Synthesis and Rheological Characterization of Latexes Stabilized by

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Synthesis and Rheological Characterization of Latexes Stabilized by Methacrylic Acid Containing Macromonomers Ingeborg Schreur-Piet, Alexander M. van Herk, Jozua Laven, and Johan P. A. Heuts Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b02794 • Publication Date (Web): 26 Aug 2019 Downloaded from pubs.acs.org on August 30, 2019

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Industrial & Engineering Chemistry Research

Synthesis and Rheological Characterization of Latexes Stabilized by Methacrylic Acid Containing Macromonomers Ingeborg Schreur-Piet,a Alexander M. van Herk,a,b Jozua Laven,a and Johan P.A. Heutsa* aDepartment

of Chemical Engineering & Chemistry, Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven, The Netherlands

bInstitute

of Chemical and Engineering Sciences, Agency for Science, Technology and Research, 1 Pesek Road, Jurong Island, Singapore 627833 *Author for correspondence: [email protected]

KEYWORDS: Emulsion polymerization, reactive surfactant, latex rheology, catalytic chain transfer

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 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 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT A range of copolymers of methacrylic acid (MAA) macromonomers prepared by cobaltcatalyzed chain transfer and methyl methacrylate (MMA) and/or butyl acrylate (BA) was synthesized and used as stabilizer in the emulsion polymerization of MMA. Although clear differences were observed in polymerization rates using the different MAAx-MMAy stabilizers, these differences were not as clearly reflected in the particle sizes, nor in the rates per particle. However, a clear difference between these systems and those stabilized by MAAx-BAy was observed. The latter systems were all characterized by much smaller particle sizes and corresponding higher rates of polymerization. In addition, the molar masses in the latter systems were all significantly larger than those obtained in the MAAx-MMAy stabilized system, in which the stabilizers act as "sulfur-free" RAFT agents. Interestingly, the prepared latexes showed a range of appearances varying from "milky" to "gel-like" depending on the used stabilizer. The MAAx-BAy stabilized latexes had in general a lower viscosity and a significantly smaller (if any) yield stress than the MAAx-MMAy stabilized latexes, and in the latter case the rheological behavior was found to depend on the block lengths in and concentration of the stabilizer.


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1


INTRODUCTION

Functional polymer latexes prepared by emulsion polymerization have found applications in many fields, including water-borne coatings, adhesives and in biomedical applications.1-4 In emulsion polymerization surfactants control the colloidal stability of the latex, the particle size and surface functionality of the formed particles and have a large influence on the rheology of the latex.5,6 If surfactants are only physically bound to the surface of the latex particles, the surfactants can migrate towards the film interface upon drying and may have a negative effect on final film properties like water sensitivity, wettability, gloss, adhesion and blocking.7-11 Hence, preferably surfactants are used which are chemically bound to the surface of latex particles.12-16 In order to avoid the chemically bound surfactants from being buried inside the latex particles or from forming water-soluble polymer chains that may cause bridging flocculation, an ideal reactive surfactant should not be too reactive at the start of the emulsion polymerization, but at the end of the emulsion polymerization all surfactant should have reacted to obtain a stable latex.13,17 Reactive surfactants containing a propenyl end-group display the right reactivity,14,15 and methacrylic oligomers containing these end-groups (called macromonomers in the remainder of this paper) are readily prepared via cobalt(II)-mediated catalytic chain transfer polymerization (CCTP).18-20 It is known that in a copolymerization these macromonomers act as addition-fragmentation chain transfer (AFCT) agents with methacrylates and that the copolymerization results in block copolymers.

In a

copolymerization with acrylates (and styrene) the mechanism is more complex and ultimately 3 ACS Paragon Plus Environment


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leads to graft copolymers.18-23 In earlier work we synthesized methacrylic acid (MAA) macromonomers via CCTP and used these directly in an emulsion polymerization to form in situ amphiphilic copolymers,24 in a mechanism similar to what is commonly known as polymerization induced self-assembly (PISA).25-28 We observed that only in cases in which these water-soluble MAA macromonomers were sufficiently quickly converted into amphiphilic copolymers stable latexes could be produced. This was the case for emulsion polymerizations of the (hydrophobic) monomers butyl acrylate (BA) and butyl methacrylate (BMA), but in the case of (the more hydrophilic) methyl methacrylate (MMA) the reaction with the MAA macromonomer was probably too slow and no stable latexes could be produced.

In order to circumvent this problem, we decided to separate the phases of

stabilizer formation and emulsion polymerization in the current study.

Hence we pre-

polymerized the MAA macromonomer with MMA and BA to yield the amphiphilic copolymers M and B, respectively, and used these as stabilizers in the emulsion polymerization of MMA.


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COOH

R' R v

z

BuOOC

COOBu

BuOOC

y COOH

x

(B)

H

The different structures of M (MAAx-MMAy) and B (MAAx-BAy) reflect the fact that methacrylates and acrylates react differently with the methacrylic macromonomers. 18,22 These different architectures (but similar overall compositions) lead to potentially different stabilizing and resulting rheological properties and these properties are discussed in the final part of this paper.

2 2.1

EXPERIMENTAL SECTION Materials

All monomers, MMA, BA and t-BMA were obtained from Sigma-Aldrich (99 %). The monomers were passed over a column of inhibitor remover (Aldrich) to remove the inhibitor. 5 ACS Paragon Plus Environment


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N,N'-azobis(isobutyronitrile) (AIBN, Merck) was recrystallized from methanol. The bismethanol complex of cobaltoxime boron fluoride (COBF) was prepared as described previously29 (chain transfer constant CT for MMA in bulk = 34·103 at 60 °C). Toluene (AR, Biosolve), dichloromethane (AR, Biosolve), tetrahydrofuran (stabilized with BHT, Biosolve) and trifluoro acetic acid (95%, Aldrich) were used as received. Potassium persulfate (KPS, p.a.) and sodium carbonate (dehydrated, p.a.) were purchased from Merck and used as received. 2.2

Synthesis of Copolymer Stabilizers

First, a range of t-BMA-macromonomers with number-average degrees of polymerization, DPn, of 4, 6 and 12 (as determined by 1H-NMR) were prepared as described previously.24 These macromonomers were then reacted with MMA or BA to give t-BMAx-MMAy and tBMAx-BAy "pre-cursor" copolymers, respectively, which were subsequently treated with trifluoro acetic acid to yield the corresponding MAAx-MMAy and MAAx-BAy copolymer stabilizers. As a typical example of the followed procedure we report here the synthesis of MAA12-MMA2; for the exact quantities used for all other copolymer stabilizers we refer to the SI. First, 4.0 g of MMA (0.04 mol) was added to a 100 mL round bottom flask containing a magnetic stirring bar. After the addition of 22 mg AIBN (0.13·10-3 mol), the flask was sealed airtight with a septum and the solution was deoxygenated by purging with nitrogen in an ice bath for 30 minutes.

