Grafting from Surfaces for“Everyone”:ARGET ATRP in the
Prence of Airtrailblazer
Krzysztof Matyjaszewski,*,†Hongchen Dong,†Wojciech Jakubowski,†
Joanna Pietrasik,§and Andy Kusumo‡
Center for Macromolecular Engineering,Department of Chemistry,and Department of Chemical Engineering,Carnegie Mellon Uni V ersity,Pittsburgh,Pennsyl V ania15213,and Institute of Polymer and Dye Technology,Department of Chemistry,Technical Uni V ersity of Lodz,Stefanowskiego12/16,90-924
Lodz,Poland
Recei V ed January22,2007
Atom-transfer radical polymerization(ATRP)is one of the controlled/living radical polymerizations yielding well-defined(co)polymers,nanocomposites,molecular hybrids,and bioconjugates.ATRP,as in any radical process,has to be carried out in rigorously deoxygenated systems to prevent trapping of propagating radicals by oxygen.Herein, we report that ATRP can be performed in the prence of limite
d amount of air and with a very small(typically ppm) amount of copper catalyst together with an appropriate reducing agent.This technique has been successfully applied to the preparation of denly grafted polymer brushes,poly(n-butyl acrylate)homopolymer,and poly(n-butyl acrylate)-block-polystyrene copolymer from silicon wafers(0.4chains/nm2).This simple new method of grafting well-defined polymers does not require any special equipment and can be carried out in vials or jars without deoxygenation.The grafting for“everyone”technique is especially uful for wafers and other large objects and may be also applied for molecular hybrids and bioconjugates.
1.Introduction
The past decade has witnesd the explosive development of controlled/living radical polymerization(CRP)process.1,2 Macromolecules with precily controlled architecture and functionality have been synthesized from a wide range of monomers under conditions that are much less rigorous than previously required for ionic living polymerizations.3-5In addition,a plethora of new functional materials have been prepared,including molecular composites,hybrids,and bioconju-gates.6-18Atom-transfer radical polymerization(ATRP)is among the most efficient and robust CRP process.19,20ATRP is especially well-suited for surface modification,bioconjugation, and the preparation of molecular brushes becau hydroxyl or amine groups on the surface or in natural pro
ducts can be easily converted to ATRP-active initiators:R-bromoesters or R-bro-moamides.Our group and many others have shown how to control the growth of well-defined polymer chains from flat silicon wafers and also from various convex,concave,and irregular surfaces.21-28 ATRP and esntially all CRP process can be carried out in bulk,solution,or disperd media(including aqueous systems).29However,becau propagating radicals are rapidly trapped by oxygen,reaction mixtures must be rigorously deoxygenated.We have previously demonstrated that in ATRP systems air can be consumed by adding a sufficient amount of an appropriate reducing agent,such as metallic copper,tin(II) 2-ethylhexanoate,which has been approved by the Food and Drug Administration(FDA),or ascorbic acid(vitamin C)30in a process called activators generated by electron transfer(AGET). ATRP is bad on the intermittent activation of alkyl halides to
*Corresponding author.E-mail:u.edu.
†Center for Macromolecular Engineering,Department of Chemistry, Carnegie Mellon University.
‡Department of Chemical Engineering,Carnegie Mellon University.
§Institute of Polymer and Dye Technology,Department of Chemistry, Technical University of Lodz.
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4528Langmuir2007,23,4528-4531
星期日英文缩写10.1021/la063402e CCC:$37.00©2007American Chemical Society
Published on Web03/20/2007
generate propagating radicals in the prence of redox-active transition-metal complexes,which are typically copper halides with polydentate N-bad ligands.In AGET ATRP,the activator (Cu(I)species)is first rapidly oxidized by oxygen to the Cu(II) species,but the latter is quickly reduced to the Cu(I)state in the prence of a reducing agent.There is an induction period during which air is consumed,and eventually the polymerization starts. This process has been successfully ud to prepare well-defined products in organic media and also in miniemulsion.31,32 Unfortunately,it is difficult to estimate the exact amount of reducing agent needed.If it is added in too large of an excess, then polymerization control is lost becau nearly all of the deactivator(Cu(II)species)is cons
umed and a relatively large amount of Cu(I)activator leads to uncontrolled ATRP that is too fast.On the contrary,an insufficient amount of reducing agent does not consume all of the air and results in no polymerization. We have recently improved the AGET process by employing a very active copper catalyst ud in minute amounts in the prence of excess reducing agent that reacts slowly.Under the conditions,activators are continuously regenerated by electron transfer(ARGET),and even parts per million amounts of catalysts
can lead to a successful ATRP.33-35ARGET ATRP can tolerate a large excess of reducing agent and therefore should be more appropriate for systems in which air must be scavenged.This system may be particularly well-suited for grafting from large wafers becau it is difficult to place them inside Schlenk flasks and carry out deoxygenation.
