Polymers with Pendant Allyl Ester Groups Prepared via Atom Transfer

Chapter 12

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Polymers with Pendant Allyl Ester Groups Prepared via Atom Transfer Radical Polymerization: Synthesis, Characterization, and Microstructuring of Thin Films 1

2

1

1

René Nagelsdiek , Petra Mela , Martina Mennicken , Helmut Keul and Martin Möller 1,2,*

1

Lehrstuhl für Textilchemie und Makromolekulare Chemie der RWTH, Aachen, Germany DWI an der RWTH Aachen e.V. Pauwelsstrasse 8, D-52056 Aachen, Germany 2

Random copolymers of styrene (St) and allyl methacrylate (ΑΜΑ) as well as of methyl methacrylate (MMA), butyl methacrylate (BMA) and ΑΜΑ were prepared by copolymerization of the mixture of monomers via atom transfer radical polymerization (ATRP). The control of the reaction depends on the monomer conversion, the monomer to initiator ratio and the concentration of ΑΜΑ in the feed. Block copolymers with a poly(carbonate) - or a poly(phenylene oxide) block and PAMA end blocks were obtained by ATRP of ΑΜΑ using the macroinitiator technique. A l l these copolymers with allyl ester side chains are soluble; films were prepared and cross-linking was achieved by irradiation in the presence of suitable photoinitiators. Moreover, microstructuring of polymer layers on solid substrates was achieved by laterally resolved photochemical cross-linking.

© 2006 American Chemical Society

In Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2006.

153

154


Introduction The prepolymer concept is a widely used concept for the preparation of cross-linked materials. It is characterized by three attributes: (i) Polymerization and cross-linking are independent processing steps; (ii) the prepolymer does not contain toxic monomers and can therefore be handled under less stringent conditions; (iii) properties of the prepolymers such as solubility, glass transition, and viscosity, can be tailored offering potential for applications like coatings, microstructuring, and 3D-lithography ("rapid prototyping"). Cross-linking of prepolymers is frequently achieved by radical addition reactions on double bonds (e.g., alkyd resins or unsaturated polyesters). These double bonds can be terminal double bonds in the prepolymer (telechelic polymer, cf. Scheme 1). However, this approach suffers from the low concentration of double bonds. Alternatively, double bonds can be located in the main chain or in side groups of a polymer chain. These in-chain groups or side groups can either be distributed in the whole polymer chain at random or in distinct sequences (cf. Scheme 1).

Telechelic with crosslinkable end groups

Copolymers with double bonds in the main chain

Random copolymers with cross-linkable side chains

Block copolymers with segments containing cross-linkable side chains

Scheme 1: Suitable polymer architectures for prepolymers which can be crosslinked via a radical process. Allyl ester side groups offer the potential of cross-linking for a variety of polymers. Soluble polymers containing these side groups can be achieved by controlled radical (co)polymerization of allyl methacrylate (ΑΜΑ) by means of ATRP. In this process, radical polymerization preferentially occurs at the methacrylic double bond whereas the allylic double bond is less reactive (1). Due to the activation/deactivation equilibrium, premature cross-linking processes are prevented up to high conversion if the polymerization is carried out as an ATRP (note that thefreeradical polymerization leads to cross-linking at low conversion) (I). In two recent papers, we reported on the homo- and



155 copolymerization of ΑΜΑ (1,2). Although available, the homopolymer of ΑΜΑ is of minor interest for application since it tends to auto-cross-link in air at room temperature (1,3). "Dilution" of the cross-linkable side groups was achieved by means of copolymerization. These random copolymers of ΑΜΑ with styrene (St) (1) or methacrylates (2), respectively, can be stored in air without the occurrence of cross-linking. Moreover, copolymerization leads to a variety of copolymers featuring different properties (e.g., glass transition temperature); enclosure of a few mole percent of ΑΜΑ repeating units into the polymer assures a potential cross-linking at a later stage. However, the atom transfer radical copolymerization of ΑΜΑ is limited to vinyl comonomers such as acrylates, methacrylates, and styrenes. We considered it to be desirable to extend the "toolbox" of cross-linkable materials to further classes of polymers, in particular high performance polymers (e.g., polycarbonates or poly(phenylene oxides)). These polymers are mainly prepared by polycondensation reactions. Hence, two different polymerization processes are required: (i) polycondensation and (ii) radical polymerization via ATRP. Consequently the macroinitiator approach appeared to be most suitable for this purpose. In this concept, the following steps are performed: (i) Synthesis of a telechelic high performance polymer by polycondensation; (ii) preparation of a bifiinctional ATRP macroinitiator by chain analogous reaction of the telechelic polycondensation product (e.g., by esterification); (iii) ATRP of ΑΜΑ using the bifbnctional macroinitiator leading to a triblock copolymer P(AMA)[polycondensate]-P(AMA). Note that in the latter step, a low monomer/initiator ratio is expected to be sufficient to assure cross-linking on demand.

