Dispersion Polymerization of Acrylate Monomers in Supercritical $CO_2$ using GMA-functionalized Reactive Surfactant

Dispersion Polymerization of Acrylate Monomers in Supercritical CO 2 using GMA-functionalized Reactive Surfactant

Kyung-Kyu Park, Chang-Min Kang*, and Sang-Ho Lee

Department of Chemical Engineering, Dong-A University, Hadan2-dong, Saha-gu, Busan, Korea

*Department of Nano Engineering, Dong-A University, Hadan2-dong, Saha-gu, Busan, Korea (Received July 20, 2010, Revised August 20, 2010, October 4, Accepted October 18, 2010)

초임계 이산화탄소에서 Glycidyl methacrylate 반응성 계면활성제를 이용한 아크릴레이트의 분산중합

박경규 ․ 강창민^*

․ 이상호

동아대학교 화학공학과,

동아대학교 나노공학과

접수일(2010년 7월 20일), 수정일(1차 : 2010년 8월 20일, 2차 : 10월 4일), 게재확정일(2010년 10월 18일)

ABSTRACT：Dispersion polymerization of methyl acrylate, ethyl acrylate, butyl acrylate, and glycidyl methacrylate were performed in supercritical CO

at 80 ºC and 346 bar. Glycidyl methacrylate linked poly(dimethylsiloxane) (GMS-PDMS) surfactant, which was prepared by linking glycidyl methacrylate to monoglycidyl ether terminated PDMS with amino- propyltriethoxysilane, was used as surfactant for the dispersion polymerization in CO

. The yield of the poly(alkyl acrylate) polymers, synthesized in CO

medium, decreased as the alkyl tail of the acrylate monomers increased. Poly(glycidyl meth- acrylate) and poly(methyl acrylate) were produced in bead form whereas poly(ethyl acrylate) and poly(butyl acrylate) were viscous liquid. The poly(glycidyl methacrylate) particles had a number average diameter of 2.45 μm and monodisperse distribution. The poly(methyl acrylate) had a number average diameter of 0.52 μm and the particle size distribution was bimodal. The glass transition temperatures (T

) of the poly(glycidyl methacrylate) and the poly(alkyl acrylate) products were 4~9 K higher than the T

of the corresponding acrylate polymers synthesized in conventional processes.

요 약：80 ℃, 346 bar 상태의 초임계 이산화탄소 내에서 methyl acrylate, ethyl acrylate, butyl acrylate와 gly- cidyl methacrylate을 중합하였다. 초임계 이산화탄소 분산매에서의 중합을 위하여 aminopropyltriethoxysilane을 사용하여 glycidyl methacrylate를 monoglycidyl ether terminated PDMS에 결합시킨 glycidyl methacrylate linked poly(dimethylsiloxane)(GMS-PDMS)를 계면활성제로 사용하였다. CO

에서 합성된 Poly(alkyl acrylate) 의 수율은 acrylate 단량체의 알킬기가 커질수록 낮아졌다. Poly(glycidyl methacrylate)와 poly(methyl acrylate) 는 구형으로 만들어진 반면, poly(ethyl acrylate)와 poly(butyl acrylate)는 점성의 액상으로 합성되었다.

Poly(glycidyl methacrylate) 입자의 수평균직경은 2.45 μm이며 입자 직경의 분포는 매우 좁았다. poly(methyl acrylate)의 수평균직경은 0.52 μm이며 입자크기는 bimodal로 분포되었다. 초임계 이산화탄소에서 중합된 poly(glycidyl methacrylate)와 poly(alkyl acrylate)들의 유리전이온도는 통상의 방법으로 중합된 poly(glycidyl methacrylate)와 poly(alkyl acrylate)의 유리전이온도보다 4~9 K 높게 측정되었다.

Keywords：Dispersion Polymerization, Supercritical CO

, Macromonomer, Functionalized Reactive Surfactant

†

Corresponding Author. E-mail: [email protected]

Ⅰ. Introduction

Dispersion polymerization in carbon dioxide has some ad- vantages over conventional solution and emulsion polymer- ization process. The dispersion medium, carbon dioxide, is easily separated from the produced polymers in carbon dioxide by simply lowering the pressure of reactor after polymeri- zation. In addition, since carbon dioxide is basically not toxic

to human, polymerization in carbon dioxide instead of organic solvent medium is environmental benign. To make high molec- ular weight polymer at high yield, it is desirable that the mix- ture of monomer and carbon dioxide is homogenous during the polymerization. However, most monomers and polymers are not soluble in carbon dioxide at moderate conditions. High pressures are applied to the reactant mixture to enhance the solubility of monomer in carbon dioxide.