In this article,we report that ARGET ATRP can be successfully carried out in the prence of limited amounts of air and control is esntially unaffected by excess reducing agent.Reactions can be carried out without any deoxygenation,in flasks fitted with rubber pta or even in simple jars.We also demonstrate that one can place functionalized wafers in the vesls and grow very den polymer brushes(∼0.4chain/nm2),including block copolymer brushes,without any deoxygenation.ATRP stops after opening the vesl to air but starts again when a sufficient amount
of reducing agent is again added.This grafting for“everyone”process does not require any special skills and is especially well-suited for grafting from larger surfaces.
2.Experimental Section
Materials.n-Butyl acrylate(Acros,99%)and styrene(Aldrich, 99%)were purified by passing the monomers through a column filled with basic alumina.The ligand,tris[(2-pyridyl)methyl]amine (TPMA),was prepared according to the reported procedure.36All other chemicals were ud as received.The initiator,1-(chlorodi-methylsilyl)propyl2-bromoisobutyrate,was grafted on the surface of a silicon wafer as described elwhere.37
Synthetic Procedures.Surface-Initiated ARGET ATRP of Butyl Acrylate from a Silicon Wafer(Figure3,points A and A2).A22mL glass vial containing an initiator-modified silicon wafer and a small stir bar was charged with butyl acrylate(15.0mL,105mmol)and ethyl2-bromoisobutyrate(32.8µL,0.224mmol).Then a solution of CuCl2(0.71mg,0.0053mmol)and TPMA ligand(6.5mg,0.013 mmol)in anisole(2mL)was added.After the vial was aled with a rubber ptum,a solution of tin(II)2-ethylhexanoate(109µL,
0.337mmol)in anisole(1mL)was injected.The initial sample was taken,and the aled vial was placed
in an oil bath thermostated at 70°C.Samples were taken at timed intervals and analyzed by GC and SEC.The polymerization was stopped after5h(M n,SEC)13900, M w/M n)1.22,conversion)18.4%)by opening the flask and exposing the catalyst to air.The silicon wafer was taken out to analyze the thickness,and the solution was capped and stored in the dark.To remove the free polymer physically adsorbed onto the surface,the resulting silicon wafer(Figure3,A)was washed with methylene chloride in a Soxhlet extractor for24h.The thickness of the dry poly(n-butyl acrylate)brushes,measured by ellipsometry in air,was12.3nm.The error in the measurement was less than0.5 nm.The grafting density(σ)was calculated using the following equation:σ)N A h F/M w,where M w is the weight-average molecular weight,N A is Avogadro’s number,and F)1.0g/cm3is the bulk poly(n-butyl acrylate)density.
After about1month,no monomer evaporation was detected in the stored solution by GC.The previously modified dry silicon wafer was put back into the solution,and the vial was again aled with a rubber ptum.The solution of Sn(EH)2(109µL,0.337mmol) in anisole(1mL)was injected to restart the polymerization.The initial sample was taken,and the aled vial was placed in an oil bath thermostated at70°C.After20.5h,the polymerization was stopped(M n,SEC)25980,M w/M n)1.31,conversion)55.4%)by opening the flask and exposing the catalyst to air.The thickness of the p
olymer layer on the surface of the silicon wafer(Figure3,A2) was22.8nm,and the error in the measurement was0.6nm. Analys.Monomer conversion was determined using a Shimadzu GC14-A gas chromatograph equipped with an FID detector using a J&W Scientific30m DB WAX Megabore column with anisole as an internal standard.Molecular weights and molecular weight distributions of the free polymers produced in solution were determined by SEC,and the measurements were conducted with a Waters515pump and a Waters2414differential refractometer using PSS columns(Styragel105,103,and102Å)with THF as the eluent (35°C,flow rate of1mL/min).Linear polystyrene standards were ud for calibration.It is believed that the values should be clo to tho of the grafted polymer brushes.The thickness of dry polymer layer was measured by a pha-modulated Beaglehole picometer ellipsometer in air.The angles of incidence were between60and 80°with1°intervals at a wavelength ofλ)632.8nm.
3.Results and Discussion
Figure1prents a general scheme for ARGET ATRP in the prence of limited amounts of air and a typical reaction tup (explained in detail in Experimental Section).