Experimental Section Materials and Methods All chemicals were commercially available and purified according to (1,2). Analytical measurements were performed as described in (1,2).

Procedures Spin coating: Thin polymer films were created on silicon and TiNi substrates by spin coating a 5 or 10 wt.% solution of the polymer in THF (Convac 1001S/ST 147 spin coater). The polymer solution already contained a certain amount of photoinitiator (P(St-co-AMA): 20 wt.% Irgacure 819, referring to the mass of the polymer; PMMA: 0,2 wt.% DMPAP and addition of 10 wt.% DegDMA). The solution was passed through a syringe filter (Schleicher & Schuell MicroScience, Spartan 13/0.2 RC58, pore diameter 0.2



156 μιη, regenerated cellulose). Spin coating was carried out at a constant rotation speed [5000 rpm for P(St-co-AMA) and 2000 rpm for P(MMA-co-BMA-coAMA)] for 30-60 s. Coated substrates were stored in the dark. Microstructuring: Copper Transmission Electron Microscopy (TEM) grids were used as shadow mask for photolithography (Agar Scientific, 400 mesh hexagon, diameter 3.05 mm). Irradiation was done with an Osram Ultra Vitalux 300 W UV lamp. The distance between the sample and the lamp was 10 cm and the exposure time was 3h for P(St-co-AMA) and 2h for P(MMA-co-BMA-coAMA). After irradiation, the P(St-co-AMA) layers were developed by dissolving the unexposed polymer in cyclohexane for 90 min at room temperature (r.T). The P(MMA-co-BMA-co-AMA) layers were developed in acetonitrile for 24 h at r.T. Substrates: Silicon wafers (375 μιη thick, CrysTek GmbH, Berlin, Germany) were cut into 15x15 mm square pieces. The pieces were cleaned in an ultrasonic bath in acetone, deionized water, and isopropanol, each solvent for two minutes. The TiNi substrates were prepared by sputter deposition (Caesar Research Center, Bonn, Germany) of 1 μιη thick Ti Ni43Hfi8 layers on singleside polished 500 μιη thick silicon wafers. The cleaning was as described for the plain silicon pieces. 39

Syntheses Poly(St-co-AMA): (l-Bromoethyl)benzene (0.93 g, 5 mmol), CuBr (0.72 g, 5 mmol), Bpy (1.95 g, 12.5 mmol), St (20.83 g, 200 mmol), and ΑΜΑ (6.31 g, 50 mmol) were stirred in BuAc (25 mL). The reaction was started by immersion into an oil bath (130 °C). After 5 h the mixture was rapidly cooled to room temperature and quenched by addition of CH C1 (20 mL) followed by stirring in air until the copper complex was completely oxidized. The copper complex was removed byfiltrationand subsequent extraction with 5 % HC1. The polymer was precipitated into methanol. Yield: 14.67 g (52 %), M„ c = 6500, MJM = 1.55, molar ratio of repeating units: 78:22 (St/AMA). Poly(MMA-co-BMA-co-AMA): M M A (22.00 g, 220.0 mmol), B M A (13.39 g, 94.3 mmol), and ΑΜΑ (3.96 g, 31.4 mmol) were dissolved in BuAc (20 mL). FPSC (0.07 g, 0.35 mmol), CuCl (0.07 g, 0.7 mmol), and PMDETA (0.14 g, 0.8 mmol) were added and thé reaction medium was stirred for 6 h at 60 °C. The workup procedure was carried out analogously to the styrene copolymer (precipitation in pentane). Yield: 16.16 g (41 %), M„ = 85100, MJM = 2.10, mlar ratio of repeating units: 62:28:10 (MMA/BMA/AMA). End-group functionalized PC and PPO: The macroinitiators were prepared according to the literature (4,5). Polymerization of ΑΜΑ using PC and PPO macroinitiatos was carried out in anisole according to the procedure described for random copolymers. Polymerization conditions are listed in Table 1. 2