Simultaneously 45 g of a toluene solution containing t-BMA

macromonomer (44 wt% macromonomer, DPn = 12) was deoxygenated in a separate 100 mL 6 ACS Paragon Plus Environment

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round bottom flask. Subsequently, the macromonomer solution was added to the monomer/initiator solution, heated to 60°C and left to react at this temperature for 16 h under continuous stirring. The obtained t-BMA12-MMA2 macromonomer was isolated by evaporation of toluene and residual monomer under reduced pressure, and dried in a vacuum oven at 60 °C for 24 h (overall yield = 90%). Subsequently 24 g t-BMA12-MMA2 was added to a 100 mL round-bottom flask containing 50 mL dichloromethane, stirred until the polymer dissolved completely after which 20 mL trifluoro acetic acid (0.3 mol) was added and stirred at room temperature for 48 h. Dichloromethane and excess trifluoro acetic acid were removed by evaporation under reduced pressure and the resulting MAA12-MMA2 polymer was dried in a vacuum oven at 60°C for 2 days (quantitative yield). 2.3

Emulsion Polymerization

Emulsion polymerizations were carried out in batch. All experiments were carried out under argon in a jacketed glass reactor (250 mL), thermostatted at 60 °C and equipped with a mechanical four bladed turbine stirrer. The monomer conversions during reaction were determined gravimetrically. First, all ingredients except for the initiator solution were charged into the reactor, the reaction mixture stirred at 350 rpm, purged with argon for 30 minutes and subsequently heated to 60 °C. Five minutes after reaching the desired temperature, the aqueous KPS solution was added with a syringe to initiate the polymerization (for composition, see Table 1). 7 ACS Paragon Plus Environment



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Table 1. Recipe of a batch emulsion polymerization at T = 60 °C Ingredient

Amount

Water

120 g

Na2CO3

0.4 g (0.02 M)

Polymeric stabilizer

Variable; 1.5 g (5 wt%a) is standard level

MMA

30 g (solids content = 20%)

K2S2O8 (KPS)

0.08 g (0.25 wt%a = 2.5·10-3 M)

a wt%

2.4

= weight percentage relative to monomer (= g/100 g of monomer).

Analysis

Nuclear Magnetic Resonance 1H

NMR spectra were recorded on a Varian MercuryVx spectrometer at 400 MHz chloroform-

d1, methanol-d4, DMSO-d6, THF-d8 and tetramethylsilane were used as solvents and internal standard, respectively. All NMR results were obtained after solvent suppression of D2Oδ=4.8. MALDI-ToF MS MALDI-ToF MS spectra were measured on a PerSeptive Biosystems Voyager-DE STR MALDI-ToF MS spectrometer equipped with 2 m flight tubes for linear mode and 3 m flight tubes for reflector mode and a 337 nm nitrogen laser (3 ns pulse). All mass spectra were obtained with an accelerating potential of 20 kV in positive ion and reflector mode with delayed extraction. Data were processed with Voyager software. Simulations were performed 9 ACS Paragon Plus Environment


with

the

MaldiAnalysis

software

by

Staal

and

Willemse.30,31

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As

matrix

2,4,6-

trihydroxyacetophenone (80 mg/mL THF or methanol) was used and diammonium hydrogen citrate (5 mg/mL THF or methanol) as cationating agent. Acrylate containing polymer samples were dissolved in THF and methacrylic acid containing polymer samples were dissolved in methanol at concentrations of 5 mg/mL solvent. Analyte solutions were prepared by mixing solutions of matrix, salt and polymer at a 4:1:4 volume ratio. Subsequently, a spot of 0.30 µL of such a mixture was put on the sample plate and dried at room temperature. Dynamic Light Scattering Dynamic light scattering (DLS) measurements were performed on a Nanotrac Ultra (Microtrac systems). The used laser was a gallium-aluminum-arsenide semiconductor diode laser with a wavelength of 780 nm and a power of 3-5 mW. Angle of incident-to-scattered light is 180° (backscatter). This technique uses the Brownian motion of the molecules. The number-average diameters (Dn) and the polydispersity indices (DPI) were determined using the cumulant algorithm according to international standards ISO22412 and ISO13321).32,33 Size Exclusion Chromatography Size exclusion chromatography (SEC) was carried out using a Waters Alliance system equipped with a Waters 2695 separation module, a Waters 2414 refractive index detector (40 °C), a Waters 2487 dual UV absorbance detector, a PSS SDV 5 µm bead size guard column followed by two PSS SDV 5 µm bead size linearXL columns in series (300 * 8 mm) at 40°C. 10 ACS Paragon Plus Environment

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THF with 1 v/v-% acetic acid was used as eluent at a flow rate of 1.0 mL min-1. The system was calibrated with polystyrene standards (Polymer Laboratories, Mn = 580 - 7.1·106 g mol-1). Potentiometric Titrations Potentiometric titrations were conducted with a Scott Titronic Titration System with a Scott pHG201 pH glass electrode. A 0.0099 M aqueous NaOH solution was used as titrant. The precise concentration of NaOH was obtained by titrating with a sodium oxalate solution of a known concentration. The titrations were performed at room temperature, in a titration vessel filled with an exactly measured quantity (ca. 10 mL) of a sample, under continuous stirring. The titration was carried out by adding titrant in doses of 0.2-0.04 mL. A 10 s time interval was allowed between two subsequent doses to ensure that the equilibrium of the reaction was reached. For the determination of the carboxylic acid content in the water phase the latex was centrifuged at 40,000 rpm for 3 h and a part of the clear top layer was used for titration. For the determination of the carboxylic acid content at the surface of the particles the latex was diluted to a solid content of 5 wt%, and titrated with the NaOH solution; the earlier obtained amount of acid in the water phase was subtracted to obtain the amount of acid on the particles surface. The amount of carboxylic acid buried inside the particles was determined by means of a mass balance, i.e., deduced from the initial amount of carboxylic monomer introduced



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and the determined amounts of carboxylic groups on the surface of the particles and in the water phase.34-37 Zeta Potential Analysis and Critical Micelle Concentration Measurements Zeta potential measurements using Laser Doppler Micro-electrophoresis were performed on a Zetasizer Nano ZS (Malvern Instruments). An electric field was applied to a highly diluted dispersion of latex particles, which then move with a velocity related to their zeta potential. The velocity was measured using PALS (Phase Analysis Light Scattering); the used laser was a 4 mW He-Ne laser with a wavelength of 633 nm. The critical micelle concentrations (CMC) were determined from the intensity of the scattered light as a function of the concentration of the copolymer. All copolymers were dissolved in an aqueous Na2CO3 solution (4·10-2 M) at pH = 10; dilutions were made with the same Na2CO3 solution.38,39

Rheology Rheological measurements were performed at 20 °C in the auto optimization mode using the Smoluchowski model, on an Anton Paar Physica MCR 301 Rheometer with the concentric cylinder CC27 system (inner and outer diameter are 26.66 mm and 28.92 mm, respectively). For each latex first dynamic measurements were performed twice with a constant frequency of 6.3 rad/s and a strain of 0.001. Before and between measurements the latex was “stirred” at 12 ACS Paragon Plus Environment

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a rate of 100 s-1 for 100 s. Then steady state viscosity measurements were performed using different shear rates between 0.001 and 100 s-1 (for these experiments we report the viscosities after the viscosities reached a constant value). The linear viscoelastic region was determined by performing a strain sweep experiment at angular frequency of 6.3 rad/s. A strain of 0.001 was chosen because this strain was sufficiently high to effectively suppress noise and small enough to stay in the linear region (see SI).