Similar as a conventional dispersion polymerization, surfac-

tant is used to make growing oligomers or polymer particles

PDMS O

C C H

C

O H

H H Si

OEt EtO

OEt

(CH

)

N H H

Si OEt EtO

OEt

(CH

)

N H

CH

C H

OH

CH

O PDMS

O H

C C

CH

C O CH

C O

CH

CH

CH

C H

C

C O

O

CH

CH

CH

HO

N C

H

OH

CH

O PDMS (H

C)

Si EtO

OEt OEt

OCH

CH

^Si = PDMS

CH

CH

O Si O

CH

CH

Si CH

CH

CH

(CH

)

+

Aminopropyltriethoxysilane Monoglycidylether terminated PDMS 1st step

80°C 13 hours

2nd step 85°C 13 hours

= OEt

adding

Glycidylmethacrylate

n n = 64

Figure 1. The molecular structure and synthesizing procedures of GMA-PDMS reactive surfactant.

stable in carbon dioxide medium. The surfactants used for dis-

persion polymerization in carbon dioxide have CO

-philic and organic-philic part.

The CO

-philic parts consist of fluoro or siloxane repeat units in their backbone or pending structures.

The organic-philic parts are compatible to the polymer synthe-

sized in carbon dioxide. For instance, DeSimone et al. used

poly(1,1-dihydroperfluorooctyl acrylate) as a surfactant for the

dispersion polymerization of MMA in CO

.

Lepilleur and

Beckman used block copolymers containing CO

-philic fluoro

repeat units as surfactant for dispersion polymerization of

MMA.

Howdle et al. demonstrated the polymerization of methylmethacrylate using an acid-terminated perfluoropoly- ether surfactant (Krytox 157FSL, DuPont)

and the surfactants grafted with fluorinated groups and containing carboxylic groups.

Due to the high price of CO

-philic fluoro materials, at- tempts replacing fluoro component by siloxane has been per- formed. Shaffer et al.

synthesized PMMA and polystyrene using poly(dimethylsiloxane) monomethacrylate as a surfactant in supercritical CO

. Because Poly(dimethylsiloxane) mono- methacrylate surfactant has not only CO

-philic parts but also functional vinyl group to react various monomers, the surfac- tant makes dispersion polymerization highly stable, as the con- sequence, high molar mass of polymer was obtained at high yield. Other researchers also conducted dispersion polymer- ization in CO

with PDMS monomethacrylate.

PDMS mac- romonomers having functional group different from acrylate, such as monocarbinol-terminated PDMS,

PDMS-g-pyrroli- done carboxylic acid,

methacryloxypropyl-terminated PDMS,

and vinyldimethylsiloxy PDMS,

were also used to polymer- ize MMA and styrene in CO

.

We performed dispersion polymerization of three alkyl acrylate and glycidyl methacrylate monomers in carbon diox- ide using PDMS macromonomer surfactant prepared by our own method. The alkyl acrylate monomers are methyl-, ethyl-, and butylacrylate, so the length of pending tail group of the produced acrylate polymers is gradually longer. We will dem- onstrate the effect of the tail length of the acrylate monomers on the yield and morphology of the polyacrylate and poly(gly- cidyl methacrylate) synthesized in CO

. The PDMS macro- monomer surfactant was prepared by linking glycidyl meth- acrylate and monoglycidyl ether terminated PDMS with ami- nopropyltriethoxysilane as a coupling agent. The prepared gly- cidyl methacrylate linked poly(dimethylsiloxane) (GMA-PDMS) has a vinyl group and CO

-philic siloxane group. Figure 1 shows the procedures synthesizing GMA-PDMS reactive sur- factant and its molecular structure. The details to synthesize and characterize GMA-PDMS were described in our previous paper.

GMA-PDMS macromonomer surfactant reacted with the acrylate monomers during polymerization and, as the re- sults, enhanced the stability of growing acrylate polymers in CO

. However, a small amount of the macromonomer was incorporated into the produced polymer and could influence on the properties of the produced polymers. We present the physical properties of the synthesized acrylate polymers, such as molecular weight, molecular weight distribution, and glass transition temperature. The yield and morphology of the acryl- ate polymers, including average particle size and particle size distribution, are also demonstrated.

