Thiol-ene grafting from polylactic acid, polycaprolactone, and polyhydroxybutyrate

Article abstract of DOI:10.1016/j.reactfunctpolym.2016.02.009

Thiol-ene additions of different monomers from multi-thiol functionalized biodegradable polymers are analyzed in this study. These monomers include quaternary ammonium groups known for their biological activities; and also methyl methacrylate (MMA) and hydroxyethyl methacrylate (HEMA), which are used to control the hydrophilic-hydrophobic balance in the issued copolymers. Monomer conversion and the resulting polymer structures are confirmed principally by ¹H nuclear magnetic resonance (NMR) spectra. Model reactions are first conducted with thiophenol, and then polylactic acid (PLA), polycaprolactone (PCL), and polyhydroxybutyrate (PHB) multi-thiol functionalized polymers are successfully used. The structures of the obtained products are analyzed by ¹H NMR spectra: all the reactions are kinetically controlled and monomer additions are controllable. Chain transfer constants of the used monomers to the multi-thiol functionalized PLA are between 1.4 and 1.9. The obtained products were also characterized by size-exclusion chromatography (SEC), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA).

Full text of DOI:10.1016/j.reactfunctpolym.2016.02.009

Reactive and Functional Polymers 101 (2016) 82–89
Contents lists available at ScienceDirect
Reactive and Functional Polymers
journal homepage: www.elsevier.com/locate/react
Thiol–ene grafting from polylactic acid, polycaprolactone,
and polyhydroxybutyrate
Kedafi Belkhir, Corinne Jegat, Mohamed Taha ⁎
a
Ingénierie des Matériaux Polymères (IMP), UMR CNRS 5223, Université de Saint-Etienne, Jean Monnet, F-42023 Saint-Etienne, France
Université de Lyon, F-42023 Saint-Etienne, France
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Thiol–ene additions of different monomers from multi-thiol functionalized biodegradable polymers are
analyzed in this study. These monomers include quaternary ammonium groups known for their biological
activities; and also methyl methacrylate (MMA) and hydroxyethyl methacrylate (HEMA), which are used to
control the hydrophilic–hydrophobic balance in the issued copolymers. Monomer conversion and the
Received 25 December 2015
Received in revised form 22 February 2016
Accepted 23 February 2016
Available online xxxx
1
resulting polymer structures are confirmed principally by H nuclear magnetic resonance (NMR) spectra.
Model reactions are first conducted with thiophenol, and then polylactic acid (PLA), polycaprolactone
Keywords:
Thiol–ene reactions
Active monomers
Functional polymers
Acrylate methacrylate addition
Chain transfer constant
(
PCL), and polyhydroxybutyrate (PHB) multi-thiol functionalized polymers are successfully used. The structures
1
of the obtained products are analyzed by H NMR spectra: all the reactions are kinetically controlled and mono-
mer additions are controllable. Chain transfer constants of the used monomers to the multi-thiol functionalized
PLA are between 1.4 and 1.9. The obtained products were also characterized by size-exclusion chromatography
(SEC), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA).
©
2016 Elsevier B.V. All rights reserved.
1
. Introduction
The reaction between thiol and C = C, or “enes,” and their high
radical polyaddition generally leads to telomers with interesting charac-
teristics and favorable material properties [25]. Among the acrylic and
methacrylic monomers that are likely to be thiol–ene added, those
with a quaternary ammonium group in their structure have attracted
much attention in polymer synthesis because of the numerous biologi-
cal and chemical properties, which they might have acquired during po-
lymerization. The presence of a quaternary ammonium group in the
structure of these monomers enhances their antibacterial [26–29], anti-
microbial [30,31], and fungicide [32,33] activities; they are also used as
surfactants [34,35], and hence known as phase transfer catalysts [36,37].
In a previous study [38], polyurethaneimide block copolymers contain-
ing quaternary ammonium groups were synthesized with easily con-
trolled polymer structures and their functional group contents, with
the resulting materials exhibiting an appreciable film-forming ability.
