined by 1H NMR, B calculated by Mn = ((mass of monomer) * (fractional conversion)/(moles of RAFT
,Th
agent)) + Mchain ends [Mchain ends = fragment AIBN + fragment CTA-1 = 235 g/mol], C determined by SEC with PMMA calibration, D not
determined due to poor solubility.
Unprecedentedly, the Mn
of PMMA obtained at a higher [AIBN] is a little closer to
,SEC
the Mn than those acquired at low [AIBN]. The MMA experiments proceeded nearly to
,Th
full conversion (91% for Entry 1) after 15 h of polymerization using the CTA-1 (Figure 2a)
this conversion is slightly higher when compared to Entry 2 that uses the conventional
CTA-2, both experiments were carried out at a typical molar ratio of RAFT polymerizations.
Moreover, a nearly linear increase of Mn as a function of the conversion using CTA-1 is
showed in Figure 2b. It suggests the “living” behavior of this polymerization, which possi-
bly is the fact that the conversion rate could be described as a first-order kinetics relation
(Figure 2a, inset). Figure 2c shows the expected complete shift of the SEC traces towards
a higher molecular weight region as the MMA polymerization in the presence of CTA-1
progresses, conserving the different chromatograms an apparent monodispersity. Such
,

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monomodality, as well as the low Ð values of around 1.3 for these PMMA samples (aliquots
at different times for Entry 1), agreeing the control of the polymerization. The MMA poly-
merization control using CTA-2 (Entry 2) was rather poor, clearly shown by the exorbitant
molecular weight value found in this sample, Mn,SEC of 189.8 Kg/mol and Ð of 2.4.
Figure 2. MMA polymerization using CTA-1: (
a) Rate of conversion (inlet: first-order kinetics), (
b) Mn
as a function of
,SEC
conversion, and (c) SEC traces of PMMA obtained at a ratio [MMA]/[CTA-1]/[AIBN] = 300:2:1.
As mentioned above, the MMA polymerization proceeded in the presence of CTA-1
at three different AIBN ratios, keeping the MMA and CTA-1 ratios constant (Entries 1,
3, and 4). As it can be observed in Table 1 for these entries, the experimental molecular
weight or Mn
is at least twice higher than the theoretical values (Mn,Th) and this
,SEC
discrepancy increases when the [AIBN] decreases. It is important to mention that the
Ð values remained low (around 1.3). Based on these results, it can be assumed that the
RAFT agent is not completely consumed during the polymerization using the studied
conditions. The partial consumption of CTA-1 throughout the MMA polymerizations
is evidenced in Figure 3. The PMMA of Entry 1 at 91% conversion was precipitated in
distilled cold hexane to isolate the polymer, then the hexane was completely evaporated,
and the residues were analyzed by 1H NMR. The spectrum of these residues is shown in
Figure 3a, which shows the presence of the aromatic protons and a methine at a
δ = 7.4,
and 6.5 ppm, respectively. The spectrum lack of any signals corresponding to the backbone
of PMMA. This result exhibits the presence of unreacted CTA-1 in the collected residues.
On the other hand, the PMMA bearing the diphenylmethyl groups from the modified
1
(or reacted according to the RAFT mechanism) CTA-1 was also analyzed by H NMR.
Aromatic protons unequivocally derived from the diphenylmethyl groups bonded to
the polymer chains can be clearly observed in Figure 3b. Other important signals, such
as the -O-CH3 from the PMMA units (3.6 ppm) were directly assigned in the spectrum.
Moreover, a signal at 6.45 ppm was observed, which could mean that part of the polymer
chains are end-thiocarbonylthio terminated (PMMA-S-(C=S)-S-CH-(C6H5)2) and not all
of them are middle-thiocarbonylthio functionalized, as stated by the RAFT mechanism
for a symmetrical trithiocarbonate. A proposed reaction scheme of the above discussed is
shown in Figure 4.
