Polymer Models for Photosynthesis - ACS Publications - American

Chapter 30

Polymer Models for Photosynthesis 1

James E . Guillet, Yoshiyuki Takahashi , and Liying G u

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Downloaded by QUEENSLAND UNIV OF TECHNOLOGY on October 31, 2014 | http://pubs.acs.org Publication Date: November 30, 1987 | doi: 10.1021/bk-1987-0358.ch030

Department of Chemistry, University of Toronto, Toronto M5S 1A1, Canada Macromolecules containing aromatic chromophores can dis play efficient electronic energy transfer to low-energy traps. By analogy with the biological process of photo synthesis, we have termed such molecules "antenna mole cules". Synthetic polymers can thus mimic the function of the light-harvesting chlorophyll pigment layers with out reproducing their exact structure. We have linked aromtic chromophroes in both organic and water-soluble polymers which provide useful antennas for solar photo chemistry. For example, antennas consisting of phenyl anthracene groups are effective in increasing the ab sorption cross-section for tetraphenyl porphine groups by at least an order of magnitude. Sulfonated poly(2vinylnaphthalene) polymers are also useful catalysts for singlet oxygen reactions in aqueous solution. As a model of the reaction center in natural photosynthesis we have prepared copolymers of acrylic acid containing small amounts of porphyrin (P) and anthraquinone (Q) moieties. The conformation of these polymers in aqueous solution is highly dependent on both the pH and ionic strength of the solvent, and this can be used to control the average distance between Ρ and Q. ESR measurements confirm the formation of separated ion pairs (Ρ · and Q ·) when the porphyrin group is irradiated in solution at -40°C. -

+

The primary photochemical step in photosynthesis i s now generally recognized to be a one-electron transfer from the singlet excited state of a chlorophyll species (Chl) to an electron acceptor. This reaction takes place within a reaction center protein that 1

Current address: Oji Paper Co., Ltd., 1-10-6 Shinonome Koto-ku, Tokyo, Japan Current address: Institute of Chemistry, Academia Sinica, Beijing 100 080, People's Republic of China

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0097-6156/87/0358-0412$06.00/0 © 1987 American Chemical Society

In Photophysics of Polymers; Hoyle, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

30. G U I L L E T E T A L .

Polymer Models for Photosynthesis

413


spans the thylacoid membrane of the chloroplast organelle o f green leaves and algae. In the simplest photosynthetic systems the electron acceptor contains the quinone moiety, such as an ubiqui none, menaquinone or plastoquinone. An e s s e n t i a l feature of t h i s process i s that the donation of an electron must lead to a separa t i o n of the charged species C h i * and Q" so that they may undergo further reactive steps i n the photosynthetic sequence. In green plants the primary charge-separation process occurs i n reaction s i t e s which contain only a small f r a c t i o n of the t o t a l pigment material. The bulk of the chlorophyll i n the chloroplast i s photochemically i n e r t , functioning as an "antenna pigment" by transferring l i g h t through non-radiant interactions to the reac t i o n centers. In t h i s way the turnover rate f o r reactive s i t e s i s increased, the occurrence o f t h i s energy-transfer process having the same e f f e c t as i f the e x t i n c t i o n c o e f f i c i e n t of the reactive center were increased by a factor of over 100. In b i o l o g i c a l pho tochemistry, t h i s e f f e c t i s known as the "antenna e f f e c t " . I t has been shown that macromolecules containing aromatic chromophores can also d i s p l a y e f f i c i e n t e l e c t r o n i c energy transfer to low-energy traps [1]. By analogy with the b i o l o g i c a l process of photosynthesis, we have termed such molecules "antenna mole cules". Macromolecules containing chromophores attached t o a p o l ymeric backbone often d i s p l a y very high e f f i c i e n c y of s i n g l e t energy transfer. The function of the connecting macromolecular chain and the plant thylacoid membrane i s s i m i l a r i n that both serve as anchors supporting high l o c a l concentrations of the chromohores. Synthetic polymers can thus mimic the function of the light-harvesting pigment layers without reproducing t h e i r exact structure. In e a r l i e r studies i n these laboratories i t has been shown that polymers containing repeating naphthalene o r phenanthrene groups and small numbers (from 0.1 t o 2%) of traps such as anthra cene, anthraquinone, or phenyl ketone, demonstrated s i n g l e t e x c i ton transfer from the absorbing s i t e i n the antenna to the trap [1]. The e f f i c i e n c y of energy transfer i n the trap can be evalu ated i n terms o f the quantity \, which i s defined as the number of photons transferred to the trap divided by the number of photons absorbed by the antenna. The e f f i c i e n c y can be calculated e i t h e r from the emission from the trap i f the trap i s a fluorescing moiety, or from the quenching o f emission from the antenna chromophores. An a l t e r n a t i v e way of expressing t h i s e f f i c i e n c y i s t o c a l c u l a t e the number of antenna donor chromophores, n, quenched by each trap. Figure 1 shows the value of η f o r antennas containing naphthalene repeating units with the three traps mentioned e a r l i e r [2]. I t can be seen that depending on the donor-trap combination, the value o f η can range from about 50 to about 150 i n these systems. This repre sents an increase i n the absorption cross-section f o r the trap o f