In a more recent study [39], a poly(arylene ether sulfone) ionomer
was synthesized, which was endowed with controlled amount of pen-
dant quaternary ammonium groups in the polymer chain, which is po-
tentially useful for elaboration of fuel cell membranes. Water-soluble
polymers based on quaternary ammonium monomers were synthe-
sized and patented; a patent [40] was published describing the inven-
tion of linear polymers with high molecular weight having a linear
chain of repeating rings with quaternary ammonium groups.
efficiency were studied for the first time in 1905 [1]. Called “thiol–
ene” reactions, they have attracted much attention in various fields.
With several characteristics of “click” reactions defined by Sharpless
and coworkers [2,3], such as modularity, quantitative yields, non-
requirement of high concentration of catalyst, rapidity in either bulk
or solution, and less sensitivity toward oxygen inhibition, these reac-
tions were considered as belonging to “click chemistry” [4,5] and repre-
sent an efficient tool for polymer functionalization [6], end group
modification [7], controlled network formation [8,9], dendrimer synthe-
sis [10], cross-linking [11], and polymer surface modification [12].
Thiol–ene click reactions were much used in the synthesis of telomers
with controlled degrees of polymerization [13,14]. Monomers contain-
ing pendant C = C, such as acrylic and methacrylic ones, can be added
to the thiol group by either (i) base- or nucleophile-catalyzed thiol-
Michael ionic addition mechanism [15,16] or (ii) anti-Markovnikov
free-radical addition [17,18]. The thiol–ene radical addition can be
photoinitiated [19–21] or carried out in the presence of a free-radical
initiator such as 2,2′-azobisisobutyronitrile (AIBN) [22–24]. Thiol–ene
Reversible addition–fragmentation chain transfer (RAFT) polymeriza-
tion is widely used as a method to synthesize polyquaterniums based on
monomers having pendant enes. This method was used in a recent study
⁎
Corresponding author at: Ingénierie des Matériaux Polymères (IMP), UMR CNRS 5223,
[41] to synthesize a poly(styrene)-b-poly(methacryloxyethyltrimethyl
Université de Saint-Etienne, Jean Monnet, F-42023 Saint-Etienne, France.
E-mail address: Mohamed.Taha@univ-st-etienne.fr (M. Taha).
ammonium iodide) diblock polymer.
http://dx.doi.org/10.1016/j.reactfunctpolym.2016.02.009
1381-5148/© 2016 Elsevier B.V. All rights reserved.

K. Belkhir et al. / Reactive and Functional Polymers 101 (2016) 82–89
83
In this study, polylactic acid (PLA), polycaprolactone (PCL), and
Table 1
Model reaction conditions.
polyhydroxybutyrate (PHB) multi-thiol functionalized polymers were
prepared similarly to our previous study [42] using a facile and original
method, resulting in potentially biodegradable materials with con-
trolled design and high thiol functionality. These polymers should be ac-
tive in thiol–ene radical addition of several double bond-bearing
monomers. It is well known that these reactions are efficient and con-
trollable in the case of small thiol functionalized molecules, producing
telomers. How, for multi-thiol functionalized polymers, the following
two aspects are considered.
Monomer
MMA
R0
Thiophenol (g)
Monomer (g)
Solvent
AIBN (g)
1
1.391
0.348
1.391
0.348
1.391
0.348
1.391
0.348
1.264
1.264
1.677
1.677
0
1
0
1
0
1
.2
THF
10 mL
HEMA
ADCl
.2
.2
0
.011
⁎
⁎
⁎
2.420
2.420
2.596
Ethanol
15 mL
MADCl
2.596⁎
0.2
Thiol–ene radical addition reactions were first carried out using a
small thiol compound, and then reactions of thiol functions of PLA,
PCL, and PHB multi-thiol functionalized polymers were performed.
The used monomers were bearing either quaternary ammonium
groups, evident from their biological activities, or methyl methacrylate
⁎
Mass corresponding to the pure product containing 20–wt.% water.
2
2
.3. Instrumentation
1
.3.1. H NMR analyses
(
MMA) to improve the processability [43] or hydroxyethyl methacry-
1H NMR analyses were performed with a Bruker Avance II spectrom-
late (HEMA) to control hydrophilic–hydrophobic balance [44]. The
obtained products were characterized by 1H nuclear magnetic reso-
nance (NMR) spectra, size-exclusion chromatography (SEC), differen-
tial scanning calorimetry (DSC), and thermogravimetric analysis (TGA).
eter with a frequency of 250 MHz. Each product (30 mg) was dissolved
in 5-mm-diameter tubes using the suitable solvent ((CD SO or D O).