3 3.1

RESULTS AND DISCUSSION Synthesis and Characterization of Copolymer Stabilizers

A range of amphiphilic copolymers of MMA and BA with MAA blocks were synthesized from t-BMA macromonomers produced via CCTP and characterized. A summary of these results is given in Table 2 (for details, see SI). The copolymer compositions were determined by 1H NMR. Since MAAx-MMAy is a block copolymer with a vinylic end group, the intensity of the signal of the vinyl end group could also be used to determine the (number average) molar mass (Mn). In the case of MAAx-BAy, the end groups are less well-defined and therefore Mn was estimated from SEC.



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Table 2. Characteristics of Copolymer Stabilizersa fMc

Mnd

Sample codeb

Ðe

FMf

(g/mol)

CMCg (M)

MAA12-MMA2

0.22

1.3·103

2.7

0.16

3.2·10-3

MAA12-MMA6

0.42

1.6·103

6.1

0.32

5.4·10-5

MAA12-MMA15

0.59

2.6·103

3.9

0.56

1.6·10-5

MAA4-BA6

0.54

2.3·103

1.9

0.60

3.3·10-3

MAA6-BA6

0.54

5.5·103

2.2

0.51

2.2·10-3

MAA12-BA2

0.20

2.1·103

1.3

0.19

1.1·10-3

MAA12-BA6

0.40

3.2·103

1.7

0.47

2.0·10-3

MAA12-BA14

0.60

1.3·104

2.1

0.61

3.1·10-4

a

Copolymers of MAAx-MMAy and MAAx-BAy synthesized from t-BMAx macromonomer DPnNMR = 4,

6 resp. 12.

b

The sample code reflects the average copolymer composition. For MAAx-MMAy, x =

number of MAA units and y = number of MMA units in the copolymer; for MAAx-BAy, x : y = number of MAA units : BA units in the copolymer. In both cases, x and y are estimated from FM using DPn = 4, 6 resp. 12 for the MAAx block.

c

fM = mole fraction of the comonomer MMA or BA in

the feed (wrt to all monomer units - all MAA units in the MAA-macromonomes are counted separately).

d

Number-average molar mass estimated via 1H NMR (for MAAx-MMAy, determined

from chain length of MAAx and FM) and SEC (for MAAx-BA)y. BMAx-BAy copolymers determined by SEC. copolymer, estimated from

1H

f

e

Dispersity of t-BMAx-MMAy and t-

FM = mole fraction of comonomer MMA or BA in the

NMR (see S.I.); standard error ≈ 0.04, all NMR results were obtained

after solvent suppression of D2Oδ=4.8; g At 25 °C, standard error ca. 5%.


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Critical micelle concentrations (CMC) of the copolymers were determined by DLS and from Table 2 it can be seen that, as expected, the CMC of the copolymers are all lower than that of sodium dodecyl sulfate (CMC = 9.0·10-3 M at 25°C),40,41 a commonly used surfactant in emulsion polymerization. For the MAAx-MMAy copolymers the CMC clearly decreases with increasing MMA block length (at fixed MAA block length), but for the MAAx-BAy copolymers no clear trend is observed, which may be caused by the fact that these copolymers consist of a more complex mixture of structures as discussed below. More detailed structural information about the stabilizers was obtained by MALDI analysis and the spectra for the MAAx-MMAy (specifically of MAA12-MMA15) and MAAx-BAy (specifically of MAA6-BA6) stabilizers are shown Figures 1 and 2, respectively. In order to aid the interpretation of these spectra, simplified reaction mechanisms for the two copolymerizations of the t-BMA macromonomer precursors are shown in Schemes 1 and 2, respectively. The copolymerization of the t-BMA macromonomer with MMA (Scheme 1) proceeds via the addition-fragmentation chain transfer process first described by Moad and co-workers23 and recently coined "sulfur-free RAFT" by Haddleton and coworkers.42,43 The main product of this reaction is a block copolymer with propenyl end group (pM2), with potential other products being macromonomers

pM1',

pM2'

and

pM3,

and possible

termination (by combination) products that we lumped into the same overall structure pM4. This is indeed observed in the MALDI spectra in Figure 1, where the simulated spectrum of M2 (i.e., the MAA-containing macromonomer that results from hydrolysis of the precursor 15 ACS Paragon Plus Environment


pM2)

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in Figure 1c shows excellent agreement with the experimental spectrum shown in Figure

1b. The majority of the peaks in Figure 1 can be assigned to M2. Also peaks of M2’ are observed, but no significant amounts of macromonomer M3 or termination products M4. (See S.I. for more details).

Scheme 1. Simplified overview of the reaction mechanism for copolymerization and possible products of t-BMA macromonomer and MMA, R and R’ = H or C(CH3)2CN. The labels pMX (with X = 1 - 4) are used to indicate the precursor molecules for stabilizers MX (with X = 1 - 4) in Figure 1.


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MMA AIBN y

y-1 CN

COOMe

CN

COOMe

COOMe

COOtBMA

pM3

H

x

H

COOtBMA

COOtBMA

x-1 COOtBMA

COOtBMA

pM1

H

x

MMA

H

y

COOtBMA COOMe

x

y-1

COOtBMA COOMe

COOtBMA

pM2 Other final products R' H

H

y COOMe

R

y

COOMe

COOMe

COOtBMA

x COOtBMA

pM2'

pM1'

pMX

Hydrolysis


pM4 MX

y COOMe

COOMe



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Figure 1. MALDI-ToF MS spectrum of MAA12-MMA15; (a) overview, red rectangle shows the range of the enlarged spectrum in (b); (b) enlarged spectrum for m/z = 970 — 1120 g/mol (same labeling as in Scheme 1) and (c) simulation of MAAx-MMAy macromonomer (M2) with Na+ as cationating agent, and DAC-THAP as salt-matrix combination.

The spectrum of the MAAx-BAy (Figure 2) is much more complicated than that of MAAxMMAy shown in Figure 1 and is virtually impossible to interpret without considering the mechanism for the copolymerization of t-BMA macromonomer with BA shown in Scheme 2 (again we used pBX to indicate the precursor to stabilizer BX).18,22 When a radical with a BA terminal unit reacts with an t-BMA macromonomer (pB1 in Scheme 2) then it will first undergo an addition-fragmentation step that results in a macromonomer containing a BA penultimate group (e.g., pB2, pB3 or pB5). This macromonomer with a BA penultimate group will now react as a true comonomer when a BA radical adds to its double bond and a graft is 19 ACS Paragon Plus Environment


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formed. The resulting propagating branched radicals will then stop growing by reaction with t-BMA macromonomer pB1, which results in a macromonomer pB5, or by termination leading to pB6 (all with a range of possible end groups and different degrees of branching). The MALDI spectrum in Figure 2 indeed shows peaks for both B5 and B6. (See SI for a more detailed analysis).

Scheme 2.

Simplified overview of the reaction mechanism for copolymerization and

possible products of t-BMA macromonomer and BA, R and R’ = H or C(CH3)2CN (= IBN). The labels pBX (with X = 1 - 4) are used to indicate the precursor molecules for stabilizers BX (with X = 1 - 4) in Figure 2.