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8767.Figure1.ARGET ATRP of BA in the prence of limited amounts of air.(a)Propod mechanism.Typical tup with silicon wafers for ARGET ATRP with limited air(b)in a large jar and(c)in a sample vial.(d)Illustration of procedures for surface-initiated ARGET ATRP with limited air.
Grafting by ARGET ATRP in the Prence of Air Langmuir,Vol.23,No.8,20074529
Table 1prents results of ARGET ATRP of n -butyl acrylate (BA)initiated by ethyl 2-bromoisobutyrate (EBiB)in the prence of a 50ppm CuCl 2/TPMA complex.Reactions were carried out under homogeneous conditions to evaluate the effect of the amount of tin(II)2-ethylhexanoate as the reducing agent.The volume of free space in a small glass vial was ∼4mL,which corresponds to 0.0375mmol of O 2.Assuming that one O 2molecule oxidizes two Sn(II)molecules,this would correspond to the molar ratio of [Sn(II)]air /[EBiB])0.34.However,no polymerization occurred at 1.5times higher concentration of the Sn(II)species.This suggests an insufficient amount of reducing agent and the prence of air dissolved in the reaction mixture as well as some additional air diffusion into the reaction mixture through the ptum.Polymerization started at the molar ratio of [Sn(II)]/[EBiB])0.75but stopped at limited conversion,plausibly becau of air diffusion through the ptum.However,when a larger excess of reducing agent was ud,polymerization proceeded very smoothly,and polymers with low polydispersity were formed.As shown in Figure 2a,b,the milogarit
hmic kinetic plots are linear,and the reaction rate is ∼0.5order with respect to the amount of effective reducing agent ([Sn(II)]0-[Sn(II)air ]).Figure 2c illustrates the evolution of molecular weights and molecular weight distributions in experiment 3from Table 1.The molecular weight smoothly incread,and the entire distribution shifted toward higher molecular weight,confirming negligible termination and transfer.Polydispersity decread with conversion,as typically obrved for CRP systems conducted with a small amount of deactivator.After 27.5h,the reaction reached 58%conversion and was stopped by exposing the reaction mixture to air.The vial was realed with a ptum,and an additional 1.5mol equiv of reducing agent was added.The reaction smoothly restarted,and the entire distribution again shifted toward higher molecular weight.This suggests the possibility of successful block copolymerization.
ARGET ATRP can also be well controlled with a large excess of reducing agent (>8-fold).In contrast,for AGET such an excess leads to poorly controlled polymerization and higher polydis-persity (entry 7in Table 1).In addition,L -ascorbic acid
(vitamin
Figure 2.(a)Kinetic plots of ARGET ATRP of BA with different amounts of reducing agent under a certain volume of air (Table 1,entries 2-4and 6).BA/EBiB/CuCl 2/TPMA )470/1/0.0235/0.06in anisole at 70°C.[BA]0)5.84M.V (reaction solution))18mL;V (free space))4mL.(b)Plot showing that the apparent rate constant k p app grows exponentially with the ([Sn(EH)2]0-[Sn(EH)2]air )/[EBiB]0ratio.[Sn(EH)2]air )2V (free space)(21%/22.4L),where 21%is the volume composition of O 2in air and 22.4L is the volume of 1mole of an ideal gas at STP.(c)Evolution of the molecular weight distribution during ARGET ATRP of BA in the prence of air (Table 1,entry 3).The dashed line is the SEC curve obtained after 27.5h when the reaction system was stopped by exposure to air and then was continued by injection of an additional 1.5mol equiv of the reducing agent.
4530Langmuir,Vol.23,No.8,2007Matyjaszewski et al.lobby>模式英文
C)can be ud as an environmentally friendly reducing agent for ARGET ATRP of BA in limited air.Acetone was ud as a solvent to ensure the partial solubility of L -ascorbic acid (entry 5in Table 1).PBA was formed in 91%yield with low polydispersity (M w /M n )1.20).The polymerization proceeded faster becau ascorbic acid is a stronger reducing agent than the Sn(II)species.ARGET ATRP proceeded successfully in large glass jars (71mL volume)as shown in entries 8and 9.