414

PHOTOPHYSICS OF POLYMERS


one t o two o r d e r s o f magnitude, depending on t h e m o l a r e x t i n c t i o n c o e f f i c i e n t o f t h e two s p e c i e s a t t h e w a v e l e n g t h o f e x c i t a t i o n . I n l a t e r work i t was shown t h a t w a t e r - s o l u b l e antennas c o u l d be made b y c o p o l y m e r i z i n g a r o m a t i c monomers s u c h a s v i n y l naphtha lene and naphthylmethyl methacrylate w i t h p o l y e l e c t r o l y t e s such as a c r y l i c a c i d [ 3 , 4 ] . The h i g h e f f i c i e n c y o f t h e s e antennas i n d i l u t e aqueous base was a t t r i b u t e d t o t h e h y p e r c o i l i n g o f t h e p o l y ( a c r y l i c a c i d ) c h a i n t o g i v e a pseudo m i c e l l a r s t r u c t u r e such a s t h a t i l l u s t r a t e d s c h e m a t i c a l l y i n F i g u r e 2. We b e l i e v e t h a t s u c h s t r u c t u r e s a r e formed s p o n t a n e o u s l y i n s o l u t i o n due t o t h e h y d r o p h o b i c i n t e r a c t i o n s o f t h e l a r g e a r o m a t i c components s t a b i l i z e d b y the i n t e r a c t i o n o f water w i t h t h e h y d r o p h i l l i c c a r b o x y l anions. I t was l a t e r shown t h a t o t h e r t y p e s o f p o l y e l e c t r o l y t e s i n v o l v i n g p a r t i a l s u l f o n a t i o n o f p o l y ( v i n y l n a p h t h a l e n e ) [5,6] a n d copolymers o f a r o m a t i c monomers w i t h s t y r e n e s u l f o n a t e w o u l d a l s o l e a d t o polymers w h i c h i n aqueous s o l u t i o n a c h i e v e d t h i s h y p e r c o i l e d conf iguration. I t t h e r e f o r e seemed l o g i c a l t o a p p l y t h e s e same p r i n c p l e s t o t h e problem o f p r o d u c i n g a model o f t h e a c t i v e s i t e i n p h o t o s y n t h e s i s . I n t h e s i m p l e s t n a t u r a l p h o t o s y n t h e t i c systems t h e f i r s t c h e m i c a l s t e p i s t h e t r a n s f e r o f a n e l e c t r o n from a p o r p h y r i n com pound t o a q u i n o n o i d s t r u c t u r e . T h i s p r o c e s s i s i n i t i a t e d b y e x c i t a t i o n t r a n s f e r from a n t e n n a c h l o r o p h y l l pigments t o a p o r p h y r i n i n t h e a c t i v e c e n t e r . A t t e m p t s have been made t o mimic t h i s p r o c e s s b y i n c l u d i n g p o r p h y r i n s a n d quinones i n f i l m s , v e s i c l e s a n d m i c e l l e s w i t h v a r y i n g degrees o f success. An a l t e r n a t i v e approach [7] i s t o l i n k a p o r p h y r i n Ρ g r o u p c o v a l e n t l y t o a quinone Q b y a c h a i n o f methylene g r o u p s . By a d