Ⅱ . Experimental

1. Materials

Methylacrylate (99%), ethylacrylate (99%), and butylacry- late (99+%) were obtained from Aldrich Co. Glycidyl meth- acrylate (99+%) was obtained from Junsei Chemical Co. All acrylate monomers were purified by vacuum distillation and stored at appropriate condition prior to use. 2,2'-Azobisisobu- tyronitrile (AIBN) (Otsuka Chemical Co. 99%+) was re- crystalized twice from methanol. Hexane (Aldrich, HPLC grade), Heptane (Aldrich, HPLC grade) were used as received.

Carbon dioxide, with purity of 99.99%, was supplied by Korea Standard Gas. The number average molecular weight ( ^ 

) of the GMA-PDMS surfactant measured by titration method and GPC were 5365.45 and 5800, respectively.

2. Polymerization in carbon dioxide

Dispersion polymerization of the acrylate monomer was car- ried out in a high-pressure, variable-volume cell, which has a working volume of ~28 cm

. The details of the polymer- ization devices and procedures are described in other refe- rences.

AIBN initiator was loaded in the high-pressure cell, then the entrapped air in the cell was purged out with CO

at 2~3 bar and room temperature. The mixture of acrylate mon- omer and GMA-PDMS surfactant was injected into the cell using a syringe. Balanced amount of CO

was gravimetrically transferred from a high-pressure cylinder into the cell. The pressure of the reactant mixture was manipulated to the desired pressure by moving a piston located in the cell. The dispersion polymerization was carried out for 4 hours at 80 ℃ and 346 bar. After polymerization, the cell was rapidly cooled in dry ice bath. The CO

in the cell slowly vented to atmospheric pressure. The acrylate polymer was washed with hex- ane/heptane solution to remove unreacted GMA-PDMS surfac- tant and then was dried for further characterization.

3. Characterization

The molecular weight was determined using gel permeation chromatography (Waters 150-C) equipped with a WATERS Styragel 3.8 x 300 mm Column (HR2X1, HR3X1, HR4X1) and a differential refractive index detector from Precision Detector Inc. Tetrahydrofuran was used as the mobile phase.

Polystyrene standards were used to obtain calibration curve.

Both the sample analysis and the calibration were conducted

at a flow rate of 1 mL/min. The morphology of the synthesized

acrylate polymers was characterized with scanning electron

microscopy (SEM) (HITACHI, S-2400). The average diameter

of the acrylate polymer particles and their particle size dis-

Table 1. The Concentrations of AIBN, Acrylate Monomer, and GMA-PDMS Surfactant Used for Dispersion Polymerization in CO

Monomer Monomer

wt%

AIBN wt%

*Surfactant

wt% **  

***  

Yield (%)

Glycidyl

methacrylate 29.6 0.53 5.12 17,060 51,820 91.6

Methyl

acrylate 28.0 0.50 5.19 45,890 140,580 85.6

Ethyl

acrylate 28.9 0.51 5.15 53,910 117,880 80.2

Butyl

acrylate 29.8 0.50 4.90 54,760 265,980 65.8

* GMA-PDMS surfactant. The surfactant wt% based on the acrylate monomer weight in the reactor. **



and ***



represent a number average and a weight average molecular weight, respectively.

tribution were calculated from around one hundred individual particles shown in each SEM micrograph of the acrylate polymers. The glass transition temperature of the acrylate poly- mers was measured with DSC (TA Instruments, model Q10).

Ⅲ. Results and Discussion

The concentrations of acrylate monomers in the reactant me- dium, composed of AIBN, acrylate monomer, GMA-PDMS surfactant, and carbon dioxide, were 28 ~ 30 wt%. Typical concentration of AIBN initiator was 0.5 wt%. The detail com- positions of the reactant mixtures are listed in Table 1. After the alkyl acrylate monomer were polymerized for 4 hours at 80 ºC and 346 bar of CO

, the produced poly(alkyl acrylate) had the ^ 

between 45,900 and 54,800, whereas the ^ 

of the poly(glycidyl methacrylate) was 17,100.

The poly(methyl acrylate) was obtained at the yield of 85.6%. As the size of the acrylate homologues increased from methyl to butyl acrylate, the yield of the acrylate polymers decreased from 85.6 to 65.8 %. The yield of poly(glycidyl methacrylate) was 91.6%, which was the highest yield for four acrylate polymer products. Since the molecular size of glycidyl methacrylate is biggest among the four acrylate monomer, the size of the acrylate monomer is not supposed the main reason to lower the yield of polyacrylate polymerized in CO

.