This advantage led itlf to reactions carried out in the prence of silicon wafers functionalized with ATRP initiators (Figure 3).The initiator density exceeded 1molecule/nm 2.13,21,38ARGET ATRP was conducted in the prence of sacrificial initiator to evaluate the molecular weight of polymers formed in solution (esntially equal to polymer growing from the wafer).The thickness of the polymer film was measured by ellipsometry on samples taken at timed intervals.Some wafers with PBA chains were reinrted into the reaction mixture and extended with additional BA,and other wafers were extended with styrene to form block copolymers.Also,the effect of stirring was examined.In all systems,a smooth and controlled growth of chains occurred regardless of stirring,intermittent exposure to air,or extension with another gment.
The high uniformity of chain growth was confirmed by the high density of chains that are strongly extended,exceeding 30%of the fully extended planar zigzag confirmation (PBA chain with M n ),DP )500has a 125nm fully extended contour length and the polymer film thickness reaches 40nm).The ultimate density of chains under highly controlled growth approaches 0.4chain/nm 2.
4.Conclusionsanimal alpha
ARGET ATRP has been successfully employed for the controlled growth of polymer chains in solution and also from various surfaces,even in the prence of limited amounts of air.Excess reducing agent slightly accelerates ATRP but does not interfere with the controlled growth.This approach virtually eliminates any requirement for the deoxygenation of reaction mixtures or the u of a vacuum line or Schlenk line.The grafting for “everyone”technique does not require any special skills and is especially well-suited for grafting from large surfaces.It is anticipated that the new ARGET ATRP method in the prence of air will facilitate commercial application and make ATRP an attractive approach for many scientists to prepare the materials with the molecular control necessary for highly value nanohybrid and bioresponsive materials.
Acknowledgment.Financial support by the National Science Foundation under grants DMR 05-49353and CTS 03-04568is acknowledged.Ke Min and Patricia Lynn Golas are acknowledged for helpful discussions.
LA063402E
(38)Huang,J.;Cusick,B.;Pietrasik,J.;Wang,L.;Kowalewski,T.;Lin,Q.;Matyjaszewski,K.Langmuir 2007,23,241-
249.
Figure 3.Relationship between the grafted PBA brush thickness measured in air by ellipsometry and M n of free PBA polymers.The polymerization was conducted with (b A,B,C,and D)or without (O E)sti
rring.BA/EBiB/CuCl 2/TPMA/Sn(EH)2)470/1/0.0235/0.06/1.5in anisole at 70°C.[BA]0)5.84M.V (reaction solution))18mL;V (free space))4mL.The open square (0A2)shows the thickness obtained after 20.5h when the reaction system of A was stopped by exposure to air and then kept for 1month and continued by reinjection of 1.5mol equiv of reducing agent.The open triangle (4D2)reprents the thickness of the PBA-b -PS copolymer brushes prepared by the chain extension of styrene from PBA-modified silicon wafer D.St/EBiB/CuCl 2/TPMA/Sn(EH)2)470/1/0.0235/0.1/0.6in anisole at 90°C.[St]0)7.27M.V (reaction solution))18mL;V (free space))4mL.
Table 1.AGET and ARGET ATRP of BA in the Prence
of Air a
entry molar ratio of [Sn(EH)2]0/
[EBiB]0volume
of free space (mL)time (h)conv (%)M n (theo)b M n (GPC)c PDI c 1
0.34 4.020.0no polymerization 20.75 4.020.024.8149309810 1.53
3 1.00 4.028.050.63050039460 1.12
4 1.50 4.028.064.53882048860 1.115d 1.50d 4.020.091.05494065960 1.2068.33 4.020.584.75102061110 1.197e 8.33 4.020.568.24108033750 1.728f 1.5013.616.051.03046035580 1.129f
14.3257.023.5
69.0
artistic4157048290
1.26
a
Reactions were conducted in a 22mL glass vial (as shown in Figure 1c)unless specified otherwi.BA/EBiB/CuCl 2/TPMA )470/1/0.0235/0.06in anisole at 70°C.[BA])5.84M.b M n (theo))([M]0/[EBiB]0)conversion.c Determined by SEC in THF on the basis of polystyrene standards.d The reaction was carried out with L -ascorbic acid as a reducing agent under the following conditions:BA/EBiB/CuCl 2/Me 6TREN/L -ascorbic acid )470/1/0.0235/0.235/1.5in acetone at 70°C.[BA])5.84M.e AGET ATRP was conducted under following conditions:BA/EBiB/CuCl 2/Me 6TREN/Sn
(EH)2)470/1/2/2/8.33in anisole at 70°C and [BA]0)5.84M.f The reactions were conducted in a 71mL glass jar (as shown in Figure 1b).
Grafting by ARGET ATRP in the Prence of Air Langmuir,Vol.23,No.8,20074531