Tetramethylsilane was used as the internal reference at 0 ppm. The tem-
perature was maintained at 300 K for 128 scans.
3
)
2
2
2
. Experimental
2
.3.2. Thermogravimetric analysis
For TGA, the MettlerToledo TGA/DSC 1 STARe System provided with
2
.1. Materials
a microbalance was used. Each sample (8–15 mg) was introduced in a
[
2-(Acryloyloxy)ethyl]trimethylammonium chloride (ADCl) and
1
6
00-μL aluminum crucible for a temperature sweeps from 25 to
00 °C at a heating rate of 10 °C min . The samples were dried under
[
2-(methacryloyloxy)ethyl]trimethylammonium chloride (MADCl)
were the quaternary ammonium active monomers, purchased as
aqueous solution of 80 wt.%. They were checked by TGA and their
−
1
vacuum at 40 °C for 48 h before TGA.
1
structures were confirmed by H NMR spectra. Two other monomers,
MMA and HEMA, were used. Thiophenol, AIBN, tetrahydrofuran
2
.3.3. Differential scanning calorimetry
DSC of the materials before and after grafting was performed with a
(
THF), and ethanol were purchased from Sigma–Aldrich.
Deuterated dimethyl sulfoxide (DMSO-d or (CD SO), purchased
from Euriso-top, and water (D O), purchased from Aldrich, were used
as solvents for H NMR analysis.
3 2
)
Q10 calorimeter purchased from TA Instruments. Samples with a
weight range of 2–3 mg were put into hermetic aluminum pans to per-
form two series of heating and cooling between −80 and 200 °C. The
rate of cooling/heating was 10 °C/min. Glass transition temperature
2
1
Multi-thiol functionalized PLA, PCL, and PHB having different de-
signed structures and different thiol functionalities were synthesized
according to the method described in our previous work [42]. An exam-
ple of two structures with three or seven biodegradable polymer arms
and respectively three or five thiol functions is illustrated in Supple-
mentary Information S-1. The synthesis protocol of these products is
also detailed in Supplementary Information S-11 and S-12.
(
T
g
) was obtained from the second heating run by identifying the inflec-
tion point.
2
.3.4. Size-exclusion chromatography
SEC analyses were conducted on a Malvern Viscotek HT GPC 350
system equipped with refractive index (RI) and viscometer detectors.
PSS-GRAM columns with a range of 100–1,000,000 g/mol were used
and the products were eluted by DMSO at 80 °C. Molecular weights
were calculated referring to a universal calibration based on polymethyl
methacrylate (PMMA) standards.
2
.2. Nomenclature
SH: thiol function.
A: number of arms in the branched structure.
F: thiol functionality (number of thiol groups in the multi-thiol func-
tionalized polymer).
g: “grafted.”
Table 2
Conditions of thiol–ene addition on multi-mercapto functionalized polymers.
Examples
Structure (number of arms Segment
R
0
Monomer (g)
PLA-SH A3 F3 = multi-thiol functionalized PLA with three arms and
three thiol functions.
PLAgAD = PLA grafted with ADCl, having three arms and three thiol
functions.
PCL-SH A1 F3 = multi-thiol functionalized PCL having one arm and
three thiol functions.
PCLgMAD A1 F3 = PCL grafted with MADCl, having one arm and
three thiol functions.
and functionality)
nature
ADCl
MADCl MMA HEMA
A1 F3
PLA
1
0
0.047 0.050
–
–
.25 0.186 0.200
0.097 0.129
0.097 0.128
PCL
PHB
0.25
1
–
–
0.034 0.036
–
–
–
–
–
–
–
–
–
–
–
–
–
–
0
1
0
1
.25 0.134 0.144
A3 F3
A7 F5
PLA
0.016 0.017
.25 0.064 0.069
0.038 0.041
PCL
PHB
0.25 0.153 0.164
0.023 0.025
0.25 0.094 0.100
PHB-SH A7 F5 = multi-thiol functionalized PHB, having seven arms
and five functions.
