Atom Transfer Radical Polymerization of
Styrene and Methyl Methacrylate from Mesoporous Ordered Silica Particles a
Maud Save,*1Gwenae¨lle Granvorka,1Julien Bernard,1Bernadette Charleux,*1Ce´dric Boissie`re,2
David Grosso,2Cle´ment Sanchez2
1Laboratoire de Chimie des Polyme`res,UMR7610-CNRS,Universite´Pierre et Marie Curie,Tour44,
1er e´tage,4place Jussieu,75252Paris Cedex05,France
Fax:(þ33)144277089;E-mail:save@ccr.jussieu.fr;charleux@ccr.jussieu.fr
2Laboratoire Chimie de la Matie`re Condense´e,CNRS UMR7574,Universite´Pierre et Marie Curie,4place Jussieu,
75252Paris Cedex05,France
Fax:þ33(0)144274769
耙式浓缩机
Received:November22,2005;Revised:January6,2006;Accepted:January9,2006;DOI:10.1002/marc.200500798 Keywords:atom transfer radical polymerization(ATRP);controlled radical polymerization;grafting from;MSU mesoporous silica;surface-initiated polymerization
Introduction
Ordered mesoporous hybrid materials have attracted much attention due to their numerous potential applications.[1] Mesoporous materials,with pores ordered in a regular ar-rangement and pore diameters ranging between2and50nm, exhibit high specific surface area(up to1300m2ÁgÀ1)and large total pore volume(up to ca.1.5cm3ÁgÀ1),which make them good candidates for sensors or catalyst supports.[2]The combination of mesoporous inorganic materials of high performance and polymer properties is of great interest.Many research teams have studied the polymerization of monomers into mesoporous silica using different strategies such as the free radical polymerization of a monomer-impregnated silica[3]or the insertion-coordination polymerization of specific monomers using a supported catalyst.[4]By this way,the polymer is not covalently linked to the inorganic materials,allowing for instance the synthesis of perfect replications of the
Summary:Mesoporous silica was used as substrate for the grafting of alkyl halides initiators.The cont
rol over the surface-initiated polymerization of styrene and MMA,in terms of molar mass and molar mass distribution,was successfully achieved using an ATRP mechanism.The occurrence of the polymerization inside the mesopores was confirmed by thermogravimetric
analysis.
Transmission electron microscopy and schematic represen-
tation of mesoporous silica functionalized by the anchored
iniator(left)and the grafted polymer (right).
Communication DOI:10.1002/marc.200500798393
a:Supporting information for this article is available at the
bottom of the article’s abstract page,which can be accessed
from the journal’s homepage at -journal.de,or
from the author.
inorganic mold such as nanocrystalline polyimide[5]or ordered mesoporous carbon for instance.[6]For other appli-cations,a stronger anchoring of the polymer is required and, in that sense,different methodologies were used to create a covalent link between organic and inorganic moieties at the nanometer scale.Afirst route consists in creating an intimate coupling between the inorganic molecular build-ing blocks and a reactive vinylic monomer containing an organosilane function,able to co-condense during the hybrid network formation in the presence of the templating surfactant.[7]Another interesting approach deals with the grafting of a functional initiator containing chloro-or alkoxy-silane groups.Controlled free radical polymeriza-tion(CRP)methods,[8]based on reversible termination [nitroxide-mediated polymerization(NMP),[8b]atom trans-fer radical polymerization(ATRP)[8c]]or reversible transfer (RAFT methodology,[8d]iodine transfer[8e]),have known a growing interest due to their ability to provide well-defined (co)polymers with controlled molar masses,low polydis-persity indexes,and controlled chain-end functionalities without recourse t