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BA

+

H

H

x-1

CN

x

COOtBMA COOtBMA

COOtBMA COOtBMA

pB1 R R

x

x

y

COOtBMA COOBu

y-1

COOtBMA COOBu

=

1 2 3 4 5 6 7 8 9 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 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


COOBu

COOtBMA

pB2

in terms of mass BA

COOtBMA COOtBMA R

R v

v

z

COOBu

COOBu

COOBu

COOBu

BuOOC

BuOOC

y

y

COOtBMA

COOtBMA

x

H

x

R

Possible end products COOtBMA

COOtBMA IBN

y'

pB3

COOBu

R IBN

v

COOtBMA

v

z'

COOH

BuOOC

COOBu

z'

COOBu

COOBu

BuOOC

BuOOC

y

y

y'

COOtBMA

COOtBMA

pB4

R1

R

R

COOBu

pB5

x

H


pB6

x

H


Figure 2. (a) MALDI–ToF MS spectrum of MAA6-BA6, m/z = 650 — 2650 g/mol, used saltmatrix combination: DAC-THAP, red rectangle shows the range of the enlarged spectrum in (b); (b) Enlarged spectrum m/z = 950 — 1100 g/mol.


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3.2


Emulsion Polymerization of Methyl Methacrylate

Batch emulsion polymerizations of MMA have been performed with MAAx-MMAy and MAAx-BAy copolymeric stabilizers. For all polymerizations the kinetics were studied and the final latexes were characterized. In Figure 3 the conversion vs. time curves of the emulsion polymerizations with the different stabilizers always at 5 wt% are shown and it is clear that all display typical ab initio emulsion polymerization behavior.

Figure 3. (a) Conversion vs. time curve of the emulsion polymerization of MMA containing 5 wt% of stabilizer, other conditions as in Table 1: (a) MAAx-MMAy copolymers: () MAA12MMA2; () MAA12-MMA6 and () MAA12MMA15. (b) MAAx-BAy copolymers: () MAA4BA6; () MAA6-BA6; () MAA12-BA2; () MAA12-BA6 and () MAA12-BA14. In Figure 3a the conversion-time curves for the emulsion polymerization of MMA with MAA12-MMAy stabilizers are shown. The curves show clearly that the overall polymerization rate increases with an increasing hydrophobic block length of the stabilizer. This would 23 ACS Paragon Plus Environment


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suggest that with increasing hydrophobic block length a larger number of particles (Np) is obtained, but this is not supported by the measured particle sizes and derived particle numbers as listed in Table 3 (it should be noted here that the polymerization using MAA12MMA2 showed significant coagulation - 7% - which was removed before further analyses). We cannot discern an obvious trend in these data and we expect the polymerization behavior to be more complicated than what can be captured by simple emulsion polymerization theory. What can, however, be concluded from Figure 3a is that nucleation is fastest for the stabilizer with the largest hydrophobic block and this is conceivably explained by the fact that these stabilizers more easily form (block copolymer) micelles that act as a "seed" for the polymerization. The results for other concentrations of the stabilizers not shown graphically are summarized in Table 3 and when considering the concentration dependencies within a series of the same stabilizer molecule, some clearer trends appear.

In general (with the

exception of the lowest concentration of MAA12-MMA2), particles sizes decrease (and Np increases) with increasing stabilizer concentration, as expected for conventional surfactants. Rates per particle are relatively constant and experimental uncertainty precludes us from drawing any definite conclusions. However, it is interesting to note that the systems with the smallest particles also have the lowest rates per particle. The conversion vs time data for a range of MAAx-BAy stabilized emulsion polymerizations are shown in Figure 3b and the first thing that one notices is that all these polymerizations are significantly faster than the MAAx-MMAy stabilized polymerizations (Figure 3a) with the 24 ACS Paragon Plus Environment

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same stabilizer contents (all 5 wt% - with similar molar quantities of hydrophilic MAA units). Comparing these two systems we may conclude that this large difference in rate is caused by different particle numbers, as the obtained particle sizes in the MAAx-BAy systems are significantly smaller than those obtained in the MAAx-MMAy systems (see Table 3). This in turn suggests that the stabilizing properties of the MAAx-BAy molecules are better than those of MAAx-MMAy.44

Within this series of MAAx-BAy experiments, no clear trends are

discernable in Figure 3b and Table 3. The only result that "sticks out" are those for MAA12BA2, which is the most hydrophilic stabilizer (and it should be noted here that MAA12-BA2 is an "average composition" denoting a range of different molecules, including those with only 1 or 0 BA units). This system is significantly slower than all the others and the particle sizes are much larger, which in retrospect, is not unexpected. Rates per particle are similar for all these systems, but are all smaller than those for the MAAx-MMAy systems. When considering all the rates per particle summarized in Table 3, then the data suggest that the rate per particle increases with increasing particle size, which would be consistent with decreasing exit and termination rates.45

Table 3. Characteristics of the Synthesized Latexes Stabilizer

MAA12-MMA2

CSa

CMAAb

Dnc

dX/dtd

Npe

(mM)

(mM)

(nm)

(min-1)

(m-3)

5.2

62

162 ± 14

7·10-3

8·1019


rppf

Mng

Mpg

Ðg

(m3/min) (kg/mol (kg/mol

9·10-23

)

)

212

830

2.5


7.0

83

Page 26 of 49

177 ± 19

1·10-2

6·1019

2·10-22

65

620

9.1

6·1019

2·10-22

70

370

5.8

10*

118

162 ± 20

9·10-3

19

230

120 ± 9

5·10-3

8·1019

6·10-23

15

140

9.0

MAA12-MMA6

5.0 7.7*

60 92

157 ± 14 141 ± 25

1·10-2 1·10-2

1·1020 1·1020

1·10-22 1·10-22

111 46

420 230

4.3 6.3

MAA12-

4.9* 7.7

59 92

161 ± 15 138 ± 17

2·10-2 1·10-2

5·1019 1·1020

3·10-22 8·10-23

116 56

1070 450

6.5 9.3

5.8* 2.4*

51 61

83 ± 22 69 ± 17

3·10-2 4·10-2

7·1020 1·1021

5·10-23 3·10-23

>104 h >104 h

MAA12-BA2

6.2*

124

118 ± 9

2·10-2

2·1020

8·10-23

>104 h

MAA12-BA6

4.1*

88

84 ± 12

4·10-2

7·1020

6·10-23

>104 h

MAA12-BA14

1.0*

55

83 ± 13

3·10-2

7·1020

5·10-23

>104 h

MMA MAA415 -BA6 MAA6-BA6

Concentration of MAAx-MMAy or MAAx-BAy stabilizer (* corresponds to 5 wt% with respect to overall monomer content), determined from used mass and Mn; b Overall concentration of MAA units, determined from the used mass and the weight fraction of MAA in the stabilizer; c Numberaverage diameter and standard deviation determined by SEM, particle count > 100; d Overall conversion rate between 20 and 60% conversion; e Number of particles per m3 water; f rate per particle = (dX/dt)/Np; g Number-average and peak molar mass and dispersity determined by SEC and reported against polystyrene standards; h estimates, exclusion limit reached. a

In Table 3 also the molar masses of the produced PMMA are shown and two things clearly stand out: the PMMA latexes stabilized by MAAx-BAy are characterized by much higher molar masses than those stabilized by MAAx-MMAy, and in the latter systems, a higher stabilizer concentration leads to lower molar masses. In Figure 4, the stabilizer concentration dependence of the molar mass distributions for a range of latexes is shown, in Figure 4a for the latexes stabilized by MAA12-MMA2 and in Figure 4b for those stabilized by MAAx-BAy.