The NMR results from Figure 3 demonstrated the partial consumption of CTA-1,
which explains the discrepancy between experimental and theoretical Mn. It is worth
mentioning that the RAFT polymerization is mainly featured by the total consumption
of CTA during the process. However, the presence of unreacted CTA-1 during the RAFT
polymerization of MMA is unusual and could be ascribed to the steric hindrance of the
leaving diphenylmethyl group. We expect that the aromatic groups led to the formation
of stable radicals which delay the β-scission during the addiction-fragmentation process,
and only a part of CTA-1 acts for generating oligomeric chains with low monomer units.
The presence of monomer units between thiocarbonylthio and leaving groups allows the
β-scission therefore the control on the growth chains.

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Figure 3.
of isolated PMMA from Entry 1 demonstrating its functionalization with aromatics groups.
(a b
) 1H NMR spectrum of residual CTA-1 recovery after purification of PMMA from Entry 1; ( ) 1H NMR spectrum
Figure 4. Plausible mechanism reaction for bulk MMA polymerization in presence of CTA-1 as
controller and AIBN as initiator.
Based on the purpose of improving the Mn,SEC/Mn,Th correlation, an experiment in-
creasing the molar ratio [MMA]/[CTA-1] to 300:4 instead of 300:2 was performed to have a
higher concentration of the controlling compound in the polymerization reaction, the AIBN

Molecules 2021, 26, 4618
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proportion was maintained in 1. At this molar ratio (under same conditions: 60 ◦C, 8 h
of reaction) the MMA conversion was 43% and unexpectedly the Mn
(12.4 Kg/mol)
,SEC
still maintained a very significant difference with the theoretically calculated (around
3.2 Kg/mol), even this difference became bigger. Moreover, Ð value was not lowered,
it was in the same range as all previous results of PMMA produced with CTA-1 (between
1.3 and 1.4). It is important to notice that in this experiment the Mn calculated by SEC
matches well with those calculated by 1H NMR (13.0 Kg/mol), indicating that most of
the PMMA chains contain the CTA-1 aromatic functionality, even when the CTA-1 was
not totally consumed during the reaction, as it was above explained. Integrated areas of
phenyls groups signal around 7.4 ppm (assignation A in Figure 3b) and the methyl group
of the PMMA signal at 3.6 ppm (assignation B in the same Figure) were used to calculate
the Mn by this equation Mn,NMR = [(Integral B/3)/(Integral A/20)] * 100 g/mol.
The unexpected lack of correlation between Mn
and Mn inspired the perfor-
,SEC
,Th
mance of three further experiments where the amount of CTA-1 was reduced (MMA and
AIBN were kept constant), to study new ratios [CTA-1]/[AIBN] of 1:1, 0.5:1 and 0.25:1; giv-
ing unexpectedly better Mn,Exp/Mn,Th correlations as the amount of CTA-1 was decreasing:
1.50, 1.37 and 1.16 values respectively. In Figure 5 these results are shown.
Figure 5. SEC traces for MMA polymerizations in presence of CTA-1 at different [CTA]/[AIBN]
molar ratios.
It was demonstrated in Figure 3 that CTA-1 was not completely consumed in the
MMA polymerization using a ratio of [CTA-1]/[AIBN] = 2:1. It is well-known that molar
ratios, where the RAFT agent is equal or lower than the initiator, are not common for
RAFT polymerizations conditions, by which a small proportion of initiator is typically
recommended to obtain low Ð values. However, the results obtained in this work indicate
that using more quantity of AIBN in relation to the RAFT agent helps to higher consumption
of the latter, and thus, the correlation Mn,Exp/Mn was improved and coming very closer
,Th
to 1. The low Ð, around 1.3, were maintained in all five experiments reported in Figure 5.