o drastic purification of chemicals.The interest of controlled polymerization for the synthesis of polymer brushes lies in the simultaneous growth of the polymer chains from the surface ensuring a high grafting density.[9]ATRP has been widely used for the synthesis of polymer brushes[10]by surface-initiated polymerization of various monomers such as(meth)acrylates,methacryla-mides,and styrenics from either colloidal[11]orflat silica surfaces[12]or porous aluminum oxide membrane.[13]As far as we are aware,only two examples reported the use of a controlled radical polymerization technique for the surface-initiated polymerization of vinylic monomers from meso-porous silica.[14]Thermoresponsive poly(N-isopropyl-acrylamide)(PNiPAM)[14a]and polyacrylonitrile,a carbonization precursor for the synthesis of mesoporous carbons,[14b]were grown from the mesoporous silica by ATRP.However,from a macromolecular chemistry view-point,neither molar mass nor kinetic data were given in these studies,[14]and hence no clear proof of any control over the polymerization occurring inside the pores was provided.The goal of the present work is to describe the synthesis of mesoporous hybrid materials via‘‘grafting from’’ATRP of styrene(S)and methyl methacrylate (MMA)from the overall surface of the ordered mesoporous silica.This work emphasizes for thefirst time the control of free radical polymerization in such a confined medium,and details concerning molar mass,molar mass distribution,and grafted initiator efficiency are described.The control over polymerization is essential to tune thefilling of the mesopores through the
volume occupied by the polymer, closely linked to chain length and grafting density.In order to facilitate the understanding of our preliminary mechan-istic study,wefirst focused our attention on model monomers such as styrene and MMA,which have been extensively studied for surface-initiated ATRP.Experimental Part
Materials
Styrene,MMA,toluene,and triethylamine were distilled on calcium hydride before use.All the other reagents,5-hexen-1-ol,2-bromoisobutyrylbromide,2-bromopropionylbromide, chlorodimethylsilane,hydrogenhexachloroplatinate hydrate, ethyl-2-bromoisobutyrate(EBiB),ethyl-2-bromopropionate (EBP),Cu(I)Cl,Cu(II)Cl2,Cu(I)Br,Cu(II)Br2,N,N,N0,N0,N00-pentamethyldiethylenetriamine(PMDETA),and dimethylfor-mamide(DMF)were used as received.The alkyl halide initiators1and2were prepared in two steps according to the method previously described.[11b,12b]Proton and carbon NMR chemical shifts are supplied in Supporting Information.One batch of MSU ordered mesoporous silica with2D hexagonal structure was synthesized as previously reported(see Support-ing Information).[15]The nitrogen adsorption measurements provided the characteristics of the initial mesoporous silica. The specific surface area(S sp)was757m2ÁgÀ1;the total pore volume of the mesoporous silica was found to be1.53cm3ÁgÀ1.The pore size distribution
was narrow with an average pore diameter of85A˚.The average particle diameter was3m m (S sp,external¼17.6m2ÁgÀ1).
Grafting of the Initiator onto Mesoporous Silica and Polymerization
Experimental details concerning the grafting of the initiator and the polymerization procedure are reported in Supporting Information.The initiator grafting density(G I)was determined by thermogravimetric analysis(TGA)(see Figure2)using Equation(1)
G I¼
W%initiatorþsilica
100ÀW%initiatorþsilicaÀ
W%silica
100ÀW%silica
M initiatorÂS sp
ÂN A ðmoleculeÁnmÀ2Þð1Þwith W%initiatorþsilica,the weight loss after initiator grafting, W%silica,the weight loss corresponding to the amount of physisorbed water in the mesoporous silica[W%silica¼4wt.-%, corresponding to the weight loss between25and2008C;the amount of chemisorbed water of9wt.-%(200<T<8008C)is not included in Equation(1)],N A is the Avogadro’s number, and M initiator is the molar mass of the initiator calculated by subtracting the molar mass of the Si and Cl atoms (M initiator¼266and280gÁmolÀ1for respectively1and2). The polymer grafting density(G P)was determined by TGA using Equation(2)
G p¼
W%polymerþsilica
100ÀW%polymerþsilicaÀ
W%initiatorþsilica
100ÀW%initiatorþsilica
M n
SEC;g
ÂS sp
ÂN A ðpolymer chainÁnmÀ2Þð2Þ
with W%polymerþsilica,the weight loss after polymerization and M n
SEC;g
,the experimental molar mass of the grafted polymer chains.