2

tGP

n

n


157 Table 1: ATRP of ΑΜΑ using bifunctional ATRP macroinitiators.

Macroinit. type CuX

No

ΑΜΑ Bpy in g (mmol)

in g (mmol) BPA-PC (3000) 1.00(0.3) TMC-BP-PC (4300) 2.00(0.5) PPO (6000) 1.00(0.2) PPO (6000) 1.00(0.2) PPO (6000) 1.00 (0.2)


1

2

3

4

5

Anisole in mL

Τ

t

°C

h

0.19 (1.3)

0.52 (3.3)

0.25 (2.0)

3

90

16

0.27 (1.9)

0.73 (4.7)

0.50 (4.0)

6

90

2

0.10 (0.7)

0.26 (1.7)

0.25 (2.0)

3

90

0.8

0.10 (0.7)

0.26 (1.7)

0.25 (2.0)

3

90

0.5

0.10 (0.7)

0.26 (1.7)

0.25 (2.0)

3

70

1.5

The polymerization was quenched by addition of CH C1 for PC or toluene for PPO. Precipitation was carried out in methanol in all cases. The analytical data are found in table 2. 2

2

NMR data: modified bisphenol A PC, carbonate repeating unit: δ = 1.68 (s, 6H, CH ), 7.16 (m, 4H, CHC-O), 7.25 (m, 4H, C#CC(CH ) ) ppm; ΑΜΑ repeating unit: 8 = 0.8- 2.8 (br, 5H, C H and C H , backbone), 4.3 - 4.7 (br s, 2H, OCH ), 5.0 - 5.5 (br m, 2H, OCH CHC# ), 5.7 - 6.1 (br m, 1H, OCH Cfl) ppm. 3

3

2

2

2

3

2

2

2

modified TMC bisphenol PC, carbonate repeating unit: δ = 0.34/0.98 (s, 3H / s, 6H, CH ), 0.87/1.37 (d, V*12 Hz, 2H, C# CHCH ax and eq), 1.16 (m, 1H, C#CH ), 1.8 - 2.1 (complex, 2H, C# C(CH ) ), 2.44/2.66 (d, J=13.2 Hz, 1H / d, 0=13.2 Hz, 1H, CH(CH )C// C(CH ) ax and eq), 6.6 - 7.5 (complex, 8H, Hématie) ppm; ΑΜΑ repeating unit: δ = 0.8 - 2.8 (br, 5H, C H and C H , backbone), 4.3 - 4.7 (br s, 2H, OCH ), 5.1 - 5.5 (br m, 2H, OCH CHC# ), 5.7 6.1 (br m, 1H, OCH C//) ppm. 3

2

3

3

3

2

3

2

3

3

2

2

2

2

2

3

2

2


158 modified PPO, phenylene oxide repeating unit: δ = 2.09 (s, 6H, CH ), 6.47 (s, 2H, CH) ppm; ΑΜΑ repeating unit: δ = 0.7 - 2.4 (br, 5H, C H and C H , backbone), 4.3 - 4.7 (br s, 2H, OCH ), 5.1 - 5.5 (br m, 2H, OCH CHC# ), 5.8 6.1 (br m, 1H, OCH C#) ppm. 3

2

2

2

3

2

2

Table 2: Results of the polymerization reactions presented in Table 1.

Macroinit. Product Downloaded by PENNSYLVANIA STATE UNIV on August 10, 2012 | http://pubs.acs.org Publication Date: September 7, 2006 | doi: 10.1021/bk-2006-0944.ch012

No.