Data from the polymerization of MMA, carried out at different concentration of CTA-1,
clearly demonstrated the impact of [AIBN] in the correlation between the experimental
and theoretical molecular weights of PMMAs. As observed, the SEC traces of PMMAs
(Figure 5) shift towards higher molecular weights with the diminution of the [CTA]. It is
well-known that the amount of AIBN can not only predict the number of dead chains
into the RAFT polymerization but also the amount of active centers produced during the
addition-fragmentation process [9,32]. Notwithstanding the above, the chemical structure
and steric hindrance of CTA-1 could greatly diminish the transfer coefficient (Ctr) of the

Molecules 2021, 26, 4618
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chain agent and cause a slow re-initiation of leaving radicals R from CTA-1 giving rise to a
retardation in the reversible deactivation process.
Besides the aromatic functionality of CTA-1 in the PMMA demonstrated by the 1H
NMR analysis, a UV analysis was also carried out to corroborate that the trithiocarbonate
(-S-(C=S)-S-) functionality of the CTA-1 was present in the polymer. The UV spectra
in Figure 6a for CTA-1 and for a purified (precipitated 3 times in cold hexane) PMMA
synthesized with CTA-1 show (both) a strong UV absorption corresponding to p/p *
transition of the thiocarbonylthio moiety in the wavelength range 280–360 nm. Absorption
of aromatic rings in the range of 215 to 275 nm is also observed in both spectra, suggesting
that the diphenylmethyl group is bonded to the polymer. To further confirm the presence
of the trithiocarbonate group in the middle of the PMMA chains, as expected for a RAFT
polymerization made with a symmetrical trithiocarbonate [24], a PMMA synthesized with
CTA-1 was treated with an excess of AIBN at 80 ◦C for 2.5 h to produce an exchange of
polymeric chains by the group -C(CH3)2CN [33,34]. The resulting polymer was analyzed
by SEC and the results showed the expected shift towards lower molecular weights region
of the chromatogram (Figure 6b). The radical attack induced a decrease of Mn from 29.2 to
18.5 Kg/mol, indicating the cleavage of the polymer in the trithiocarbonate moiety.
Figure 6. (a) UV analysis, and (b) AIBN cleavage of a PMMA synthesized in presence of CTA-1.
2.3. Glycidyl Methacrylate and Ethyl Methacrylate
Both CTA-1 and CTA-2 were also utilized in the polymerization of derivative methacry-
lates (GMA and EMA). The reactions were performed under similar conditions than poly-
merization of MMA and the ratio [Monomer]/[CTA]/[AIBN] was maintained constant
(300:2:1). Data from SEC analysis (Entry 5) from the polymerization of GMA with CTA-1
showed a Mn
= 45.1 Kg/mol and Ð = 1.9, while the theoretical Mn was calculated to
,SEC
,Th
14.6 Kg/mol. Similar to the case of MMA, this Mn,SEC/Mn,Th correlation was not close to
1 value, and the resulting Ð cannot be considered as a good value for RAFT. Nonetheless,
to the best of our knowledge, this is the first example reported for a homogeneous (bulk or
solution) GMA polymerization kind of controlled by a symmetrical trithiocarbonate, in
absence of an auto-acceleration effect. The term “controlled” was daring to be used in this
case since in the polymerization of GMA with the reference CTA-2 (Entry 6), the resultant
PGMA was insoluble in THF, chloroform, acetone, and toluene. We presume the formation
of gel due to crosslinking and therefore the sample is neither analyzed by SEC nor NMR.
Crosslinked GMA is typically obtained through conventional radical polymerization in
bulk conditions [35], and thus, highlights the relevance of being able to synthesize PGMA
in a rather controlled way by employing a symmetrical trithiocarbonate, in this case, CTA-1.
For the polymerization of EMA with CTA-1 and CTA-2, data from SEC analyses
were displayed in Entries 7, and 8, respectively (See Table 1). Similar to the polymer-
ization of MMA with CTA-1, results from SEC analysis of PEMA obtained with CTA-1
showed a Mn
= 30.5 Kg/mol, which is approximately twice higher than the theoretical
,SEC

Molecules 2021, 26, 4618
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(Mn = 13.5 Kg/mol) and a low Ð = 1.3 with a good conversion ca. to 80%. The low Ð
,Th
value indicates a certain control exercised by the CTA-1 on the polymerization of EMA.
