Synthesis, Characterization And Application Of

SYNTHESIS, CHARACTERIZATION AND APPLICATION OF POLYMERIC PHOTOINITIATORS PREPARED BY ATOM TRANSFER RADICAL POLYMERIZATION AND RING-OPENING POLYMERIZATION

under nitrogen in previously flamed and nitrogen- purged schlenk tubes equipped with magnetic stirrer. The ε-CL polymerizations were carreid out in bulk at 110oC. After a given time the mixtures were diluted with CH2Cl2 and poured into ten-fold excess of cold methanol. The polymers were collected after filtration and drying at room temperature in a vacuum for three days.

Mustafa Degirmenci, Ioan Cianga, Gurkan Hizal and Yusuf Yagci

Results and discussion Istanbul Technical University, Department of Chemistry, Maslak, Istanbul, 80626, Turkey

The following precursor initiators used for in the controlled polymerizations. O CH3

Introduction Polymers possesing side- or main- chain photoreactive groups capable of initiating polymerization reaction receive continuous interest due to their application in various fields of UV curing and in polymer synthesis1,2. In UV curing applications, the advantages expected from the polymeric photoinitiators include good compatibility, low migration, and low volatility which reduces odor problems associated with the low molar-mass photoinitiators3,4. Polymeric photoinitators are also precursors for block or graft copolymers depending on the position of the photoinitiator moiety incorporated into the polymer chain. Many side- and main-chain photoinitiators have been synthesized and their photochemistry and utilization in both applications have been reviewed extensively1-5. The aim of this paper is to prepare new kind of macrophotoinitiators of well-defined polystyrene and poly(ε-caprolactone) that have a potentiality in initiating light induced free radical polymerization. The synthetic strategy followed in this study is depicted in Scheme 1.

C 1

O CH3

C O C

O CH3

CH

Br

C

CH3

O C

4 O

O CH3 CH O C

CH

Br

C

O CH3

CH C O

CH OH

5

2 CH3 O Br

C OH CH3

CH2 O

O CH3

C C O C

2

CH

O Br

HO

CH2 O

C OH

2

CH3

3

CH3

C

CH3

6

ATRP initiators

ROP Initiators Scheme 2

The ATRP of St was carried out by using 1 and 2 and 3 as mono and bifunctional initiators, respectively. 1

2

or

or

3 o

Styrene CuBr/Bpy, 110 C, Bulk O

O CH3

O

CH3

C

C

CH CH2 CH

C O CH3

Br n

C

O CH3 CH O C

CH

or

CH2 CH

Br n

8

7

Scheme 1 CH3 O

Experimental Materials. Benzoin (B), (5) (Aldrich) was recrystalized from ethanol. 2hydroxy-2-methyl-1-phenyl propan-1-one (HMPP), Darocure 1173, and 2hydroxy-1-[4-(2-hydroxyethoxy)phenyl]-2-methyl propan-1-one (HEHMPP),(4), Irgacure 2959,(6), photoinitiators were received from Ciba Specialty Chemicals and used without further purification. Initiators for ATRP (1-3) were synthesised and described previously6. CuBr (Aldrich), 2,2’bipyridine (Merck) and pyridine (Lab-scan) were used as received. Stannous 2-ethyl-hexanoate (stannous octoate) (Sigma) was used as received. Styrene (St) methyl methacrylate (MMA) and ε-Caprolactone (CL) (Aldrich) was vacuum distilled over calcium hydride just before use. Instrumentation. 1H-NMR spectra were recorded on a Bruker 250 MHz spectrometer with CDCl3 as the solvent and tetramethylsilane as the internal standard. IR spectra were recorded on a Jasco FT/IR-3 spectrometer. UV-vis spectra were recorded on a Perkin-Elmer Lambda 2 spectrophotometer. Gel permeation chromatography (g.p.c) analyses were performed with a set up consisting of an Agilent 1100 RI apparatus equipped with three Waters ultrastyragel columns (HR series 4, 3, 2 narrow bore), with THF as the eluent at a flow rate of 0.3 mL/min and a refractive index detector. Molecular weights were calculated with the aid of polystyrene standards. Elemental analysis results were performed on a CHNS-932 LECO instrument. Polymerizations. Atom Transfer Radical Polymerization (ATRP). A round bottomflask equipped with magnetic stirrer and a lateral neck with tap was used. The system was vacuumed and back-filled with dry nitrogen several times. Catalyst (CuBr), ligand bipyridine (bpy), initiator (1 or 2 or 3) and St were introduced under inert atmosphere. The flask was placed in an oil bath warmed at 110oC and stirred at that temperature. After a given time, the mixture was diluted with THF and poured into ten-fold methanol. The solid was collected after filtration and drying at 40oC in vacuum overnight. In order to remove the complex salts from the polymers they were redissolved in THF and passed through a silicagel column followed by precipiation in methanol. Ring-opening Polymerization (ROP). Certain amounts of monomer (εcaprolactone), stannous octoate and photoinitiators (4 or 5 or 6) were added