1
0.033 0.043
PHBgAD A7 F5 = PHB grafted with ADCl, having seven arms and five
functions.
ADCl and MADCl additions were done in a 50/50 vol. mixture of THF/ethanol, while THF
was used as a solvent in the case of MMA and HEMA.

84
K. Belkhir et al. / Reactive and Functional Polymers 101 (2016) 82–89
1
Fig. 1. H NMR spectra of model reactions to obtain oligomers: (1) SH-ADCl(R
0 0 0 0 2
=1), (2) SH-ADCl (R =0.2), (3) SH-MADCl(R =1), and (4) SH-MADCl(R =0.2). Reaction conditions: D O,
400 MHz and 300 K.
2
.4. Model thiol–ene reactions
where [SH]
0
is the concentration of thiol functions and [C = C]
0
is the
initial monomer concentration.
Model thiol–ene additions of acrylic (ADCl) and methacrylic
MADCl, MMA, and HEMA) monomers were performed with
The reaction between thiophenol and ADCl with R =1 is given
as an example to detail the procedure: 1.3912 g of thiophenol
0.013 mol) was dissolved in 7.5 mL of ethanol, and 3.0261 g of
80 wt.% ADCl (0.013 mol) was dissolved in the same amount of eth-
anol. The two solutions were introduced into a 25-mL Schlenk tube
0
(
(
½
SHꢀ
0
R0 ¼
¼ 1 and 0:2
ð1Þ
½
C ¼ Cꢀ
0

K. Belkhir et al. / Reactive and Functional Polymers 101 (2016) 82–89
85
T
Fig. 3. C determination during thiol–ene reaction of ADCl with thiophenol (70 °C).
2.6. Monitoring of reaction kinetics and determination of transfer constants
The conversions of both thiol functions and monomers were moni-
tored during the thiol–ene addition in the case of a model reaction
and the thiol–ene addition of the cited monomers on a multi-thiol func-
tionalized PLA.
Fig. 2. 1H NMR spectra of (a) thiophenol and (b) SH-HEMA (R
0
=1) model reaction
As a model reaction, thiol–ene addition of ADCl on thiophenol is
given as an example: 8.473 g of ADCl (0.035 mol) and 0.779 g of
thiophenol (0.007 mol) were dissolved in a 50-mL round-bottom two-
neck flask with DMSO-d (25 mL) as a solvent and AIBN (0.03 g) as an
product. Reaction conditions: 7.1–7.6 ppm, 300 K, and 400 MHz. Showing thiophenol
conversion calculation.
equipped with a magnetic stirrer, and mixed until homogenization.
−
3
Then, 0.011 g of AIBN ([AIBN]/[monomer] = 5 × 10 ) was added.
The reactor was purged by a nitrogen flow and immersed in an oil bath
at 70 °C for 8 h. The solution was then dried using a rotary evaporator,
and the remaining solvent was removed under vacuum in an oven
initiator. In these proportions, the initial thiol/ene ratio was R
The flask was first purged by a nitrogen flow pipe and the reaction
was carried out at 70 °C using an oil bath with magnetic stirring. 1
0
=0.2.
H
NMR tubes (5 mm diameter) were previously prepared and contained
0.65 mL of DMSO-d6. Samples (0.75 μL) were taken from the reaction
mixture at different times and introduced in the prepared 1H NMR
tubes, which were quickly shacked and immersed in an ice/water bath
−
3
(
10 mmHg, 40 °C) until constant weight was attained.
All the model reaction conditions are reported in Table 1.
1
2
.5. Thiol–ene reactions using multi-thiol functionalized PLA, PCL, and PHB
For the thiol–ene addition of monomers on multi-thiol functional-
at 0 °C to allow freezing of the reaction. The DMSO-d volume in the H
NMR tubes and the sample were set to obtain a concentration of ap-
−
1
1
proximately 30 mg mL in the H NMR tubes (recommended value
for the analyses). After that, the collected samples were analyzed to de-
termine the thiol and monomer conversions enabling the calculation of
the transfer constant.