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Figure 4. Molar mass distributions of latexes made different stabilizers: (a) MAA12-MMA2 at different concentrations (Cs); (b) Different MAAx-BAy stabilizers at a concentration of 5 wt%. Data are reported against polystyrene standards.

The observed differences in molar mass distributions are easily explained by the differences in molecular structure of the used stabilizers M and B, i.e., M is representative for the MAAxMMAy series and B for the MAAx-BAy series. In the emulsion polymerization of MMA, M will act as an addition-fragmentation chain transfer agent,28 and as with any other chain transfer agent, this will lead to a molecular weight reduction. B on the other hand will largely behave as an unreactive surfactant (except for the small fraction of the molecules that still contains a propenyl endgroup, i.e., B1, B2, B3 and B5) and high molar masses, typical of emulsion polymerization are expected. The unreacted stabilizers are visible in the molar mass distribution as the low molar mass peak in Figure 4b, whereas the small "bumps" in some of the molar mass distributions around M = 106 g/mol may be indicative of some chain transfer going on (because of the presence of a small amount of molecules with propenyl endgroups).



Page 28 of 49

We conclude this section by stating that both the MAAx-MMAy and MAAx-BAy series of stabilizers are suitable for emulsion polymerization, be it that the former series in general leads to slower polymerizations, larger particles and lower molar masses than the latter series. Since the used stabilizers are not fully homogeneous in terms of molar mass and chemical composition distributions, it is difficult to identify clear trends within each series of experiments. 3.3

Latex Characterization

In the previous section we discussed the use of a range of short MAA-containing copolymers as stabilizers in the emulsion polymerization of MMA and concluded that all these copolymers in principle resulted in stable latexes. We have not, however, commented yet on the appearance of these latexes, which varied from very liquid, "milky", to very viscous, "gellike". It is well known that the rheological behavior of a latex is determined by the volume fraction and particle size of the polymer particles, and by the amount and type of stabilizer used.46 Thus, some differences in rheological behavior are expected, but we did not anticipate the differences we observed. To facilitate the further discussion, we subdivide the latex appearance into three types which are schematically shown in Figure 5. Type I is a colloidally stable, liquid-like, latex (low viscosity, containing no visible sediment/coagulum), type II is a flocculated latex which may


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contain some redispersible sediment and type III is a highly viscous latex with internal network structure (gelated).

I

II

III

Figure 5. Schematic representation of the three types of appearances of the latexes produced in this work: I) fluid-like latex; II) viscous latex with reversible flocculation; III) highly viscous, gelated latex.

In Table 4 the appearances of the final latexes with corresponding stabilizers and concentrations are listed in combination with the zeta potentials and the distributions of the stabilizer in different phases of the latex (water, surface or inside particle).

What is

immediately clear from these data is that the MAAx-MMAy-stabilized latexes (L1-L8) are in general more viscous (mainly type II) than those stabilized by MAAx-BAy (L9-L13, mainly type I). This difference is conceivably explained47,48 by the fact that in general (except for L11) the amount of water-soluble molecules of stabilizer B is lower than the amount of watersoluble molecules of stabilizer M, as shown by the titration results (i.e., the values for CMAA,w 29 ACS Paragon Plus Environment


Page 30 of 49

in Table 4). Notable exceptions are L4 (type I) and L11 (type II), but these results are not inexplicable. L4 is the latex with the highest MAA12-MMA2 concentration, which has a very high CMAA,w, but also the highest number of stabilizing groups per m2 surface area (n"MAA,s). L11 is the latex with the best water-soluble stabilizer, MAAx-BAy (which contains a large fraction of molecules with only one or even no BA groups, see SI) and therefore results in a high CMAA,w and the lowest n"MAA,s. Additionally, it is important to stress here that all the copolymer stabilizers in this study have relatively broad molar mass and chemical composition distributions, so preferential adsorption of only certain molecules is highly likely and this complicates the exact interpretation of our results.


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Table 4 Characteristics and MAA Distributions of the Synthesized Latexes CSa

CMAAb

(mM)

(mM)

L1

5.2

62

III

-56

L2

7.0

83

II

10*

118

L4

19

L5

Latex

L3

L6 L7 L8

Stabilizer

MAA12-MMA2

MAA12-MMA6

MAA12-MMA15

Typec

CMAA,we

nMAA,sf

n'MAA,sg

n"MAA,sh

nSi

MAA,pj

(mM)

(mmol)

(mmol/part)

(mmol/m2)

(1/part)

(%)

8.1

3.6

0.5

5·10-17

8·10-8

3103

87

-56

7.2

13.6

0.5

1·10-16

9·10-8

5103

78

II

-52

6.2

46.8

1.0

1·10-16

2·10-7

5103

54

230

I

-55

5.3

121.9

4.8

5·10-16

8·10-7

2104

29

5.0

59

II

-50

7.0

4.2

2.7

2·10-16

4·10-7

1104

55

7.7*

92

II

-62

7.0

16.2

1.2

7·10-17

1·10-7

4103

71

4.9*

59

II

-55

8.2

1.8

0.06

6·10-18

1·10-8

3102

96

7.7

92

I/II

-58

7.0

10.8

7.5

7·10-16

1·10-6

4103

19

pHe

d (mV)

L9

MAA4-BA6

5.8*

51

I

-45

8.0

1.5

1.0

1·10-17

6·10-8

5102

82

L10

MAA6-BA6

2.4*

61

I

-49

8.4

0.6

0.7

5·10-18

4·10-8

3102

90

L11

MAA12-BA2

6.2*

124

II

-55

6.5

22.0

1.2

7·10-18

1·10-8

4102

68

L12

MAA12-BA6

4.1*

88

I

-47

8.8

0.6

0.3

5·10-18

2·10-8

3102

96

L13

MAA12-BA14

1.0*

55

I

-43

8.1

0.1

0.09

2·10-18

6·10-9

1102

98

a

Concentration of MAAx-MMAy or MAAx-BAy stabilizer (* corresponds to 5 wt% with respect to overall monomer content),

b

Overall concentration of MAA

units, determined from the used mass and the weight fraction of MAA in the stabilizer; c Appearance of latex, see text and Figure 5 for definition; d -potential in diluted solutions, pH ≈ 7; e pH of final latex; f Concentration of MAA in the water phase determined by titration;

31

ACS Paragon Plus Environment

g

Amount of MAA on particle surface

expressed in mmoles determined by titration; surface in mmol per 1 2 3 4 5 6 7 8 9 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 36 37 38 39 40 41 42 43 44 45 46

m2; j

h

Amount of MAA units on the particle surface in mmol per particle; i Amount of MAA units on the particle Industrial & Engineering Chemistry Research

Number of stabilizer molecules on surface per particle, ignoring any preferential adsorption;

buried inside the particles.