In the case of the polymerization of EMA with the reference CTA-2, data from SEC analysis
exhibited a Mn
ca. to 200 Kg/mol and broad molecular weight distribution when com-
,SEC
pared with the resulting parameters using CTA-1. These last results indicate the expected
poor (or non-existent) control of the polymerization of EMA using the reference CTA-2.
2.4. Styrene and Butyl Acrylate
Additionally, CTA-1 and CTA-2 were also utilized in the polymerization of St and
◦
BuA. Reactions were performed under similar conditions than PMMA (T = 60 C, t = 15 h)
and the ratio [Monomer]/[CTA]/[AIBN] = 300:2:1 was maintained constant to compare
the results from each experiment. Data are summarized in Table 2.
Table 2. Reaction conditions and result of styrene and butyl acrylate polymerizations.
A
B
C
Entry
CTA
Monomer
Mn
(Kg/mol)
Ð
Conversion (%)
Mn
(Kg/mol)
,SEC
,Th
9
CTA-1
CTA-2
CTA-1
CTA-2
St
St
BuA
BuA
3
51
3
0.7
8.1
0.7
0.4
3.6
0.3
1.1
1.3
1.1
1.2
10
11
12
95
18.4
21.3
Molar ratio [Monomer]/[CTA]/[AIBN] = 300:2:1, T = 60 ◦C, t = 15 h, A determined by 1H NMR, B calculated by Mn,Th = ((mass of monomer)
* (fractional conversion)/(moles of RAFT agent)) + Mchain ends [Mchain ends = fragment AIBN + fragment CTA-1 = 235 g/mol], C determined
by SEC with PSt calibration.
As observed in Entries 9 and 11, the polymerization of both monomers (St and BuA)
in the presence of CTA-1 exhibited an extended inhibition period featured by a conversion
of 3% after 15 h of reaction and Mn
values lower than 0.5 Kg/mol, see Figure 7 for
,SEC
St case. As abovementioned, the great stability of the leaving group (diphenylmethyl
radical) does not re-initiate the propagation step due besides to the steric effect. For this
reason, high inhibition was observed and only oligomers were acquired. The monomers
St and BuA were polymerized in the presence of reference CTA-2 prepared in our lab,
see Entries 10 and 12 in Table 2. Data from SEC analyses of both experiments revealed
a good control of CTA-2 in the polymerization of St (Mn
= 3.6 Kg/mol, Ð = 1.3) and
,SEC
BuA (Mn,SEC = 21.4 Kg/mol, Ð = 1.2). These results matched with reported data from the
polymerization of St and BuA in the presence of the well-studied CTA-2 [31,36–39].
Figure 7. Polymerization of St using CTA-1 and CTA-2; (a) rate of conversion and (b) Mn as a function
of conversion.
2.5. Block Copolymerizations
The RAFT technique is featured by the capacity of previously synthesized macro-CTA
to be chain extended for obtaining copolymers with defined structure and special properties.
For highlighting this characteristic, the PMMA from the Entry 4 (Mn
= 21.7 Kg/mol,
,SEC
Ð = 1.3) was utilized as macro-CTA for preparing copolymers by addition of St and BuA.

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Considering that thiocarbonylthio functionality is mainly localized in the middle of the
macro-CTA chains, triblock copolymers (ABA) PMMA-b-PSt-b-PMMA and PMMA-b-PBuA-
b-PMMA were expected. Table 3 summarizes the results related to the synthesis of the
block copolymers using different reaction conditions.
Table 3. Reaction conditions and results of the chain extension and block copolymerizations.