0
ized PLA, PCL, and PHB, two values of R (Eq. (1)) were chosen for
each monomer. An example of the addition of MADCl on the three-
thiol functionalized PLA including three biodegradable arms with
R
0
=0.25 has been provided: in a 25-mL Schlenk tube under nitrogen
Concerning thiol–ene addition on the multi-thiol functionalized PLA,
four monomers were investigated. The latter comprised of five arms of
−
5
flow, 0.2 g of PLA (8.268 × 10 mol of thiol groups) was dissolved in
5 mL of a 50/50 vol. mixture of THF/ethanol, then 0.086 g of MADCl
80 wt.% in water) was added. The reaction mixture was stirred until
−
1
1
(
1100 g mol and endowed four thiol functions. The multi-thiol func-
−
3
tionalized PLA (3.5 g including 1.6 × 10 mol of thiol functions) was
−
3
homogenization, and then 5 × 10 g of AIBN was added. The system
was immersed in an oil bath at 70 °C with magnetic stirring for 12 h.
After the reaction was complete, the solution was dried using a rotary
evaporator. The remaining solvent was removed under vacuum in an
dissolved in a 50-mL round-bottom two-neck flask using 25 mL of
−
3
DMSO-d, then 8 × 10
1.550 g, MADCl: 1.662 g, MMA: 0.801 g, HEMA: 1.041 g). In these pro-
portions, the initial thiol/ene mol ratio was R =0.2. The initiator was
mol of the monomer was added (ADCl:
0
−
3
−2
oven (10
mmHg, 40 °C) until constant weight was attained. The
AIBN (7 × 10 g). The reaction was carried out under nitrogen atmo-
sphere at 70 °C in an oil bath. Samples (175 μL) were taken from the re-
action mixture at different times and directly introduced in previously
1
product was then analyzed via H NMR. Reaction conditions for
equivalent reactions are listed in Table 2.
Table 3
1
Model reaction results concerning the conversions of thiol functions and monomers when they react with thiophenol, conversion-based DP
according to the product structure.
n
evaluation, and its H NMR–confirmed values
0
n⁎
n
preliminary evaluation by 1
DP
n
confirmed by 1H
Monomer
R
Expected DP
Conv SH
Conv C=C
DP
H
NMR-determined conversions
NMR-analyzing structure
ADCl
1
0
1
0
1
0
1
0
1
5
1
5
1
5
1
5
0.99
1
1
0.98
0.87
0.88
0.96
0.98
1
1
1
0.99
0.97
0.95
0.93
0.92
1.0
5.0
1.0
5.1
1.1
5.4
1.0
4.7
1.1
5.4
1.0
4.5
1.1
4.8
1.1
5.2
.2
.2
.2
.2
MADCl
MMA
HEMA
⁎
This value of DPn is expected for Conv SH = and Conv C=C = 1.

86
K. Belkhir et al. / Reactive and Functional Polymers 101 (2016) 82–89
Fig. 4. 1H NMR spectra in DMSO at 300 K and 400 MHz of (4.1) the PLA-based polymer (three arms, three thiol functions) from 6.4 to 7.2 ppm, (A) before thiol–ene addition, (B) after
addition of ADCl, showing the calculation of thiol group conversion and (4.2) (C) ADCl, (D) PHB-based polymer (seven arms, five thiol functions) before adding ADCl and (E) after
addition, from 4.1 to 6.7 ppm.
1
prepared H NMR tubes (containing 0.65 mL of DMSO-d) before being
quickly shacked and immersed in an ice/water bath for freezing. Then
Spectra of the obtained oligomers and the peak attributions are shown
in Fig. 1 for ADCl and MADCl, and Supplementary Information S-6 and
S-7 for HEMA and MMA, respectively. The peaks of protons located on
ortho “o”, meta “m”, and para “p” positions in thiophenol were used
for titration and observation of its related shifts.
Thiophenol conversion (Conv SH) was calculated by integrating
unreacted proton peaks and comparing them with the reacted ones;
for example, Fig. 2 shows this calculation in the case of SH-HEMA oligo-
1
H NMR spectra of the samples were recorded and the conversions of
both thiol functions and monomers were calculated. The obtained
values were used to determine the transfer constant of each monomer
on the multi-thiol functionalized PLA.