394M.Save et al.
三诺n20gConversions were determined by 1H NMR.All the chara-cterization methods are described in Supporting Information.
Results and Discussion
Mesoporous Silica Functionalization
Scheme 1summarizes the general strategy developed to synthesize the functional alkyl halide initiator,able to self-condense with the silica surface.The monofunctional chlorosilane group was cho
sen to favor a monolayer formation during the condensation of the initiator with the surface hydroxyl groups.FTIR spectroscopy was employed to confirm the grafting of the initiator with the appearance of the characteristic carbonyl and aliphatic vibration bands (Figure 1).The detection of bromine atom by elemental analysis corroborated the anchoring of the halogenated initiator.The characteristics of the initiator-functionalized mesoporous silica are reported in Table 1.Considering the total specific surface area of 757m 2Ág À1,the initiator grafting density ranged from 0.78to 1.16molecule Ánm À2[Equation (1)].The grafting density of 1was limited to 1.16molecule Ánm À2using two different initial n initiator /n OH ratios (Table 1,Expt.1,2).These values are similar to those observed for the grafting of either a monochlorosilane-terminated ATRP initiator [11b,16]or a triethoxysilane-terminated alkoxyamine initiator to silica nanoparticle surface.[17]Nevertheless,they are lower than those reported by Patten [11a]and Fukuda [11c]for the surface coverage of colloidal silica particles,respectively,modified by mono-and tri-alkoxysilane molecules.
‘‘Grafting from’’ATRP
FTIR spectra of the recovered silica after styrene or MMA polymerization gave clear evidence of polymer grafting.Increase of aliphatic and carbonyl vibration band intensity was thus observed after MMA polymerization (Figure 1)while characteristic bands of the aromatic groups appeared
OH
Si CH Cl
中国
检验检疫3
Toluene, NEt 31h, 0°C + 4h, 25°C
Br CH 3
O Toluene, NEt 3Mesoporous silica
Si-O-Si-O-Si O Si
CH 3
H 3C
(CH 2)6O
O
R
H 3C
Br
Surface-initiated ATRP of styrene (initiator 1)
or MMA (initiator 2)90°C, CuX/CuX 2, PMDETA
3
1(R=H); 2 (R=CH 3)
a a
Scheme 1.Synthetic route to the hybrid materials:initiator synthesis,condensation step on the mesoporous silica particle,and MMA or styrene polymerization by ATRP.
Figure 1.IR spectra of (a)mesoporous silica,(b)mesoporous silica functionalized with 2(Expt.3,Table 1),and (c)mesoporous silica after MMA polymerization (Expt.3,Table 2and 3).
Atom Transfer Radical Polymerization of Styrene and Methyl Methacrylate from ...
395
after styrene polymerization (Supporting Information).Moreover,in comparison with the bromopropionate-functionalized silica (Si-BP-1a),the increase of carbon content and the decrease of bromine content,detected by elemental analysis after styrene polymerization,confirmed the growth of polystyrene chains (see Supporting Informa-tion).The polymerizations of MMA and styrene were performed either in the presence of ‘‘free’’sacrificial ini-tiator or without this additional initiator but always in the presence of deactivating copper(II)species.Indeed,authors stated that the addition of sacrificial initiator was essential
拉碗
to ensure a sufficient concentration of deactivating species,enabling the polymerization to be controlled.[12b]However,Matyjaszewski [18]showed that the control over film thick-ness could be maintained through a sufficient concentration of deactivator in the absence of untethered initiator.Concerning ATRP initiated from the functionalized meso-porous silica particles,both styrene
and MMA polymeriza-tions exhibited the characteristics of a controlled polymerization using free sacrificial initiator.Indeed,controlled molar masses,narrow molar mass distributions (M w =M n <1:3),and a good agreement between the molar mass of the free and grafted polymers were observed in all cases (Table 3,Expt.1,3).As predicted by Matyjas-zewski,[18]the polydispersity indexes were slightly higher for the surface-grafted chains when compared to the free chains (Table 3,Expt.1,3).The experimental molar masses were higher than the theoretical ones,likely due to the low efficiency of the grafted initiator (1%<f g <29%,Table 3).Low values of f g have already been mentioned by several groups [11c,19]and could explain the decrease of the overall initiator efficiency (f T )observed when the proportion of grafted initiator increased (Table 2and 3,Expt.1–2,3–4).The noticeable lower value of the grafted initiator efficiency in the case of MMA polymerization with respect to that of styrene might be due to a higher propagation rate (the product propagation rate constant Âequilibrium constant,k p K eq ,is two orders of magnitude higher for MMA than for styrene polymerization [20]),inducing a larger steric
Table 1.Characteristics of the initiator-grafted mesoporous silica particles.