M^GPC

M„,GPC

(MJMJ 1 2

•3

J

4 5

4400 (1.60) 4500 (1.31) 7800 (1.62) 7800 (1.62) 7800 (1.62)

5400 (1.15) 5000 (1.69)

Yield in g

Mol ratio ofRU Average number of initial block ofAMA-RU per chain end and ΑΜΑ

0.56

72:28

2.4

1.68

66:34

3.0

0.83

87: 13

3.4

0.83

89: 11

2.7

crosslinked 16500 (4.22) 12500 (2.11)

Results and Discussion Random Copolymers In contrast to other monomers (e.g., MMA), in the atom transfer radical polymerization and copolymerization of ΑΜΑ, side reactions have to be taken into consideration. Beside regular chain growth, the following reactions are discussed (cf. Scheme 2): (i) The allyl group of an ΑΜΑ monomer is added to the active chain end resulting in a methacrylic side group, (ii) A hydrogen atom of the allyl side group in the copolymer is transferred from the allyl position to the radical chain end leading to chain termination (the allyl radical formed is not reactive enough to continue chain growth), (iii) The radical chain end is added to the allyl ester side group of the polymer resulting in an additional branch; in free radical polymerization, this reaction results in cross-linking, whereas the radical species formed is terminated irreversibly in an ATRP. The importance of these and other side reactions depending on conversion and monomer/initator ratio are discussed elsewhere (1). Side reactions become less significant if the


159

AMA/comonomer ratio is decreased. A kinetic investigation of the ATRP of a 20/80 mixture of ΑΜΑ and styrene using (l-bromoethyl)benzene as the initiator and CuBr/Bpy as the catalyst at 130°C exhibit linear semilogarithmic plots up to ca. 70% overall conversion for both monomers (cf. Figure la).


further chain growth or irreversible termination (macromonomer formation)

(i) Irregular ΑΜΑ addition + ΑΜΑ

Scheme 2: Proposed side reactions in the atom transfer radical polymerization in the presence ofΑΜΑ under participation of the allyl ester groups.

The increase of M„ with conversion shows a linear dependence although values are slightly higher than expected at low conversion. This was attributed to the preferential incorporation of ΑΜΑ during early stages of the polymerization and the higher mass of the ΑΜΑ repeating unit compared to the St repeating unit. However, reactivity ratios of St and ΑΜΑ were. not determined. The controlled nature of the polymerization process was indicated



^

25

50 / / min

75

100

125 150 0.0

1000

S 2000

s.

C L 3000-I

4000

5000 -|

6000

'—~—ι 0.2

1 « 1 0.4 0.6 tota! conversion (GC)

'

0.8

Figure 1: Copolymerization of a 80/20 mixture ofstyrène (St) and allyl methacrylate (ΑΜΑ) using the system (1-bromoethyl)benzene, CuBr/2,2 '-bipyridine in η-butyl acetate at 130 °C ([StJo/fAMAJo/flJo = 40:10:1). (a) First order plots (closed boxes represent values for St, open boxes for AMA); (b) molecular weight as a function of total monomer conversion.

0.5-

1.0-

2.0-

2.5-


161 by monomodal molecular weight distributions for all experimental points presented in Figure 1 (MJM < 1.37). P(St-co-AMA) can be cross-linked "on demand" using initiators such as 2,2'-azobis(2-methylpropionitrile) (AIBN) or bis(2,4,6-trimethylbenzoyl)phenyl-phosphine oxide (Irgacure 819) as reported earlier (1). In further studies, it turned out that in photochemical cross-linking, the extent of cross-linked material can be greatly increased upon addition of a small amount of a low molecular weight cross-linking agent, e.g., diethyleneglycol dimethacrylate (DegDMA): Results found for P(St-co-AMA) containing 10 mole% of ΑΜΑ repeating units are presented in Figure 2. It is obvious that at the same concentration of DegDMA (including the absence of DegDMA) an increase on the concentration of the photoinitiator from 1 to 10 wt.% leads to an increase of cross-linked material. However, at the same concentration of photoinitiator, the situation is different below 1 wt.% DegDMA and above this concentration. It must therefore be concluded that a low concentration of photoinitiator is the limiting parameter for the amount of cross-linking reaction. For P(MMA-co-BMA-co-AMA) the same tendencies during cross-linking were observed (2).