Molar Ratio
[Monomer]/
[macro-CTA]/
[AIBN]
A
T (◦C),
t (h)
Mn
,Th
B
C
Macro-
Mn
(Kg/mol), Ð
Mn
,SEC
,SEC
Entry
CTA
Monomer
Ð
Conversion
(%)
(Kg/mol)
(Kg/mol)
13
14
Entry 4
Entry 4
21.7, 1.3
21.7, 1.3
St
BuA
60, 15
60, 15
608:1:1.3
509:1:1.3
65
96
63.9
84.1
66.9
76.4
1.2
1.4
A
C
Determined by 1H NMR, B calculated by Mn = ((mass of monomer) * (fractional conversion)/(moles of macro-CTA)) + Mmacro-CTA
PMMA calibration.
,
,Th
The block copolymerizations using the CTA-1 functionalized PMMA from Entry 4 as
macro-CTA were performed at a molar ratio of [Monomer]/[macro-CTA]/[AIBN] = 608:1:
1.3 in the case of St and 509:1:1.3 in the case of BuA (or a mass ratio = 300:100:1 in both
cases), at a temperature of 60 ◦C for 15 h. The consumption of St (65%) and BuA (96%)
was calculated by 1H NMR. For the case of St as the monomer, data from SEC analyses
of the resulting copolymer demonstrated that the Mn
= 66.9 Kg/mol is close to the
,SEC
Mn
= 63.9 Kg/mol with a Ð = 1.2. The composition of this triblock copolymer was
,Th
calculated by 1H NMR resulting in 69.5 wt.% of PSt and 30.5 wt.% of PMMA, which agrees
with the expected composition according to the recipe and the monomer consumption. On
the other hand, in Entry 14 BuA was used to synthesize a triblock copolymer PMMA-b-
PBuA-b-PMMA with Mn
= 76.4 Kg/mol and a Ð = 1.4. In this experiment, the value
,SEC
for Mn experimental is also closed to the theoretical one. The composition of the resultant
triblock copolymer was 76 wt.% of PBuA and 24 wt.% of PMMA as calculated by 1H NMR.
The SEC traces of both synthesized copolymers exhibited in Figure 8 showed a
monomodal distribution with a shift towards a higher molecular weight region, which
corroborate the insertion of novel blocks to the PMMA “first” block and the formation of
the expecting triblock copolymers. In the case of PMMA-b-PSt-b-PMMA, the SEC traces
from the refractive index and UV detectors are overlapped indicating the presence of PSt
in all the detected chains (Figure 8a). For the PMMA-b-PBuA-b-PMMA, only the trace
from the refractive index detector was observed because PBuA is undetected under this
condition (Figure 8b). These findings represent evidence of the “living” characteristic of
PMMA obtained by the RAFT conditions polymerization of this methacrylic monomer in
the presence of the novel CTA-1.
Figure 8. SEC traces for block copolymerizations; (
PMMA-b-PBuA-b-PMMA.
a) entry 13, PMMA-b-PSt-b-PMMA; (b) entry 14,

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3. Materials and Methods
Reagents: the monomers methyl methacrylate (MMA, 99%), ethyl methacrylate (EMA,
99%), glycidyl methacrylate (GMA, 97%), styrene (St, 99%), and butyl acrylate (BuA,
99%) were distilled under vacuum before use. Carbon disulfide (99.9%), resin amberlyst
A-26(OH), benzyl bromide (98%), bromodiphenylmethane (95%), and anhydrous magne-
sium sulfate ( 99.5%) were used as received. AIBN (98%) was recrystallized twice from
≥
≥
≥
ethanol before use. CDCl3 and THF (HPLC grade) were also used as received. All reagents
were purchased from Sigma-Aldrich (Toluca, Mexico).
Characterization: Size exclusion chromatography (SEC). The molecular weight charac-
teristics of polymers were determined by SEC using a Hewlett-Packard instrument (HPLC
series 1100) equipped with UV light and refractive index detectors. A PLGel mixed column
was used. Calibration was carried out with polystyrene and poly(methyl methacrylate)
standards and THF (HPLC grade) was used as eluent at a flow rate of 1 mL/min.