3
3
3
. Results and discussion
0
mer with R =1. We can see the remaining ortho-located protons “o”
peak at 7.50–7.55 ppm (unreacted thiophenol), and a new peak
appeared at 7.15–7.25 ppm, corresponding to the reacted thiophenol
.1. Thiol–ene model reactions: thiophenol addition
.1.1. Synthesis and structure analyses
o
para-located proton “p'.” With A and Ap' denoting the peak areas of
Thiophenol was chosen for additions on ADCl, MADCl, MMA, and
“o” and “p'” protons, respectively, thiophenol conversion can be
HEMA (Supplementary Information S-2).
expressed as
The products were analyzed after removing solvents, and the con-
versions and polymerization degrees (DP
NMR spectra. The oligomers of ADCl and MADCl were analyzed in
O, and those of MMA and HEMA were analyzed in DMSO-d.
) were determined by 1
H
n
2
ꢁ Ap0
Conv SH ¼
:
ð2Þ
2 ꢁ Ap0
þ Ao
D
2
Table 4
Results of monomers thiol–ene addition on multi-thiol functionalized polymers.
Structure
A1 F3
Segment nature
PLA
R
0
Monomer
ConvC = C
ConvSH
Determined DP
n
Expected DP
n
1
ADCl
MADCl
ADCl
MADCl
MMA
HEMA
MMA
HEMA
ADCl
MADCl
ADCl
MADCl
ADCl
MADCl
ADCl
MADCl
ADCl
MADCl
ADCl
MADCl
ADCl
MADCl
ADCl
MADCl
MMA
HEMA
0.89
0.85
0.97
0.84
0.91
0.69
0.98
0.78
0.86
0.76
0.96
0.77
0.97
0.90
0.95
0.93
1.00
0.82
0.96
0.77
1.00
0.72
0.89
0.70
1.00
0.85
0.64
0.84
0.63
0.97
70.1
0.77
0.77
0.83
0.68
0.78
0.73
0.83
0.92
0.96
0.76
0.83
0.82
0.90
0.78
0.93
0.80
0.89
0.72
0.90
0.81
0.90
1.4
1.0
6.2
3.5
5.1
3.5
5.1
3.7
1.3
1.0
5.3
3.7
1.1
1.0
5.0
4.5
1.2
5.3
4.9
3.3
1.3
1.0
4.9
3.1
4.9
3.8
1
1
4
4
4
4
4
4
1
1
4
4
1
1
4
4
1
1
4
4
1
1
4
4
4
4
0
.25
PCL
0.25
1
PHB
0.25
1
0.25
1
0.25
1
0.25
A3 F3
A7 F5
PLA
PCL
PHB

K. Belkhir et al. / Reactive and Functional Polymers 101 (2016) 82–89
87
Fig. 5. Determination of chain transfer constant for the telomerization of ADCl, MADCl, MMA, and HEMA using the multi-thiol functionalized PLA at 70 °C.
For monomer conversion(Conv C=C), the remaining amount was
reflected by double-bond proton peaks; these two protons show
two separated peaks: MADCl: 6.15 and 5.76 ppm and ADCl: 6.44
and 6.05 ppm (each one is split into two parts). Another peak is
exploitable for ADCl, which appears at 6.23 ppm and corresponds
to the tertiary carbon proton; MMA: 6.02 and 5.67 ppm; HEMA:
peak). For SH-HEMA oligomers, the “s” (unreacted) and “s'”
(reacted) methylene proton peaks were exploited.
Once the conversions of thiophenol and monomers were calculated, a
first approximation of DP
n
of the model products could be expressed as
½c ¼ cꢀ0 ꢁ Conv C¼C
DPn ¼
ð3Þ
6
.06 and 5.67 ppm. As it can be observed from Fig. 1spectra (1) and
2), there is no trace of unreacted double-bond protons. This indi-
cates a total consumption of ADCl. However, these peaks appear in
SH-MADCl (R =1) spectra (Fig. 1, spectra (3) and (4)), but with
½SHꢀ ꢁ Conv SH
0
(
where [C = C] and [SH] are respectively the initial mole concentrations
0
0
0
of the monomer and thiophenol in the reaction mixture.