n

Block Copolymers Polycondensation processes yielding high performance materials frequently involve reactive monomers with nucleophilic functional groups (e.g., OH or NH ) and electrophilic functional groups (e.g., -COOR, -COC1, -NCO, OCOC1). Under suitable reaction conditions, amino or hydroxy telechelics are obtained. These telechelics can readily be converted into macroinitiators by amidation or esterification (6) using suitable acid halides such as 2-bromoisobutyryl bromide or a-chlorophenylacetyl chloride. In the present paper, the corresponding macroinitiators were applied for the ATRP of ΑΜΑ yielding cross-linkable polycondensation products (cf. Scheme 3). Only a few ΑΜΑ repeating units at each chain end are expected to be sufficient to achieve a cross-linkable polycondensation product. Therefore, the structures ought not to be termed as "block copolymers" but instead be referred to as chain end functionalized or modified polymers, i.e., the properties of the polycondensation product are not changed significantly. In our group, recent focus was laid on the preparation of polycarbonate (PC) (4) and polyphenylene oxide (PPO) (5) macroinitiators. Hydroxy telechelic bisphenol A polycarbonate was prepared by melt polycondensation of bisphenol A and diphenyl carbonate using La(acac) as transesterification catalyst (4). Analogously, hydroxy telechelic TMC bisphenol polycarbonate was achieved 2

3


162 100

80

φ o>

Φ

Ο)

Β

60

c Φ

5


CO

1

40 20

Z2L wt.% DegDMA wt.% 1-819

iiimm ο

1

5

10

10

10

10

10

10

1

5

10

Figure 2: Photochemical cross-linking ofP(St-co-AMA) (10 mol% ΑΜΑ repeating units) using variable concentrations of photoinitiator (Irgacure 81 and a low molecular weight cross-linking agent (DegDMA), irradiation time: h; the weight percent ofgel is referred to the total mass of the prepolyme photoinitiator and DegDMA.

Scheme 3: Concept for the preparation ofprepolymers with allyl ester side groups via macroinitiators.


163


from diphenyl carbonate and TMC bisphenol (3,3,5-trimethylcyclohexyl bisphenol, (cf. Scheme 4, top); TMC bisphenol units cause a significant increase of the glass transition temperature compared to bisphenol A polycarbonate (7,8). The hydroxy telechelic polycarbonates were then converted into bifunctional ATRP macroinitiators by esterification with 2-bromoisobutyryl bromide (BiBB). Bifunctional hydroxy telechelic PPO was prepared by phase transfer catalyzed oxidative coupling of 4-bromo-2,6-dimethylphenol based on a procedure reported by Percée (cf. Scheme 4, bottom) (9,10). According to the literature (5), the PPO was esterified using BiBB to yield a bifunctional macroinitiator.

BPA and TMC-BP Polycarbonate with ΑΜΑ End Groups Due to the good solubility of BPA polycarbonate in many organic solvents, we started our investigation of the ATRP of ΑΜΑ using a BPA-PC macroinitiator. To prevent premature cross-linking, Bpy was employed as the ligand and the reaction was carried out at mild conditions (16 h at 90 °C in anisole). According to H NMR spectroscopy, the macroinitiator employed had an M„ of ca. 3000. Compared to the mass of the macroinitiator, 25 wt.% ΑΜΑ were used for the polymerization reaction. The presence of ΑΜΑ repeating units in the product is proven by the *H NMR spectrum. The ratio of repeating units was determined by comparison of the integrals of the aromatic protons of the carbonate repeating units (6.5 - 7.6 ppm) and the -C/f=CH proton of the allyl group (5.7 - 6.1 ppm). From these integrals, a molar ratio of 72:28 of carbonate/ΑΜΑ repeating units was calculated. The average number of ΑΜΑ repeating units per chain end is thus 2.4. GPC reveals a bimodal molecular weight distribution indicating branching reactions during polymerization (reactant: M„= 4400, MJM = 1.60; product: M„ = 5400, MJM = 2.13). The obtained end group functionalized BPA-PC was cross-linked photochemically. Therefore, polymer and photoinitiator were dissolved in CH C1 and - after evaporation of the solvent - the film prepared was irradiated for 9 h at a distance of 10 cm between the UV lamp and the sample. Since BPA polycarbonate repeating units contain aromatic groups and hence show significant UV absorption, Irgacure 819 was used as the photoinitiator analogously to the styrene copolymers. To assure a sufficient generation of radicals, 20 wt.% of initiator were used referring to the polymer mass (for technical application, this amount should be reduced). Furthermore the influence of a low-molecular weight cross-linking agent (DegDMA) on the extent of the cross-linking reaction was investigated. It turned out that the addition of l

