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Solid Phase Modular Synthesis of Polymacromer Brushes and Their Isolation as Molecular Products by Photocleavage from the Substrate AUTHOR NAMES Benjamin I. Dach †, ‡, Xia Li †, Nicholas J. Turro †, ‡, and Jeffrey T. Koberstein ‡ ,*

AUTHOR ADDRESS † Department of Chemistry and ‡ Department of Chemical Engineering, Columbia University, 3000 Broadway, New York, New York 10027 CORRESPONDING AUTHOR FOOTNOTE E-mail: [email protected]. Tel.: (212) 854-3120. Fax: (212) 854-3054 KEYWORDS Polymacromer, Solid Phase Synthesis, Polymer Brushes, Click chemistry, Photocleavage, Macromonomer

ABSTRACT. We report a novel solid phase method for the while segmented block copolymacromers of any desired sequence sequential coupling of heterobifunctional macromonomers to can be prepared using different macromonomers in each cycle. form a new class of polymeric materials that we refer to as Azide-alkyne click chemistry, via the Hüisgen 1,3-dipolar cy- cloaddition mechanism, is employed as the coupling chemistry polymacromers. Starting from an azide functional substrate, - because the alkyne group can be readily protected and subse- azido, -protected-alkyne macromonomers are added step-by quently deprotected using relatively mild conditions. The click re- step by thermally initiated click reactions. After each addition action can be initiated at room temperature with a copper catalyst step, the terminal alkyne group is deprotected to allow addi- (CuAAC) or thermally at temperatures as low as 70 °C without tion of another macromonomer. The use of highly chemose- catalyst. The high chemoselectivity of the reaction enables the lective click chemistry for the coupling reactions allows virtu- coupling of virtually any macromonomer, regardless of its chemi- ally any macromonomer to be employed, regardless of its cal nature.14-16 The required heterobifunctional macromonomers, chemical nature. The polymacromers may be left as polymer comprising one azide end group and a second protected alkyne brushes on the substrate, or can be isolated as molecular prod- end group, are prepared by atom transfer radical polymerization ucts by photocleavage of an ortho-nitrobenzyloxycarbonyl (ATRP).17 A limitation of the SPS method in this application is (NBOC) linkage incorporated at the substrate interface. The that the harsh reagents required for release of the final polymer method is illustrated by forming homopolymacromers by se- product from the surface can potentially degrade the polymer. To quential coupling of poly(tert-butyl acrylate) macromonomers. circumvent this problem, a photochemical cleavage method is de- The results of characterization of the polymacromer bushes by veloped for release of the PM product from the substrate under ellipsometry, contact angle analysis and x-ray photoelectron extremely mild and universal conditions. The process for solid phase synthesis of a PM on a sili- spectroscopy, and direct measurements of the molecular con wafer (i.e., SiO surface) is illustrated in Scheme 1 for four weights of isolated products by gel permeation chromatogra- 2 addition cycles of an -azido, -trimethylsilane-protected-alkyne phy demonstrate that polymacromers can be prepared with a poly(tert-butyl acrylate) [PtBA] macromonomer. The process coupling efficiency approaching 100%. consists of: TEXT “Solid phase synthesis” (SPS) is a method by 1. attachment of an amine terminated primer (1, Chart 1) which molecules are bound to a solid surface (commonly a bead) to the SiO surface; and then synthesized step-by-step in a reactant solution. Com- 2 2. reaction of the amine terminus with a nitrobenzyloxy- pared to conventional solution synthesis, it is easier to remove ex- carbonyl (NBOC) photo-cleavable linker possessing an cess reactant and by-products because the desired product is cova- alkyne terminus (2, Chart 1); lently bound to the solid surface. SPS is a modular method that generally employs building blocks with two complementary func- 3. click reaction of the alkyne terminus with an -azide, tional groups, one of which is protected. The synthetic pathway is -TMS-protected-alkyne macromonomer (M1, 3, Chart controlled by the order of addition (and subsequent deprotection) 1); of these building blocks to a solid substrate that is modified to 4. removal of the alkyne protecting group and repetition of present unprotected surface functional groups. SPS has become step 3 to add a second polymer (M2, Chart 1); an important method for the synthesis of peptides,1-3 DNA4-8 and 5. repetition of the cycle to add two more polymer units other biopolymers9,10 for which a certain asymmetric predeter- (M3 and M4, Chart 1); mined sequence is desirable, and has also been employed recently 6. photocleavage and characterization of the final product in combinatorial chemistry.11-13 PM1-4 (4, M1-M2-M3-M4, Chart 1). We describe herein a new SPS method through which a The chemistry involved in Scheme 1 is shown explicitly in specific, asymmetric sequence of macromonomers can be joined Scheme 2. It is evident that SPS of PMs produces first a polymer to produce a new class of polymers, polymacromers (PM). Ho- brush tethered by its end to the substrate. If desirable, the PM can mopolymacromers with integral molecular weights can be pre- subsequently be released from the substrate and isolated as a pared by multiple addition cycles using a single macromonomer molecular polymer product by applying the photocleavage step. 1 Chart 1. Materials used in the step-by-step synthesis of a PM.