very small amplitude, and hence there is a very high consumption
of MADCl (conversion values are presented in Table 3). Concerning
MMA and HEMA (Supplementary Information S-6 and S-7),
double-bond proton peaks are more marked in the model products'
spectra, particularly for HEMA (“q”). This reveals a lower monomer
conversion than ADCl and MADCl. These unreacted monomer peaks
were compared, where appropriate, to peaks of reacted monomer
protons, except for SH-ADCl oligomers (total consumption). For
SH-MADCl oligomers, “h'” and “i'” peaks, which also contain “h”
and “i” peaks of unreacted monomers, were chosen. For SH-MMA
oligomers, the “m'” peak was used (it includes unreacted “m” proton
This preliminary evaluation was confirmed by studying the structure
of the obtained oligomers by H NMR as follows:
1
For the SH-ADCl oligomer with R 0=1 (Fig. 1, spectrum (1)), the cal-
culation was done by comparing the peak denoted as “p'” corresponding
to the para-located proton in reacted thiophenol (at 7.3 ppm) with the
methylene proton peak “c'” of reacted ADCl at 4.44 ppm. The same result
was obtained when the “d'” methylene proton peak was exploited in-
stead of “c'”. In the case of SH-ADCl oligomer with R 0=0.2 (Fig. 1, spec-
trum (2)), the DP n was calculated by dividing the “c'” peak area at
4.40–4.64 ppm (corresponding to the methylene-reacted protons of
ADCl) by the “p'” peak area at 7.3 ppm corresponding to the reacted
g d
Fig. 6. (A): Examples of DSC curves showing T values before and after grafting, (B) examples of TGA curves showing T variations due to grafting monomers on the multi-thiol
functionalized polymers.

88
K. Belkhir et al. / Reactive and Functional Polymers 101 (2016) 82–89
thiophenol para-located proton. For SH-MADCl with R
0
=1 (Fig. 1, spec-
3.2.1.1. SH conversion. For thiol conversion, the aromatic ortho proton
(z) toward the SH at 6.53 ppm is deshielded during the thiol–ene reac-
tion, producing (z') peak (Fig. 4.1). The SH conversion thus becomes
trum (3)), all the peaks were exploitable and resulted in convergent
values, and the ratio of the “p'” proton peak area at 7.3 ppm to the “h'”
one at 4.35 ppm (corresponding to the methylene proton peak) was
calculated. An equivalent calculation procedure was used for SH-MADCl
0
z
ConvSH ¼
:
ð5Þ
with R
0
=0.2 to calculate DP
n
(Fig. 1, spectrum (4)), in which the “p'”
0
z þ z
proton peak was compared with the “h'” one at 4.27–4.65 ppm.
Thiophenol and monomer conversions with the determined DP
values are summarized in Table 3.
Table 3 shows that high monomer conversion rates were obtained
for quaternary ammonium monomers (ADCl and MADCl), which
present a higher conversion than MMA and HEMA. The transfer agent
n
3.2.1.2. Monomer conversion. As an example, Fig. 4.2 shows the determi-
nation of ADCl conversion by comparing the “b” residual proton peak at
6
4
2
.23 ppm to total corresponding –O–CH – proton peak (“c” + “c'”) at
.29–4.74 ppm. The following equation was used for this calculation:
0
(
thiophenol) conversion rate is also higher for these monomers.
c þ c −2b
ConvADCl
¼
:
ð6Þ
A good agreement was found between the expected DP
n
values and
c þ c0
those calculated from the 1H NMR evaluation of monomer and
The determination of conversions of other monomers (MADCl,
MMA, and HEMA) and the spectra of other products are described in
Supplementary Information (S-5, S-6, and S-7) and (S-8), respectively.
Both monomers and SH conversion values are reported in Table 4.
As shown in Table 4, the maximum conversions of monomers are
variable (from 0.70 to 1.00). This is probably due to dead-end polymer-
ization [45,46]:
thiophenol conversions. This was also the case when the expected
1
DP
n
values were compared with those determined by H NMR analysis
of the structure of model molecules. This indicates that the experimen-
tally determined DP values are in accordance with the expected ones
n
and that the two methods are consistent and produce equivalent
results.