2

n

2

n

2


164

La(acac) Downloaded by PENNSYLVANIA STATE UNIV on August 10, 2012 | http://pubs.acs.org Publication Date: September 7, 2006 | doi: 10.1021/bk-2006-0944.ch012

3

200-220 °C -PhOH

BiBB pyridine THF, 80 °C

Scheme 4: Preparation of bifunctional ATRP macroinitiators based on TMC bisphenol polycarbonate (top) andpolyfphenylene oxide) (bottom) facac = acetyl acetonate, BiBB = 2-bromoisobutyryl bromide].



165 DegDMA is not necessary to obtain a nearly quantitative degree of cross-linking (degrees of cross-linking higher than 90 % were obtained in all cases), most probably due to the high concentration of photoinitiator. It is remarkable that only 2-3 ΑΜΑ repeating units are sufficient to obtain nearly quantitative crosslinking. To assure that cross-linking is really attributed to the presence of ΑΜΑ repeating units (and not by a reaction at the PC aryl units caused by UV radiation) the PC macroinitiator (without ΑΜΑ units) was treated under the same conditions. Cross-linked product was not formed under these conditions. Moreover, we compared the thermal properties of the end group funtionalized BPA polycarbonate before and after cross-linking. Before crosslinking, the T of the polymer is 98 °C. This is remarkably lower than the literature value of BPA-PC (ca. 150 °C) which was attributed to the following: (i) presence of the short PAMA segments (partial miscibility appears possible); (ii) residual amounts of the photoinitiator (plasticizer effect); (iii) most important, the low degree of polymerization of the polycarbonate reduces its T . After cross-linking, T rises to 148 °C (i.e., literature value) because the molecular weight is now significantly increased. DSC reveals still a sharp glass transition of the cross-linked material (and no broadening) which was attributed to the low cross-linking density with the PC segments working as spacers between the cross-linking sites. Due to the successful application of the ΑΜΑ end group concept to bisphenol A polycarbonate, we decided to apply the analogous strategy to TMC bisphenol polycarbonate. The polycarbonate used had a M of 4300 (NMR). Again, polymerization was carried out using 25 wt.% ΑΜΑ (referring to the mass of the macroinitiator) in anisole at 90 °C (catalyst: CuBr/Bpy, t = 2 h). The ratio of repeating units in the copolymer obtained is 66:34 (carbonate to ΑΜΑ, according to H NMR), i.e., 3.0 repeating units of ΑΜΑ are attached to each chain end on average. The GPC curve of the product is multimodal (reactant: M = 4500, MJM = 1.31; product: M = 5000, MJM = 1.69) as a result of branching reactions. Photochemical cross-linking was carried out analogously to the procedure described before. Two parameters were varied: the amount of DegDMA added (0, 5, and 10 wt.%); in the absence of DegDMA, the concentration of photoinitiator was varied to investigate whether a lower amount of initiator is still sufficient. Figure 3 reveals the following trends: (i) cross-linking of functionalized TMC-BP polycarbonate is obviously less efficient than the one of functionalized BPC polycarbonate, although the number of ΑΜΑ repeating units at the chain ends is similar, (ii) To obtain a high amount of cross-linked product, a high concentration of photoinitiator is required. The amount of cross-linked material exceeds 60 % only if 20 wt.% of photoinitiator is added. At the same initiator concentration, the cross-linked product was raised to 74 % by addition of 5 wt.% of DegDMA. A further increase of the DegDMA concentration does not result g

g

g

n

!

n

n

n

n


166 in a significant increase of cross-linked material (77 % insoluble product using 10 wt.% DegDMA). As in the case of BPA polycarbonate, the glass transition temperature of the modified TMC bisphenol polycarbonate increased upon cross-linking, although less pronounced (before cross-linking: 161 °C, afterwards: 172 °C).