Br

HC

H2C

CH C O CH2

n/2

CH2

O

O CH3

O

CH3

C

C

CH

C O

CH2 CH

CH3

Br n/2

9

Scheme 3 Table 1. Synthesis of macrophotoinitiators of polystyrene by ATRPa

a

Run

Initiator

[I]x102 (mol.L-1)

Time (hour)

PSt

1 2

1 1

8.5 3.5

6 7

3

1

39

4

1

7.0

5

1

6 7 8 9

7 7

Conversion, (%) 82 57

Mntheo.

8957 14694

7

7

70

3

7

53

7.0

4

7

2

17.5

3

2 3

8.75 4.4

5 5

3

4.4

6

MnGPC

Mw/M n

MnH-NMR

9600 16200

1.17 1.14

10350 nd b

2096

2491

1.12

2500

7125

7700

1.20

8200

68

9175

9700

1.12

10500

8

89

4962

4716

1.18

4857

8 9

79 25

8521 5694

8551 5649

1.17 1.14

8115 6000

9

80

17160

18000

1.15

nd b

Temp.110°C, [St]o=8,75 mol.L-1(in bulk), [I]/[CuBr]/[Bpy]: 1/1/3 for the initiator 1 and 2, [I]/[CuBr]/[Bpy]: 1/2/6 the initiator 3,b nd: not determined

The synthesis of macrophotoinitiators of poly(ε-caprolactone) depicted in scheme 4, involved the reaction of photoinitiators, namely benzoin (B).(4), 2-hydroxy-2-methyl-1-phenyl propan-1-one (HMPP),(5), and 2-hydroxy-1-[4(2-hydroxyethoxy)phenyl]-2-methyl propan-1-one (HE-HMPP),(6) with εcaprolactone (ε-CL) in the presence of stannous octoate catalyst. In view of the reported role of hydroxyl groups as initiators of the ring-opening polymerization, this reaction was expected to produce polymers containing a photoiniator group on one end or on the middle of the chain, derived from a single or two terminal units of the photoinitiators, respectively (Scheme 4).

Polymer Preprints 2002, 43(2),22

4 or 5 or 6 O

Sn(Oct)2, 110 oC, bulk

O

O CH3

O

O

C C O C CH2CH2CH2CH2CH2 O H or n CH3 10 or

CH O C CH2CH2CH2CH2CH2 O

n

H

11 O CH3

O H O CH2CH2CH2CH2CH2 C

O

C

The g.p.c. traces are unimodal and narrow indicating that no side reactions occurred. Moreover, dual detection by refractive index and UV measurements provides clear evidence for the complete functionalization. As can be seen from Figure 4, gpc trace of poly(ε-caprolactone) prepared from benzoin measured by UV (λ=330 nm) and refractive index appear at the same elution volume.(Figure 4)

O

C C O C CH2CH2CH2CH2CH2 O H n/2 CH3

O CH2 CH2 O n/2 12

Scheme 4 Table 2. Synthesisa of macrophotoinitiators of poly(ε-Caprolactone) by ROP Time (hour) 48

PCL

4

[I]x102 (mol.L-1) 41

5

45

72

12

5

45

13

6

22

Run

Init.