ꢀ
ꢁ
3
.1.2. Chain transfer constant determination
Chain transfer constant C was determined by the following relation
20]:
f ½Iꢀ0
lnð1−Conv C¼CÞ ¼ −2kp
:
ð7Þ
ktkd
T
[
The dead-end conversion depends not only on the initiator's initial
concentration[I] , but also on other parameters that change with the
and k ) and the initiator (k ) as well as the initiator's
ꢀ
ꢁ
α
0
log 1−
¼ CT logð1−αÞ
ð4Þ
used monomer (k
efficiency f.
p
t
d
DPnR0
All these parameters and constants change with the used reaction
composition and conditions; the dead-end reaction occurs at different
conversions. This is obtained in the aforementioned experiments.
The used multi-thiol functionalized polymers were efficient in thiol–
ene radical addition of the monomers. The structures of the obtained
products conformed to the expected ones.
where α is the monomer conversion.
α
Thus, by plotting logð1−
ed as shown in Fig. 3.
Þ versus log(1−α), C
T
can be calculat-
DPnR0
C
T
of thiophenol to ADCl at 70 °C was 1.44; the trend line shows
good linearity with a correlation coefficient of 0.985. This value of C is
coherent with those cited in the literature for reactions of thiol and
acrylate functions.
T
It is possible to conduct thiol–ene reactions using these multi-thiol
functionalized polymers with acrylate monomers and obtain either
n
monoaddition or longer chains with controlled DP .
3
3
.2. Thiol–ene grafting reactions on multi-thiol functionalized polymers
3.2.2. Chain transfer constants of multi-thiol functionalized polymers
.2.1. Synthesis and structure analyses
Thiol–ene radical additions of ADCl, MADCl, MMA, and HEMA on
Chain transfer constants of the four monomers (ADCl, MADCl, MMA,
and HEMA) were also determined. Fig. 5 shows the plots corresponding
to the thiol–ene addition of these monomers on the multi-thiol func-
tionalized PLA.
multi-thiol functionalized PLA, PCL, and PHB were carried out. The reac-
tion conditions are presented in Table 2. An example of a three-thiol
functional prepolymer is illustrated in Supplementary Information S-4.
T
A good correlation between the calculated values of C and the linear
1
The products were analyzed by H NMR. The monomer and thiol
2
correlation coefficient R was achieved for each plot (Fig. 5). The kinetic
monitoring allowed the determination of chain transfer constants. The
reactions of multi-thiol functionalized polymers were kinetically con-
trolled when small thiols were used.
n
group conversions were measured, and DP was calculated according
to Eq. (3). Examples of thiol and monomer conversions determined by
1
H NMR are provided in the following subsections.
3
.2.3. Thermal properties
s of the products before and after thiol–ene grafting were deter-
mined by DSC, and the thermal degradation temperature (T ) was de-
T
g
d
termined by TGA. Fig. 6 shows some examples of the obtained results,
and other values are provided in Supplementary Information S-9.
As shown in Fig. 6(A), T
the monomers; several studies [47–49] investigated the effect of cross-
linking density on the T of the polymer and concluded that the T in-
g
of the materials increases due to grafting of
g
g
creases with the increase of the cross-linking density. In this study, by
grafting monomers on the end chains of the branched structures, the
cross-linking density increases according to the structure of the poly-
mer, and hence it is obvious that the T
after grafting monomers. On the contrary, the T
g
of these materials increases
depends on the nature
g
of the pendent groups in the branched structure [49]; thus, because of
grafting monomers with quaternary ammonium sites, the chemical
and physical properties of the pendant groups, that is, thiol groups,
Fig. 7. SEC traces showing the evolution of the refractive index before (PLA-SH) and after
(PLAgHEMA) grafting. This result concerns the sample with 7 arms and 5 thiol
functions.R =0.25.
0

K. Belkhir et al. / Reactive and Functional Polymers 101 (2016) 82–89
89
change, thereby affecting the T
Fig. 6(B)), T decreases after the grafting of monomers. The values of
are still acceptable for eventual applications in molten phase.
g
. Regarding with thermal degradation
Appendix A. Supplementary data
(
T
d
d
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.reactfunctpolym.2016.02.009.
3
.2.4. SEC results
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Acknowledgments
[
[
The authors are grateful to the French Ministry of Higher Education
and Scientific Research for the PhD scholarship of Kedafi Belkhir. They
also thank the NMR Polymer Center of the Institut de Chimie de Lyon
[
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(
FR3023) for assistance and access to the NMR facilities.