Nuclear magnetic resonance (NMR). The chemical structure of CTA, polymers, and the
conversion rate of the monomers was tracked via 1H NMR using a Bruker Avance III HD
400N spectrometer (with a 5 mm multinuclear BB-decoupling probe, direct detection with
Z grad). The analyses were performed at 25 ◦C and the samples were diluted in CDCl3.
Ultraviolet spectroscopy (UV) was carried out in a Varian Cary 100 UV/Vis spec-
trophotometer (Agilent Technologies) to corroborate the presence of thiocarbonylthio
groups [-S-(C=S)-S-] within the polymer chains of functionalized RAFT polymers.
Synthesis of di(diphenylmethyl) trithiocarbonate (CTA-1): 15 g of amberlyst A-26 (OH)
previously dried at 110 ◦C were collocated into a three-neck round flask equipped with a
magnetic stirring, condenser, and addition funnel under argon flow. 50.40 g (0.663 mol) of
carbon disulfide were added, and the resulting suspension was stirred at room temperature
for 10 min. The color of the resin changed from yellowish to red indicating the formation
of thiocarbonylthio group. After that, 3.76 g (0.015 mol) of bromodiphenylmethane were
added and the reaction mixture was stirred under reflux for 10 h in an inert atmosphere.
Then, the mixture was filtered and washed three times with THF. The filtrate was dried
over anhydrous magnesium sulfate and the solvent evaporated under reduced pressure to
obtain the crude product. The CTA-1 was acquired by recrystallization in hexane. Yield
of 92% (3.05 g, 6.9 * 10−3 mol) after purification was obtained. 1H NMR.
δ
: 6.5 (s, 2H,
S-CH-(C6H5)2), 7.2-7.4 (m, 20H, ArH). 13C NMR.
219 (C=S).
δ: 59 (S-CH-(C6H5)2), 127-139 (ArC),
The synthesis◦of di(benzyl) trithiocarbonate (CTA-2): 15 g of amberlyst A-26(OH)
before dried at 110 C were added into a three-neck round flask equipped with a magnetic
stirring, addition funnel, and condenser under argon flow. 56.70 g (0.746 mol) of carbon
disulfide were added and the suspension was stirred at room temperature for 10 min.
The color of the resin turns from yellowish to red, pointing out the formation of thiocar-
bonylthio group. After that, 5.11 g (0.030 mol) of benzyl bromide were added and the
reaction mixture was stirred under reflux for 10 h. Then, the mixture was filtered and
washed three times with THF. The filtrate was dried over anhydrous magnesium sulfate
and the solvent evaporated under reduced pressure to afford the crude product. The
trithiocarbonate was purified by column chromatography using a mixture of petroleum
ether:benzene (9:1) as eluent. Yield of 21% (0.913 g, 3.15 * 10−3 mol) after purification was
1
obtained. H NMR.
δ
: 4.6 (s, 4H, S-CH2-C6H5), 7.2–7.4 (m, 10H, ArH). 13C NMR.
δ: 41
(S-CH2-C6H5), 127–135 (ArC), 223 (C=S).
Hompolymerization reactions: in a typical RAFT polymerization reaction, Entry 1 in
Table 1, a stock solution of MMA (9.0 g, 0.09 mol), CTA-1 (0.265 g, 5.99 * 10−4 mol) and
AIBN (0.049 g, 2.99 * 10−4 mol) (molar ratio of 300:2:1) was prepared. Aliquots of 1.5 g
approximately were transferred to six ignition tubes, degassed with three freeze-evacuate-
◦
thaw cycles, and sealed with flame under vacuum. The tubes were then heated to 60 C in
an oil bath and left for different reaction times (15 h was the final one). An aliquot of each
reaction crude was analyzed by 1H NMR to calculate monomer conversion. The isolated

Molecules 2021, 26, 4618
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polymer was analyzed by SEC to estimate the number-average molecular weight (Mn) and
molecular weight dispersity (Ð) of the resulting polymer.