To obtain high yield of the acrylate polymer during the dis- persion polymerization in CO

, it is desirable that the acrylate monomer, the oligomers, and the growing acrylate polymer particles are well dissolved in the dispersion medium without phase separation. The dispersion medium, CO

, has quadruple moment. The polar interactions between CO

and the growing acrylate polymer are favorable to dissolve the growing polymer into CO

.

It was reported that at temperatures lower than 100 ºC, the

demixing pressure of poly(alkyl acrylate) in CO

greatly in- creased as the alkyl tail of alkyl acrylate monomer increased from methyl to butyl, since the effective polarity of the acrylate decreased with increasing the alkyl tail.

In other words, the solubility of poly(alkyl acrylate) decreased as the alkyl tail of the alkyl acrylate repeat unit increased at temperatures lower than 100 ºC, where the polar interactions between CO

and poly(alkyl acrylate) governed the solubility. At 80 ºC and 346 bar, it is supposed that the solubility of the growing poly(alkyl acrylate) polymer decreased as the alkyl tail of the alkyl acryl- ate monomer increased from methyl to butyl group. The grow- ing poly(ethyl acrylate) more fell out of CO

dispersion me- dium and precipitated than the poly(methyl acrylate) that is the most polar poly(alkyl acrylate). As the consequence, the yield of the poly(ethyl acrylate) was lower than the poly(meth- yl acrylate).

Whereas alkyl acrylate monomers have one polar ester group, glycidyl methacrylate has two polar groups, ester and epoxide group. Due to the two polar groups, glycidyl meth- acrylate has a large effective polarity, and the polar interactions between CO

and growing glycidyl methacrylate polymer are stronger than the interactions between CO

and growing alkyl acrylate polymers. Thermodynamic interpretation suggests that at the reaction condition, the growing glycidyl methacrylate polymer is more soluble in CO

than the growing alkyl acrylate polymers. Therefore, more stable dispersion polymerization was performed with glycidyl methacrylate than alkyl acrylate, and the higher yield was obtained for the dispersion polymer- ization of glycidyl methacrylate.

Figure 2 is SEM images showing the morphology of the

synthesized poly(glycidyl methacrylate) and poly(methyl acry-

late) product. The poly(ethyl acrylate) and the poly(butyl acryl-

ate) were obtained as viscous liquid. The less soluble the grow-

ing polyacrylate is in CO

, the more it falls out of the dis-

A (x 2000) A (x 5000)

B (x 2000) B (x 10000)

Figure 2. SEM images of poly(glycidyl methacrylate) (A) and poly(methyl acrylate) (B) synthesized at 80 ºC and 346 bar of CO

. The detail compositions of reactant mixtures are presented in Table 1.

persion medium. The precipitated polyacrylate makes the dis- persion medium unstable. If the precipitation is severe, the polymerization is performed at phase separation state. At 80 ºC and 346 bar, the growing poly(ethyl acrylate) and poly(bu- tyl acrylate) are not soluble in CO

, so that the polymerization occurs in unstable dispersion medium. We expect that at 80 ºC the growing poly(glycidyl methacrylate) and poly(methyl acrylate), which their acrylate repeat units have larger effective polarity than ethyl- and butyl acrylate, were so soluble that the polymerization occurred in stable dispersion and the acryl- ate products were obtained as a particle. While the particle size of the poly(glycidyl methacrylate) was regular, the particle size of the poly(methylacrylate) showed bimodal distribution.

Using the SEM images in Figure 2, we determined a number average particle diameter ( ^ 

), a weight average particle diam- eter ( ^ 

), and a particle size distribution (PSD). PSD is de- fined as the ratio of weight average particle diameter to number average particle diameter ( ^ 

/ ^ 

), which is used to figure out how the particle size is distributed. Table 2 lists the appear-

ance of the acrylate products. The ^ 

, ^ 

, and PSD of the poly(glycidyl methacrylate) and poly(methyl acrylate) are also listed in Table 2.

The ^ 

and PSD of the poly(glycidyl methacrylate) product are 2.45 μm and 1.1, respectively. The low PSD value, 1.1, represents that the poly(glycidyl methacrylate) product has reg- ular and narrow particle size distribution. The ^ 

of the poly(methyl acrylate) product is 0.52 μm, which is much smaller than that of the poly(glycidyl methacrylate). However, the SEM image of the poly(methyl acrylate) product showed that the poly(methyl acrylate) particles size is bimodal.