Experiment
Grafted initiator
Initiator-grafted silica n initiator n OH a)W %TGA G I
Graft efficiency b)
Initiator Ánm À2
%1(CH 3)2SiCl(CH 2)6OCO CH (CH 3)Br Si-BP-1a 1.0830 1.16232(CH 3)2SiCl(CH 2)6OCO CH (CH 3)Br Si-BP-1b 0.5430 1.16233
(CH 3)2SiCl(CH 2)6OCOC (CH 3)2Br
Si-BiB-2
0.21
24
0.78
16
a)
The ratio between the number of moles of the initiator introduced and the number of moles of hydroxyl functions is calculated assuming 5OH Ánm À2for both the inner and the outer surfaces.b)
Graft efficiency ¼(n grafted initiator /n OH )Â100¼(G I /5)Â100.
W )
%( t h g i e Temperature (°C)
200
600
800
400
60
40
80
100
120
(a)
(b)
(c)
Figure 2.TGA of (a)mesoporous silica,(b)mesoporous silica functionalized with 2(Expt.3,Table 1),and (c)mesoporous silica after MMA polymerization (Expt.3,Table 2and 3).
Table 2.
Experimental conditions of styrene and MMA surface-initiated ATRP polymerizations carried out at 908C.
Experiment
Initiator-grafted silica a)Monomer
[M]0Free initiator Catalyst
[M]0:[Cu I ]0:[Cu II ]0:[PMDETA]0:[I]f 0
[I]f 0b)[I]g 0c)½M 0½I f 0þ½I g 0
mol ÁL À1
mol ÁL À1mol ÁL À11Si-BP-1a Styrene 7.9EBP CuBr:CuBr 2195:0.9:0.1:1:1 4.0Â10À2
1.8Â10À21342Si-BP-1b Styrene 7.9–CuBr:CuBr 2190:0.9:0.1:1:00 1.8Â10À24323Si-BiB-2MMA 4.5EBiB CuCl:CuCl 2292:0.8:0.2:1:1 1.5Â10À2
5.8Â10À32124Si-BiB-2MMA 4.2–CuCl:CuCl 2298:0.9:0.3:1:00 5.7Â10À3
7845
–
MMA
4.5
EBiB
CuCl:CuCl 2
299:0.9:0.3:1:0.4
5.7Â10À3
–
790
a)The weight concentration is equal to 10and 17g ÁL À1for MMA and styrene polymerization,respectively.b)[I]f 0Corresponds to the free initiator concentration.
c)
[I]g 0Corresponds to the grafted initiator concentration calculated from ½I g 0¼m silica ÂS sp ÂG I
N A ÂV (with N A ,the Avogadro’s number,m silica ,the mass of the mesoporous silica,and V ,the total volume of the polymerization solution).