100


80

60

40

1 20

0 wt.% DegDMA wt.% 1-819

Figure 3: Photochemical cross-linking of end-group modified TMC bispheno PC using variable amounts ofphotoinitiator (Irgacure 819) and DegDMA; the weight amounts are referred to the total mass of the prepolymer reactant an DegDMA.

PPO with ΑΜΑ End Groups Investigations were carried out using a bifunctional PPO macroinitiator (Μι,ΜΛΓ^ΟΟΟ). Reaction conditions of the ΑΜΑ polymerization were analogous to the conditions described for polycarbonates (CuBr/Bpy, 90 °C, anisole, 25 wt.% of ΑΜΑ referring to the initiator mass). Gelation was observed after only 50 min (cf. Table 1 and 2, entry 3). Obviously, the tendency towards cross-linking is increased in the presence of PPO segments. The reasons for that are found in hydrogen abstraction at the methyl group of the dimethylphenylene oxide repeating unit and in radical combination (basically these observations coincide well with results reported for atom transfer radical coupling of polymers containing PPO segments (11)). To prevent premature cross-linking, two variations of the polymerization procedure


167 were carried out: (i) decrease of the polymerization time (cf. Table 1, entry 4) and (ii) a lower polymerization temperature (Table 1, entry 5). When the reaction was quenched after 30 min (Tables 1 and 2, entry 4), a soluble product was obtained with a molar phenylene oxide/ΑΜΑ ratio of 87:13 (NMR), i.e., 3.4 ΑΜΑ repeating units are located at each chain end. The polymerization is accompanied by branching reactions (a preliminary step to the gelation observed after 50 min) as indicated by the multimodal molecular weight distribution of the product and a significant increase of the M„ in comparison to the macroinitiator used (reactant: M = 7800, MJM = 1.62; product: M = 16500, MJM = 4.22). In an alternative experiment, the polymerization temperature was decreased from 90 °C to 70 °C (entry 3) while the reaction was carried out under analogous conditions for 90 min. The composition of the product is similar to the one obtained at 90 °C/30 min: The molar ratio of phenylene oxide to ΑΜΑ is 89:11 (i.e., 2.7 ΑΜΑ repeating units at each chain end). However, GPC reveals that cross-linking is less significant in this case than at 90 °C: The weight distribution remains monomodal even though slightly broadened if compared to the macroinitiator (reactant: M„ = 7800, MJM = 1.62; product: M = 12500, MJM = 2.11). The product obtained at 70 °C is more comparable to the modified polycarbonates described before. It was therefore used for further investigation. Photochemical cross-linking of end-group modified PPO was carried out on films prepared from a solution of the polymer and of Irgacure 819 (20 wt.% referring to the polymer) in CH C1 /CHC1 (1:1 v/v). Irradiation was carried out for 9 h at a distance of 10 cm between the UV lamp and the sample. Even if DegDMA was not added to the sample, the polymer was quantitatively reacted to an insoluble product. In contrast to similar reactions involving the polycarbonates (vide supra), a change in color was observed (reactant: bright yellow, most likely the color of the photoinitiator; product: dark brown). This is an indication for reactions at the aromatic rings. This assumption is supported by DSC measurements: In contrast to the modified polycarbonates, the T of the modified PPO did not increase upon cross-linking, but decreases from 199 °C to 180 °C. This is explained by the formation of branched structures and the addition of flexible groups by radical reactions at the PPO repeating units. In a control experiment, the procedure was carried out using the pure PPO macroinitiator (irradiation of a PPO film using 20 wt.% Irgacure 819 in the absence of ΑΜΑ repeating units). The reaction was carried out both in the absence and presence of 5 and 10 wt.% DegDMA. The formation of crosslinked product was not observed. However, the color changed into brownish upon irradiation as well. The product obtained was investigated by means of GPC. An increase in molecular weight and polydispersity was detected in all cases, indicating branching reactions caused by radical attacks at the PPO repeating units (reactant macroinitiator: M = 12700, MJM = 1.61; product obtained after 9 h irradiation without addition of DegDMA: M = 20000, MJM = 2.63). n