Figure 3, which was plotted with the SEM image (x 10,000)

of Figure 2B, demonstrates that the particle size distribution

of the poly(methyl acrylate) is bimodal. The diameter of the

poly(methyl acrylate) particles was apparently divided into

smaller and larger than 0.62 μm. The ^ 

and PSD of the

small poly(methyl acrylate) particles are 0.31 μm and 1.7,

respectively, whereas those of the large poly(methyl acrylate)

particles are 1.8 μm and 1.4. The ^{ }

of the large particles

Table 2. The Particle Size, Particle Size Distribution, Appearance, and Glass Transition Temperatures of the Poly(glycidyl meth- acrylate) and the Poly(alkyl acrylate) Products

Product

 

(µm)

 

(µm)

PSD Appearance

Poly(glycidyl methacrylate) 2.45 2.73 1.1 powder

Poly(methyl acrylate) 0.52 2.54 4.9 powder

Poly(ethyl acrylate) - - - viscous liquid

Poly(butyl acrylate) - - - viscous liquid

a 



and



represent a number average and a weight average particle diameter, respectively.

PSD represents particle size distribution (



/



).

0 5 10 15 20 25 30

Nu mb er o f Partic le

Diameter of particle (μm) 0.5 1.0

0.0 1.5 2.0 2.5 3.0 3.5

D

= 0.31 μm PSD = 1.7

D

= 1.8 μm PSD = 1.4

Figure 3. The particle size distribution of the poly(methyl acrylate) synthesized in CO

with GMS-PDMS surfactant.

Table 3. The Glass Transition Temperatures of the Poly(gly- cidyl methacrylate) and the Poly(alkyl acrylate) Synthesized with GMS-PDMS Surfactant

Polymer

Glass transition temperature (K) Our method Conventional

methods Poly(glycidyl methacrylate) 352 347

Poly(methyl acrylate) 292 283

Poly(ethyl acrylate) 257 249

Poly(butyl acrylate) 228 224

were six times as large as that of smaller particles. However, the PSD values of both small and large particles of the poly(methyl acrylate) are lower than 2, representing that al- though the particle size has bimodal distribution, each portion of the bimodal is relatively narrow distribution.

Table 3 lists the glass transition temperatures (T

) of poly (glycidyl methacrylate) and the poly(alkyl acrylate) synthe- sized with our GMS-PDMS surfactant in supercritical CO

.

The T

of the acrylate polymers synthesized in conventional methods was also listed in Table 3. Since GMS-PDMS surfac- tant is capable and expected to react with the acrylate mono- mers, the poly(glycidyl methacrylate) and poly(alkyl acrylate) synthesized with GMS-PDMS surfactant are copolymers that contain small amount of GMS-PDMS less than 5 wt%. The T

of PDMS is 150 K.

Therefore, if the effect of incorporat- ing GMS-PDMS into the poly(alkyl acrylate) is manifested, it is supposed that the T

decreases. Interestingly, the T

of the poly(alkyl acrylate) products synthesized with GMS-PDMS surfactant were 4 ~ 9 K higher than T

of the acrylate polymers synthesized in conventional methods. At this moment, we do not have a clue what increases the T

of the acrylate polymer products.

Ⅳ. Conclusions

We demonstrated the synthesis of poly(methyl acrylate), poly(ethyl acrylate), poly(butyl acrylate), and poly(glycidyl methacrylate) in supercritical CO

with GMA-PDMS function- alized reactive surfactant. Poly(methyl acrylate) was produced at 85.6 % yield as a bead form. The yield of poly(ethyl acryl- ate) and poly(butyl acrylate), obtained in viscous liquid, was lower than 81 %. The poly(glycidyl methacrylate), which was produced in bead form and it's particle size distribution was fairly monodisperse, was obtained at the highest yield of 91.6

%.

The T

of the acrylate polymers synthesized with GMA- PDMS surfactant were 4 ~ 9 K higher than the T

of the corre- sponding acrylate polymers synthesized in conventional pro- cesses. Since T

of PDMS is 150 K, GMA-PDMS surfactant is expected to lower T

of the acrylate polymer products. More work needs to figure out the reason increasing the T

.

Acknowledgements

This research was financially supported by the Ministry of

Education, Science Technology (MEST) and Korea Institute

for Advancement of Technology (KIAT) through the Human

Resource Training Project for Regional Innovation.

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