32


Page 32 of 49

k

Percentage of added MAA units

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The data in Table 4 also show that, in this study, the measured -potential (of the highly diluted latex) is not necessarily a good reflection of colloidal stability in the latex: in general, the MAAx-BAy stabilized latexes are characterized by a somewhat smaller negative -potential, but exhibit a more fluid-like, flocculate-free, behavior.

We also

cannot observe any direct correlation between the -potential and the number of MAA units on the surface of a particle (n"MAA,s), but for most MAAx-BAy stabilized latexes this number is lower than those observed for the MAAx-MMAy stabilized latexes. What is also interesting to note is the fact that a very large fraction of the surfactant molecules is buried inside the particles (MAA,p denotes the fraction of all MAA units buried inside the particle) irrespective of the type of used stabilizer.

Although for the series of MAAx-MMAy

stabilized latexes the trend appears to be that the buried fraction (MAA,p) decreases with increasing surfactant concentration (CS or CMAA), this trend is less clear when considering the absolute number of moles that is buried (e.g., for the series L1-L4, the number of buried MAA units is relatively constant : ~ 7 mmol). With similar amounts of surfactant buried inside the particles, the observed (and expected) trends of increasing aqueous phase concentrations and adsorbed polymer on the interface with increasing surfactant concentration are then fully consistent. In order to get a more quantitative insight into the appearances and the rheological properties of the prepared latexes we measured the steady state viscosities as a function of the shear rate and probed the mechanical microstructure by measuring the dynamic moduli G' and G'' for a period of 1000 s at a fixed frequency (i.e., we performed a dynamic 33 ACS Paragon Plus Environment


Page 34 of 49

time sweep) with an amplitude small enough to not disturb any possible structures. In order to check the recoverability of the structure after such time sweep, the sample was stirred (using a shear rate of 100 s-1 for a duration of 100 s), after which another dynamic time sweep was recorded. In Figure 6, the results are shown for L2, L6 and L8, as an example for the influence of the length of the MMA block on the rheological properties.

Figure 6. Rheology of latexes stabilized with 7-8 mM MAAx-MMAy copolymers. (a) Steady state viscosity; (b) Storage modulus G' (symbols) and loss modulus G" (lines) as function of time; strain = 0.001, ω = 6.3 rad/s; at 1000 s the sample was agitated using a shear rate of 100 s-1 for 100 s. () MAA12-MMA2 (L2), () MAA12-MMA6 (L6) and () MAA12-MMA15 (L8). Values of G' ~ 0 are plotted as 10-2 Pa. In Figure 6a a comparison of the flow curves is made for latexes stabilized with 7-8 mM MAAx-MMAy copolymer with different MMA block lengths (L2, L6 and L8). All latexes show shear thinning behavior and at high shear rates the viscosities of the latexes tend to level off, suggesting that internal structures are no longer broken up by increasing shear rates.

The relatively high viscosities at which the levelling off take place, however,

suggest that large agglomerates or flocs with strong interactions still exist.46 At lower 34 ACS Paragon Plus Environment

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shear rates the viscosities increase, and approach a slope of -1 in double-log plot, indicative of plastic behavior; this means that large agglomerates stick to each other and form a soft solid with yield stress values from ~ 0.001 Pa (for L8) to ~ 0.2 Pa (for L2). In Figure 6b the results are shown of two sequential time sweeps (duration = 1000 s,  = 6.3 rad/s and strain = 0.001), separated by a period of 100 s with stirring at 100 s-1. The three latexes show all a different behavior.

The latex stabilized with MAA12-MMA2 (L2)

displays a solid like behavior (G' > G'') and the initial increase in G' is indicative of build up of some internal structure over time. Stirring breaks up this structure, but after stirring the structure is restored, albeit at a lower modulus level, which suggests that the mobility of the flocs or particles is still high. In the case of MAA12-MMA6 the latex (L6), which is more fluid-like (G" > G') also some internal structure is built up in time, and this structure is immediately restored after stirring at the same viscosity level. G'' values give a yield stress of ~ 0.1 Pa, which implies that network formation is minor. For the latex stabilized with MAA12-MMA15 (L8), which also is more fluid-like (G'' > G'), it can be seen that the internal network is destroyed after stirring after the first measurement and not built up anymore (G'  0). The results shown in Figure 6 suggest that there is a significant effect of hydrophobic block length on the final viscosity and stability of the latex. If we compare these results with the results in Table 4 for the amount of MAA on the surface (nMAA,s) we can conclude that higher amounts of MAA units on the surface of the particles give less network formation and a lower viscosity of the latex, so at a concentration of 8 mM MAA12-MMA15 appears to be a better dispersant than MAA12-MMA6 and MAA12-MMA2, 35 ACS Paragon Plus Environment


Page 36 of 49

but the relatively high viscosities at high shear suggest that still higher concentrations are required for a good colloidal stabilization.

Figure 7. Rheology of MAA12-MMA2 stabilized latexes of pMMA (a) Steady state viscosity; (b) Storage modulus G' (symbol) and loss modulus G'' (dotted line) as function of time, strain = 0.001, ω = 6.3 rad/s. Different concentrations of MAA12-MMA2 stabilizer: () 5 mM (L1), () 7 mM (L2), () 10 mM (L3), () 19 mM (L4). The effect of stabilizer concentration on latex rheology is shown in Figure 7 for the MAA12MMA2 systems (L1-L4). The results of the steady state viscosity measurements are shown in Figure 7a. The viscosity of the latex stabilized with 19 mM MAA12-MMA2 (L4) was so low that it could not be determined at low shear rates. Plastic behavior is observed for the other latexes, comparable to what is observed in Figure 6a. The high viscosities at high shear rates are indicative of large agglomerates with strong interactions, which is supported by the dynamic time sweep results shown in Figure 7b. All latexes behave like soft solids (G' > G") and the behavior of L1 to L3 is consistent with a space-filled flocculated packing where the flocs break up the bonds with their neighboring flocs when sheared, but on rest immediately restore these bonds. 36 ACS Paragon Plus Environment

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Figure 8. Rheology of latex stabilized with 5 wt% MAA-BA: (a) Steady state viscosity; (b) Storage modulus G' (symbols) and loss modulus G" (dotted lines) as a function of time; strain = 0.001, ω = 6.3 rad/s; () MAA4-BA6 (L9); () MAA6-BA6 (L10); () MAA12-BA2 (L11); () MAA12-BA6 (L12);() MAA12-BA14 (L13). Finally, in Figure 8 the results of the steady state viscosity and the results of the dynamic time sweep experiments for the latexes stabilized with MAAx-BAy copolymers are shown. The low shear rate data for the low viscous systems were left out because they were below the equipment sensitivity limit. From Figure 8a can be seen that none of these latexes show noticeable plastic behavior (slope  -1 in a double-log plot), with the exception of L13 (stabilized with MAA12-BA14). From the Newtonian plateau at around 0.005 Pa·s in the shear rate range of 0.1-100 s-1 it can be concluded that these non-plastic latexes are wellstabilized, non-aggregating dispersions. L13 clearly shows a slight plasticity with a yield value of about 0.2 Pa. We consider this as indication that the colloidal stability may be not perfect. At shear rates > 100 s-1 an increase of the viscosity is observed due to Taylor turbulences.49,50 The results of the oscillatory deformation tests shown in Figure 8b further confirm that the extent of agglomeration at rest is very limited. The elastic component of 37 ACS Paragon Plus Environment


Page 38 of 49

the dynamic modulus is for all latexes larger than the viscous component (G' > G"). The loss moduli (G'') give a value of around 0.1 Pa and in combination with a low viscosity these latexes could be suitable to be used in a coating formulation.47 When comparing the appearances of latexes stabilized with MAAx-MMAy and MAAx-BAy it can be seen that the latter are all characterized by a low viscosity and a low yield stress (types I or II), whereas the former show more flocculation, higher viscosities and higher yield stresses (types II or III). These differences in appearances and rheological behavior can partly be explained by the final distribution of the stabilizers over the different phases in the polymer latex.