Based on similar conditions, other MMA experiments were performed varying the
[CTA-1]/[AIBN] ratio (Entries 3 and 4 in Table 1). Moreover, polymerizations of derivative
methacrylate monomers such as EMA and GMA were performed using CTA-1 under similar
reaction conditions. Further, all monomers were tested using CTA-2 as a reference controller.
Block copolymerization reactions: in a typical reaction, Entry 13 in Table 3, a solution
of functionalized PMMA as macro-CTA (0.50 g, 2.30 * 10−5 mol), styrene (1.50 g, 0.014 mol),
and AIBN (5 mg, 3.05 * 10−5 mol) was prepared and transferred to a tube, which was
degassed with three freeze-evacuate-thaw cycles and sealed with flame under vacuum.
The tube was heated to 60 ◦C in an oil bath and left for 15 h. Reaction crude was analyzed
1
by H NMR to calculate monomer conversion. The resulting copolymer was isolated
by precipitation in cold hexane and vacuum-dried, after that, it was analyzed by SEC to
estimate macromolecular parameters.
4. Conclusions
The di(diphenylmethyl) trithiocarbonate or CTA-1 was successfully synthesized in a
yield of 92%, purified and characterized by 1H and 13C NMR. This symmetrical trithiocar-
bonate was used for the first time as chain transfer agent under RAFT polymerization condi-
tions for methacrylic monomers, as well as styrene and butyl acrylate. Methyl methacrylate
polymerization in presence of CTA-1 showed low molecular weight dispersity values, be-
low 1.5 and a “living” behavior. Contrary to previously reported for RAFT, atypically high
[AIBN]/[CTA-1] molar ratios were required for accessing predictable molecular weights.
We also provided important insights towards the methacrylates controlled polymeriza-
tions in homogeneous media using a symmetrical trithiocarbonate as RAFT agent. It was
proved that CTA-1 inhibited the polymerization of styrene and butyl acrylate because of
the great stability and steric effect of the leaving group (diphenylmethyl radical), which
did not re-initiate the propagation step in the reaction. Poly(methy methacrylate)s, pre-
pared in presence of CTA-1, were successful macro-CTAs to synthesize well-defined block
copolymers by sequential polymerization using styrene and butyl acrylate as co-monomers.
Author Contributions: Conceptualization, F.J.E.-M.; data curation, A.L.R.G. and J.L.O.-R.; formal
analysis, L.V. and F.J.E.-M.; funding acquisition, F.J.E.-M.; investigation, R.D.d.L. and F.J.E.-M.;
methodology, A.L.R.G. and H.M.-T.; project administration, F.J.E.-M.; resources, H.M.-T. and F.J.E.-M.;
supervision, L.V.; Validation, J.R.T.-L.; visualization, J.L.O.-R.; writing—original draft, C.S.T., L.V. and
F.J.E.-M.; writing—review and editing, C.S.T., L.V. and F.J.E.-M. All authors have read and agreed to
the published version of the manuscript.
Funding: The authors acknowledge the financial support of the Mexican National Council of Science
and Technology (CONACyT) through the Basic Science project A1-S-29092.
Data Availability Statement: The data that support the findings of this study are available within
the article.
Acknowledgments: The authors are grateful to Judith Cabello, Alejandro Diaz, Ricardo Mendoza
and Maricela Garcia for their technical assistance in the SEC and NMR characterization. We also
acknowledge CONACyT for the scholarship assigned to Alvaro Leonel Robles Grana and for the
financial support through the project A1-S-29092.
Conflicts of Interest: The authors declare that they have no known competing financial interests or
personal relationships that could have appeared to influence the work reported in this paper.
Sample Availability: Samples of the compounds CTA-1 and CTA-2 are available from the authors.

Molecules 2021, 26, 4618
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