n

n


n

n

n

n

2

2

3

g

w

n

w


n

168


The described procedure is not limited to PC or PPO structures. The concept can easily be transferred to a wide variety of polymers, e.g., polyethylene oxide, polypropylene oxide, poly(tetrahydrofuran), polysulfones, polyethersulfones, polyesters, polysiloxanes, polyamides, polyimides, and polyetherketones. Thus, a whole tool box of cross-linkable materials is now accessible. Transfer to new polymer classes is only limited by the availability of appropriate macroinitiators which might be affected by solubility issues. However, in the case of polyethylene oxide (8), poly(tetrahydrofuran) (9), polysulfones (2), and polysiloxanes (10), macroinitiators are readily available according to the literature. Microstructuring of Polymer Films by means of Photolithography Microstructuring of polymer layers on solid substrates was achieved by laterally resolved photochemical modification of the layers. The polymer layer functions as a negative resist (Scheme 5).

a)

b)

Scheme 5: Microstructuring of a prepolymer layer by means of photolithography: a) spincoating of the layer on the substrate; b) UV irradiation through a mask; c) resist development. Irradiation of the layers with UV light through a lithography mask results in the cross-linking of the polymer in the exposed areas. By subsequently placing the substrate in a solvent able to dissolve the non cross-linked polymer (resist development), only the UV exposed polymer pattern is left on the substrate. In order to prevent swelling of the cross-linked material during development, a theta solvent should be used. Microstructuring experiments were performed with P(St-co-AMA) and P(MMA-co-BMA-co-AMA) layers on silicon and Titanium-Nickel alloy substrates. Cross-linking of both polymers was supported by addition of photoinitiators and cross-linking agents: 20 wt.% Irgacure 819 was added to P(St-co-AMA), 0.2 wt.% DMPAP and 10 wt.% DegDMA were added to P(MMA-co-BMA-co-AMA). TEM grids with hexagonal holes where positioned on the spincoated polymer layers to act as a shadow mask during the irradiation step. Figure 4 shows the result of the photolithographic patterning of a (MMA-co-BMA-co-


16?


ΑΜΑ) layer. The hexagonally shaped areas represent the exposed polymer left on the surface after resist development in acetonitrile. The areas in between are bare TiNi substrate. The dimensions of the obtained polymer structures are in good agreement with the pattern on the TEM grid. Similar results were obtained on silicon substrates and with P(St-co-AMA).

Figure 4: Microstructured P(MMA-co-BMA-co-AMA) polymer layer (molar ratio MMA:BMA:AMA = 62:28:10) on a TiNi substrate. After irradiation for 2 h, the non cross-linked polymer was removed using acetonitrile (24 h at r. T Conclusions A new approach to cross-linkable polymers by atom transfer radical copolymerization of ΑΜΑ was reported. Tailored prepolymers containing ΑΜΑ repeating units were prepared by two concepts: (i) random copolymerization of ΑΜΑ and a vinyl comonomer (styrene or methacrylates), (ii) ATRP of ΑΜΑ using a macroinitiator. The latter approach offers the potential for a whole range of prepolymers starting from hydroxy or amino telechelic polycondensation products (e.g., high performance materials). Random copolymers with allyl ester side groups were successfully used for thin film microstructuring. The presented prepolymers therefore represent a novel class of photoresists for photolithography and 3D-lithography. The results presented in this report complement earlier research on functional well defined polymers prepared via Atom Transfer Radical Polymerization (15-17).

Acknowledgements The authors would like to thank Dr. G. Schmidt/Ciba Specialty Chemicals for the donation of Irgacure photoinitiators. Financial support of the Center of


170 Advanced European Studies and Research (caesar) and of the Fonds der Chemischen Industrie is gratefully acknowledged.

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Polymers with Pendant Allyl Ester Groups Prepared via Atom Transfer

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