NH2 NO2 O NO2 O

N O O H

Si O O 2 O

1 NBOC: photocleavable linker to M Primer 1

O O NO O O

N3 O N N O 98 H 98 TMS N N TMS green = primer. yellow = NBOC photocleavable linker. O O O O 1 - 4 4 Scheme 1. Schematic description of the solid phase synthesis of a 3 four-step polymacromer on a silicon wafer and isolation of the Photocleaved PtBA Polymacromer product by subsequent photocleavage. PtBA (M1-M2-M3-M4) Macromonomer Silicon wafers (prime grade N-doped [100] purchased from Addison Engineering) were cleaned with piranha solution, washed with Millipore distilled water and cut into 1 in x 1 in substrates. The primer layer, (aminohexyl)aminomethyl-triethoxysi- lane (AHAMTES) was deposited by dipping the substrates into a toluene solution of 1 (Chart 1) and the surface was functionalized with the photocleavable alkyne functionality according to Scheme 2. -azido, -TMS-protected-alkyne PtBA macromonomers (MW = 10,000), prepared by atom transfer radical polymeriza- tion18 were spin coated onto the alkyne functional substrates from toluene and were subsequently heated to 110 °C for 18 hrs to ef- fect a thermal click reaction.19 The terminal TMS groups were then removed by treatment with a solution of MeOH:DCM (1:10) 20 with excess K2CO3 for 2 hr. A total of four spin coating/deprotection/thermal click cycles were used to prepare the four step PtBA polymacromer, which was photocleaved from the surface using UV light at a wavelength of 360 nm. Details regarding the solid phase synthesis procedure, macromonomer synthesis and synthesis of the photocleavable linker are described in the supporting information.

Scheme 2. Reaction scheme for solid phase synthesis of a PtBA PM on a SiO2 surface followed by photochemical cleavage of the product from the surface.

To evaluate the efficiency of the SPS method, four - azido, -TMS-protected PtBA macromonomers were coupled se- quentially to silicon wafers as described above. After each cycle, 2 the resultant polymer brushes and the product isolated by photocleavage were both characterized by a battery of methods: X-Ray photoelectron spectroscopy (XPS), water contact angle (WCA), ellipsometry (ELP) and Gel Permeation Chromatography (GPC). The results of all of these independent methods confirm successful step-by-step covalent attachment of PtBA macromonomers to form the final PM product (4, M1-M2-M3-M4, Chart 1). The XPS analysis (PHI 5500 spectrometer, Al Kα monochromator X-ray source at 15 kV and 23.3 mA, 45° take off angle, penetration depth of 4.6 nm) of the surface was performed (Figure 1) at each addition step in the overall process, M 1, M1-M2, M1-M2-M3 and M1-M2-M3-M4 and after the photocleavage. Con- trols of the bare SiO2 surface and the surface primed with AHAMTES were also characterized. The results are summarized in Figure 1. The qualitative features of the data are: (1) compared to the bare SiO2 surface, the carbon signal (C 1s, 285 eV signal) increases as the primer and subsequent four layers of PtBA are at- Figure 2. WCA measurements on bare, amine, NBOC, 1 PtBA tached to the surface; (2) upon photocleavage, the signals are sim- macromonomer, 2 PtBA macromonomers, 3 PtBA ilar to that of the surface with the primer. These results are con- macromonomers, 4 PtBA macromonomers, and photocleaved sistent with the successful sequential step-by-step additions of substrates (point on far right). four PtBA macromonomers and the complete removal of the final tetramacromer by photochemical cleavage. The results of the WCA measurements are shown in Figure 2. The hydrophilic SiO2 surface yields a low water contact angle of ~ 5°. Upon adding the less hydrophilic layers of primer, NBOC and macromonomers, the CA monotonically increases and approaches a limiting value of 90°, consistent with the value for pure PtBA.21 Upon photocleavage of the polymer film, the contact angle returns to a value close to that found for the primer. ELP measurements (ALPHA-SE® J.A. Woollam Co., Inc., USA. 70° fixed angle of incidence, Cauchy model) of the film thicknesses provide a quantitative means to evaluate the efficiency of the overall SPS process (Figure 3). The film thickness increases upon addition of the primer, the NBOC and the four PtBA macromonomers as expected.

Figure 3. Measured (filled squares) and theoretical (open circles) film thicknesses (nm) from ellipsometric measurements for bare silicon oxide, AHAMTES, NBOC, PtBA1, PtBA2, PtBA3, Pt- BA4, and photocleaved substrates (point on far right). The ellipsometric thickness scales linearly with the number of macromonomer addition cycles and compares well with the theoretical thickness calculated assuming 100% conver- sion for the click reaction for each cycle. The value of the thickness for the photocleaved surface returns to a value close to that for the surface with just the primer. Film thicknesses determined by X-Ray reflectivity measurements, to be reported in a paper to follow, agree well with the ellipsometric thicknesses. The efficiency of coupling can be probed in more depth by analyzing polymacromers that have been isolated by cleavage from the substrate. GPC analyses (Knauer GPC system with a Knauer K-2301 refractive index detector, three Polymer Laborato- Figure 1. XPS spectra of bare silica, silica-amine, silica-NBOC, ries 5 μ m particle size PLgel columns, linear polystyrene calibra- silica-PtBA4, and silica substrate after photocleavage. tion standards ranging in molecular weight from 580-377,400 Da, THF eluant with a flow rate of 1.0 mL/min at room temperature) of isolated products are shown in (Figure 4). The signal quality is low because each silicon substrate yields only a limited amount of product however it is evident that the molecular weight increases in integral multiples of the number of macromonomers added, consistent with the linear behavior found in the ellipsometry data.

3 SUPPORTING INFORMATION PARAGRAPH Experimental details, AFM images, and XPS multiplex spectra are included. This material is available free of charge via the Internet at http://pubs.acs.org.

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