Due to the more hydrophilic nature of the MAAx-MMAy

copolymeric stabilizers, more molecules may dissolve in the water phase thus increasing the viscosity of the solution. These molecules may form aggregates, be present on the surface of the polymer particles thus increasing the effective volume fraction of particles, or form links between particles and thus form an internal network which can be destroyed at higher shear rates.47,48 These effects will be less in the case of the more hydrophobic MAAx-BAy stabilizers.

4

CONCLUSIONS

Copolymers of MMA or BA with MAA macromonomers prepared via catalytic chain transfer polymerization were prepared, characterized and used as efficient stabilizers in the emulsion polymerization of MMA. The MAAx-MMAy copolymers (M) have a blocky 38 ACS Paragon Plus Environment

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structure and act as (reversible) chain transfer agents in the emulsion polymerization. The MAAx-BAy copolymers (B) have a more complex structure, which on average resembles more that of a graft copolymer, and are (in general) unreactive in the emulsion polymerization. This difference in behavior manifests itself in the polymerization kinetics (B leads to smaller particles and much faster rates), molar masses of the emulsion polymer (M leads to significantly reduced molar masses) and distribution of the stabilizer molecules over the three different locations, i.e., the water phase, the particle surface and the particle interior. In the case of M, more molecules are located in the water phase, and this may be the cause for the significant difference in appearances of the B- and Mstabilized latexes.

The former latexes are all characterized by a low viscosity and no

significant aggregation (low yield stresses), whereas the latter latexes are characterized by higher viscosities, stronger aggregation and yield stresses, which depend on block length and concentration.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xxxx 

Details related to characteristics (NMR, MALDI-ToF-MS, SEC, CMC, DLS) of synthesized macromonomers and latexes (pdf).



xxxx.pdf AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] ORCID Johan PA Heuts: 0000-0002-9505-8242 Ingeborg Schreur-Piet: 0000-0002-2079-4333 Alex M van Herk: 0000-0001-9398-5408 Jozua Laven: 0000-0002-1860-8677

Funding This work was partially funded by the Stichting Emulsion Polymerization Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS We gratefully acknowledge financial support by the Stichting Emulsion Polymerization. Electron microscopy was performed at the Center for Multiscale Electron Microscopy at Eindhoven University of Technology.


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(1) Urban, D.; Takamura, K. (Eds.); Polymer dispersions and their industrial applications; Wiley-VCH, Weinheim, 2002. (2) Elaissari, A.; Colloidal polymers: synthesis and characterization; Marcel Dekker, New York, 2003, 115. (3) Lovell, P.A.; El-Aasser, M.S. (Ed.), Emulsion polymerization and emulsion polymers, Wiley, Chichester, 1997. (4) Gurnani, P.; Sanchez-Cano, C.; Abraham, K.; Xandri-Monje, H.; Cook, A.B.; Hartlieb, M.; Lévi, F.; Dallmann, R.; Perrier, S. RAFT emulsion polymerization as a platform to generate well-deﬁned biocompatible latex nanoparticles, Macromol. Biosci. 2018, 18, 1800213. DOI: 10.1002/mabi.201800213 (5) Zhang, X.; Fu, H.; Huang, H.; Chen, H. Influence of carboxyl groups on the particle size and rheological properties of polyacrylate latices, J. Wuhan Univ. Technol. Mat. Sci. Edit. 2010, 25, 492. DOI.org/10.1007/s11595-010-0030-2. (6) Russell W.B.; Saville, D.A.; Schowater, W.R. Colloidal dispersions, Batchelor, G.K. (Ed.), Cambridge University Press, Cambridge, 1991. (7) Holmberg, K.; Jönsson, B.; Kronberg, B.; Lindman, B. Surfactants and polymers in aqueous solution, John Wiley & Sons, Ltd, 2nd Edition, 2003. (8) Aramendia, E.; Mallégol, J.; Jeynes, C.; Barandiaran, M.J.; Keddie, J.L.; Asua, J.M. Distribution of surfactants near acrylic latex film surfaces: A Comparison of Conventional and reactive surfactants (Surfmers), Langmuir, 2003, 19, 3212–3221.



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(9) Evanson, K.W.; Thorstenson,T.A.; Urban, M.W. Surface and interfacial FTIR spectroscopic studies of latexes. II. Surfactant–copolymer compatibility and mobility of surfactants. J. Appl. Polym. Sci., 1991, 42, 2297–2307. (10) Steward, A.; Hearn, J.; Wilkinson, M.C. An overview of polymer latex film formation and properties, Adv. Colloid Interface Sci., 2000, 86, 195–267. (11) Winnik, M.A. Latex film formation, Curr. Opin. Colloid & Interface Sci., 1997, 2, 192–199. (12) Vidal, F.; Guillot, J.; Guyot, A., Surfactants with transfer agent properties (transurfs) in styrene emulsion polymerization, Colloid Polym. Sci., 1995, 273, 999–1007. (13) a. Asua, J.M.; Schoonbrood, H.A.S. Reactive surfactants in heterophase polymerization, Acta Polym., 1998, 49, 671–686; b. Dimonie, V.L., Sudol, E.D., El-Aasser, M.S. Role of Surfactants in Emulsion Polymerization Polymers by design, Rev. Chim. (Bucureºti), 2008, 59 (11), 1218-1221. (14) Guyot, A.; Tauer, K.; Asua, J.M., Van Es, S.; Gauthier, C.; Hellgren, A.C.; Sherrington, D.C; Montoya-Goni, A.; Sjoberg, M.; Sindt, O.; Vidal, F.; Unzue, M.; Schoonbrood, H.; Shipper, E.; Lacroix-Desmazes, P. Reactive surfactants in heterophase polymerization, Acta Polym., 1999, 50, 57-66. (15) Monteiro, M.J.; Bussels, R.; Wilkinson, T.S. Emulsion polymerization of methyl methacrylate in the presence of a novel addition-fragmentation chain-transfer reactive surfactant (transurf), J. Polym. Sci., Polym. Chem., 2001, 39, 2813-2820. (16) Guyot, A. Advances in reactive surfactants, Adv. Colloid Interface Sci., 2004, 108-109, 3-22.