396
栽培技术
M.Save et al.
hindrance at the surface and limiting the initiation site accessibility.In the absence of free sacrificial initiator (Table 2and 3,Expt.2,4),the behavior of both monomers strongly differed in the sense that the polymerization was well-controlled for styrene (M w =M n ¼1:24)but uncon-trolled for MMA (M w =M n ¼2:59and M n ;SEC )M n ;theoretical ).It is important to note that the PMMA recovered from the model polymerization,performed in solution using similar experimental conditions,displayed a narrow molar mass distribution (Table 3,Expt.5).Special attention was given to the MMA polymerization kinetics.The logarithmic monomer concentration versus time plots depicted in Figure 3indicate a linear trend and a drastic decrease of the polymerization rate for the surface-initiated ATRP of MMA carried out in the absence of free initiator compared to the model ATRP of MMA initiated by EBiB in solution.Actually,the initial initiator concentrations were similar in both experiments,but as we noticed above,the efficiency of the grafted initiator was considerably lower than that of the free initiator (Table 3,Expt.4,5).Assuming
that the slope of the linear plots is proportional to the polymeric radical concentration,it was then sati
sfying to observe a good agreement between the initiator efficiency ratio and the slope ratio for both experiments (f T,expt.4/f T,expt.5¼0.23,see Table 3and slope ratio ¼0.18,see Figure 3).
We performed nitrogen adsorption experiments on the PMMA-functionalized silica (see Supporting Information).The BET surface area has been reduced down to 35m 2Ág À1with the funtionalization of the surface with PMMA (Table 3,Expt.3).The total pore volume decreased from 1.53to 0.053cm 3Ág À1.The filled volume being higher than the PMMA volume in the hybrid materials,it was deduced that a part of the pores might be blocked up by the PMMA at their entrance.Considering the polystyrene grafting density calculated on the basis of the total specific surface area,including internal and external surfaces,the values obtained for our experiments (Table 3)were in the range of those previously reported for CRP initiated from colloidal particles.[11c,19,21]It is noteworthy that this result indirectly indicates that the polymerization most likely took place inside the mesopores.Indeed,the calculation of the polymer grafting density,based on the sole external specific surface area of the silica particles (S sp,external ¼17.6m 2Ág À1),would give unrealistic values ranging between 9and 14polysty-rene chains Ánm À2.Moreover,the SEC chromatogram of the cleaved polystyrene chains displayed a narrow monomodal molar mass distribution (see Supporting Information),which is in accordance with a simultaneous growth of the polymer from both the internal
surface of mesopores and the external surface of particles.In conclusion,the experimental data extracted from TGA provided the proof of the occur-rence of the polymerization process inside the mesopores.
Conclusion
New mesoporous hybrid materials made of mesoporous silica and covalently grafted polymer with controlled chain length were successfully synthesized.
Surface-initiated
Figure 3.Logarithmic monomer concentration versus time plots for surface-initiated ATRP of MMA from mesoporous silica in the absence of free initiator (},Expt.4,Table 2)and for model ATRP of MMA in solution initiated by EBiB (&,Expt.5,Table 2).
Table 3.Results of styrene and MMA surface-initiated ATRP.
Experiment
Initiator-grafted silica
Time Conversion M n theo :a)M n SEC ;g
(M w =M n )b)f T c)
M n SEC ;f
(M w =M n )b)W %polymer þsilica
(TGA)
G P
f g d)
h g Ámol À1g Ámol À1g Ámol À1
%Chain Ánm À2
1Si-BP-1a
240.851210014130(1.23)0.8317250(1.18)
810.2150.182Si-BP-1b 240.311420030440(1.24)0.46–
930.3360.293Si-BiB-2160.42930016390(1.13)0.5316620(1.08)
530.0390.054Si-BiB-2
60.131060063440(2.59)
0.16–
490.0080.015
–
2
0.22
17750
–
0.7024990(1.08)–
–
–
a)Theoretical average molar mass:M n theo :¼M initiator þ½M 0
½I
g 0
Âconv :ÂM monomer
b)
M n SEC ;g and M n SEC ;f are the experimental molar mass (SEC)of the grafted polymer chains and the free polymer chains,respectively.
c)Overall initiator efficiency,f T ¼M n theo :nSEC ;g
¼½I exp ½I f 0þ½I g 0
.d)
Grafted initiator efficiency,f g ¼
G p
G I .
Atom Transfer Radical Polymerization of Styrene and Methyl Methacrylate from ...
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