10 11

Mntheo

MnGPC

10

Conv. (%) 90

MnH-NMR

MnUV

2600

Mw/ Mn 1.08

2420

11

84

2150

3650

2560

2500

1.13

4200

96

11

100

3050

2500

3300

1.57

4170

90

12

100

4000

4800

4400

1.56

5100

5300

a

Temp.110°C, [ε-CL]o = 9.02 mol.L-1(in bulk), [I]/[ε-CL]: 1/20 and [Sn(Oct)2]/ [I]: 1/400 for the initiator 4 and 5, [I]/[ε-CL]: 1/40 and [Sn(Oct)2]/ [I]: 1/200 for the initiator 6

As can be seen from Table 1, and Table 2, the measured and calculated Mn values are in good agreement indicating that each initiator added to the solution generates one or two growing ends depending on the initiator functionality. By modifying the initiator concentration and polymerization time, macroinitiators with various molecular weights and low polydispersities were obtained. In the 1H-NMR spectra of the related samples can be found not only the specific signals of polystyrene and poly(ε-caprolactone) (PCL) but also absorptions belonging to the rests of initiators.

Figure 4.GPC trace of PCL, 5; refractive index signal (____) and UV signal at λ = 330 nm (----)

Figure 5. GPC traces of PSt, 9, before (a) and after photolysis

Even more convincing evidence for the presence of the alkoxy phenyl keton group was obtained from the photodegradation of polymers produced by means of ATRP. As can be seen from the GPC traces in Figure 5, after photolysis of PSt (9) in methylene chloride in the presence of hydroquinone as the radical scavenger, a significant reduction in the molecular weight was observed. The number of chain scission per macromolecule, Ns= (Mn0 / Mnt)1, is found to be 0.80, where Mn0(18000) and Mnt(10000) denotes the number avarage molecular weight before and after photolysis.

1,0

1,0

1,0

0,8

0,8

0,8

0,6

0,6

1 7

0,6

1,0 0,8 4

Absorbance

Absorbance

The incorporation of alkoxyphenyl ketone groups into polymers was also evidenced by UV absorption measurements. Figures 1 and 2 show the absorption spectra of precursors, 1 and 4, together with the Pst, 7, and PCL, 10, obtained. It can be seen that all spectra contain a characteristic absorption band of the precursor photoinitiators.

10

0,6

0,4

0,4

0,4

0,4

0,2

0,2

0,2

0,2

0,0

0,0 380

0,0

260

280

300

320

340

360

0,0 250

275

300

325

350

Figure 1. Absorption spectra of 1 (4 x 10-4 mol L-1) and 7 (2.6 g L-1) in CH2Cl2

Figure 2. Absorption spectra of 4 (2.9 x 10-4 mol L-1) and 10 (1.6 g L-1) in CH2Cl2

Figure 3 and shows the fluorescence emission of the related compounds, 5 and 11, in chloroform at room temperature. Both spectra show the vibrational structures of the benzoin chromophore. Similar behavior was noted in the fluoroscence emission studies of the other photoiniators and the corresponding polymers. These spectroscopic investigations suggest that the photochromophoric phenyl ketone groups were conserved under the polymerization conditions. 1,5 8 5 11

1,0

Intensity

Figure 6. GPC traces of PCL, 12, (a) and PCL-b-PMMA

375

Wavelength(nm)

Wavelength(nm)

6

4 0,5 2

0,0

We have also tested the photoinitiation capability of these polymers. Photochemically induced polymerization of bulk methyl methacrylate (MMA) at 25 oC with 12 produced 21 % conversion of MMA after 75 min of irradiation time. A control experiment without the polymeric initiator gave only negligible amount of polymer after the same irradiation time. Succesful blocking has been confirmed by a strong change in the molecular weight of the prepolymer and the block copolymer (Figure 6) as well as NMR spectral measurements. References (1) Yagci, Y.; in “Macromolecular Engineering: Recent Aspects”, Mishra, M. K.; Nuyken, O.; Kobayashi, S.; Yagci, Y., Eds., Plenum Press, New York, 1995, ch. 11. (2) Yagci, Y.; Macromol. Symp., 2000, 161, 19. (3) Carlini, C. ; Angiolini, L. ; Adv. Polym. Sci., 1995, 123, 12. (4) Davidson, R.S.; J.Photochem. Photobiol., A-Chem., 1993, 69, 263 (5) Yagci, Y.; Schanbel, W; Prog. Polym.Sci., 1990, 15, 551 (6) Degirmenci, M.; Cianga, I.; Yagci, Macromol. Chem., Phys., in press

0 320

340

360

380 400 420 Wavelength(nm)

440

460

Figure 3. Fluorescence spectra of 5 (1.4 x 10-3 mol.L-1) and 11 (10.7 x 10-3 mol.L-1) in CHCl3, λexc = 310 nm

Polymer Preprints 2002, 43(2),23