Page 43 of 49 1 2 3 4 5 6 7 8 9 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 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(17) Gregory, J. Polymer adsorption and flocculation in sheared suspensions, Colloids Surfaces, 1988, 31, 231–253. (18) Heuts, J.P.A.; Smeets, N.M.B. Catalytic chain transfer and its derived macromonomers, Polym. Chem., 2011, 2, 2407. (19) Gridnev, A.A.; Ittel, S.D. Catalytic chain transfer in free-radical polymerizations, Chem. Rev., 2001, 101, 3611–3660. (20)

Slavin, S.; McEwan, K.; Haddleton, D.M. Cobalt-Catalyzed Chain Transfer Polymerization: A Review, Polym. Sci. Compr. Rev., 2012, 3, Ch. 9

(21)

Haddleton, D.M.; Maloney, D.R.; Suddaby Adam Clarke, K.G.; Richards, S.N. Radical-addition fragmentation and co-polymerization of methyl methacrylate macromonomers from catalytic chain transfer polymerization (CCTP), Polymer, 1997, 38, 6207–6217.

(22) Yamada, B.; Oku, F.; Harada, T. Substituted propenyl end groups as reactive intermediates in radical polymerization, J. Polym. Sci., Polym. Chem., 2003, 41, 645–654. (23) Krstina, J.; Moad, C.L.; Moad, G.; Rizzardo, E.; Berge, C.T.; Fryd, M. A new form of controlled growth free radical polymerization, Macromol. Symp., 1996, 111, 13–23. (24) Schreur-Piet, I.; Heuts, J.P.A. In situ stabilizer formation from methacrylic acid macromonomers in emulsion polymerization, Polym. Chem., 2017, 8, 6654-6664. (25) Canning, S.L.; Smith G.N. ; Armes, S.P. A critical appraisal of RAFT-mediated polymerization induced self-assembly, Macromolecules, 2016, 49, 1985-2001.



Page 44 of 49

(26) Charleux, B. Delaittre, G.; Rieger J.; D'Agosto, F. Polymerization-induced selfassembly: from soluble macromolecules to block copolymer nano-objects in one step, Macromolecules, 2012, 45, 6753-6765. (27) Lotierzo, A.; Schofield, R.M.; Bon, S. Towards sulfur-free RAFT polymerization induced self-assembly, ACS Macro Lett., 2017, 6, 1438-1443. (28) Zhou, D.; Kuchel, R.P.; Zetterlund, P.B. A new paradigm in polymerization induced self-assembly (PISA): exploitation of "non-living" addition-fragmentation chain transfer (AFCT) polymerization, Polym. Chem., 2017, 8, 4177. (29) Bakač, A.; Brynildson, M.E.; Espenson, J.H. Characterization of the structure, properties, and reactivity of a cobalt(II) macrocyclic complex, Inorg. Chem., 1986, 25, 4108–4114.; DOI: 10.1021/ic00243a012. (30) Staal, B.B.P. Characterization of (co)polymers by MALDI-TOF-MS, Thesis, University of Technology Eindhoven, 2005; DOI: 10.6100/IR583506. (31) Willemse, R.X.E., New insights into free-radical (co)polymerization kinetics, Thesis, University of Technology Eindhoven, 2005. (32) International Standard ISO22412, Particle size analysis - dynamic light scattering, International Organisation for Standardisation (ISO); 2008. (33) International Standard ISO13321, Methods for determination of particle size distribution part 8: photon correlation spectroscopy, International Organisation for Standardisation (ISO), 1996.


Page 45 of 49 1 2 3 4 5 6 7 8 9 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 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(34) Zwetsloot, J.P.H.; Leyte, J.C. Conductometric titration of latex with acidic surface groups, J. Colloid. Interface Sci., 1994, 163, 362–371. (35) Wang, C.; Tam, K.C.; Jenkins, R.D.; Bassett, D.R. Potentiometric titration and dynamic light scattering of hydrophobically modified alkali soluble emulsion (HASE) polymer solutions, Phys. Chem. Chem. Phys., 2000, 2, 1967–1972. (36) Oliveira, M.; Giordani, D.; Santos, A. The role of itaconic and fumaric acid in the emulsion copolymerization of methyl methacrylate and n-butyl acrylate, Eur. Pol. J., 2006, 42, 5, 1196-1205. (37) Kawaguchi, S.; Yekta, A.; Winnik, M.A. Surface characterization and dissociation properties of carboxylic acid core–shell latex particle by potentiometric and conductometric titration, J. Colloid Interface Sci., 1995, 176, 362–369. (38) Topel, O.; Çakır, B.A.; Budama, L.; Hoda, N. Determination of critical micelle concentration of polybutadiene-block-poly(ethyleneoxide) diblock copolymer by fluorescence spectroscopy and dynamic light scattering, J. Molecular Liq., 2013, 177, 40-43, DOI.org /10.1016/j.molliq.2012.10.013. (39) Gaillard, N.; Claverie, J.; Guyot, A. Synthesis and characterization of blockcopolymer surfactants with specific interactions with associative thickeners, Prog. Org. Coatings, 2006, 57, 98–109. (40) Riess, G.; Labbe, C. Block copolymers in emulsion and dispersion polymerization, Macromol. Rapid Commun., 2004, 25, 401–435, DOI. 10.1002/marc.200300048.



Page 46 of 49

(41) Berthod, A.; Garcia-Alvarez-Coque, C. Micellar liquid chromatography, 83, Marcel Dekker Inc., New York, 2000, 33-35. (42) Nurumbetov, G.; Enelis, N.; Godfrey, J.; Hand, R.; Anastasaki, A.; Simula, A.; Nikolaou, V.; Haddleton, D.M. Methacrylic block copolymers by sulfur free RAFT (SF RAFT) free radical emulsion polymerisation, Polym. Chem., 2017, 8, 1084-1094. (43) Engelis, G.; Anastasaki, A.; Nurumbetov, G.; Truong, N.P.; Nikolaou, V.; Shegiwal, A.; Whittaker, M.R.; Davis, T.P.; Haddleton, D.M. Sequence-controlled methacrylic multiblock copolymers via sulfur-free RAFT emulsion polymerization, Nature Chemistry, 2017, 9, 171-178. (44) Müller, H.; Leube, W.; Tauer, K.; Förster, S.; Antonietti, M. Polyelectrolyte block copolymers as effective stabilizers in emulsion polymerization, Macromolecules, 1997, 30 (8), 2288–2293, DOI: 10.1021/ma9615516 (45) Thickett, S.C.; Gilbert, R.G., Emulsion polymerization: State of the art in kinetics and mechanisms, Polymer, 2007, 48, 6965-6991, DOI:10.1016/j.polymer.2007.09.031 (46) Fitch, R.M. (Ed.) Polymer colloids: a comprehensive introduction, Acadamic Press, London, 1997, Ch. 7, 10. (47) Koleske, J.V. (Ed.) Paint and coatings testing manual, 15th Edition, ASTM Manual Series, Bridgeport, 2012, Ch. 33. (48) Mewis, J.; Wagner, N.; Colloidal suspension rheology (Cambridge Series in Chemical Engineering), Cambridge, Cambridge University Press, 2011. Ch. 1, DOI:10.1017/CBO9780511977978.004.


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(49) Davey, A.; Di Prima, R.C.; Stuart, J.T. On the instability of Taylor vortices, J. Fluid Mech., 1968, 31, 17–52. (50) Taylor, G.I., Stability of a viscous liquid contained between two rotating cylinders, Philos, Trans R. Soc., London, Ser. A, 1923, 223, 289–343.


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Graphical Abstract


Synthesis and Rheological Characterization of Latexes Stabilized by

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