j u s t i n g t h e l e n g t h o f t h e c h a i n i t was p o s s i b l e t o o b t a i n compounds w h i c h a r e s o l u b l e i n o r g a n i c s o l v e n t s i n w h i c h p h o t o e l e c t r o n t r a n s f e r from Ρ t o Q c a n be ob s e r v e d . However, a f u r t h e r r e q u i r e m e n t i s t h a t t h e r a d i c a l i o n s p e c i e s c r e a t e d i n t h i s f i r s t s t e p be i s o l a t e d from each o t h e r s o t h a t t h e y do n o t recombine a n d l o s e t h e e x c i t a t i o n e n e r g y a s h e a t . I t was f o u n d e x p e r i m e n t a l l y t h a t i f t h e p o r p h y r i n a n d quinone a r e t o o c l o s e t o g e t h e r b a c k - t r a n s f e r w i l l o c c u r r e a d i l y and d e t e c t i o n o f t h e s t a b i l i z e d i o n r a d i c a l s p e c i e s w i l l be d i f f i c u l t . However, t h e r e a p p e a r s t o be a d i s t a n c e o f a p p r o x i m a t e l y 10 A a t which e l e c t r o n t r a n s f e r c a n occur from t h e e x c i t e d p o r p h y r i n t o t h e quinone w h i l e t h e b a c k - t r a n s f e r p r o c e s s i s s u f f i c i e n t l y s l o w t h a t r a d i c a l i o n s c a n be o b s e r v e d b y ESR a n d o t h e r t e c h n i q u e s . F o r example, M c i n t o s h et al. [8] have d e t e c t e d a c h a r g e - s e p a r a t e d s p e c i e s b y e l e c t r o n p a r a m a g n e t i c resonance s p e c t r o s c o p y o f a num b e r o f c o v a l e n t l y l i n k e d P-Q compounds. I n o u r r e c e n t work we d e c i d e d t o t a k e advantage o f t h e h y p e r c o i l i n g e f f e c t s observed i n p o l y ( a c r y l i c a c i d ) c o n t a i n i n g l a r g e h y d r o p h o b i c groups t o s e e i f we c o u l d f i n d c o m p o s i t i o n s c o n t a i n i n g p o r p h y r i n s a n d quinones where t h e d i s t a n c e between t h e Ρ a n d Q groups c o u l d be a d j u s t e d b y c h a n g i n g t h e pH o r i o n i c s t r e n g t h o f t h e s o l u t i o n . A c c o r d i n g l y , s e v e r a l polymers were s y n t h e s i z e d b y c o p o l y m e r i z a t i o n w i t h a c r y l i c a c i d w i t h monomers o f t h e s t r u c t u r e


30.


GUILLET ET AL.

415


200

η 100

-ο 2

F i g u r e 1. Number o f n a p h t h a l e n e donors quenched p e r t r a p η a s a f u n c t i o n o f t r a p c o n c e n t r a t i o n f o r ( O ) 1NMMA-2AQMMA, ( Δ ) 2VNPVK, a n d (•) 2NMMA-9AMMA i n 2MeTHF a t 77K ( ~ 4 χ 1 0 ~ M i n naph t h a l e n e ) . ( R e p r i n t e d from Ref. 2. C o p y r i g h t 1985 American Chemical S o c i e t y . ) 4

po-coTcoo-

F i g u r e 2. P r o p o s e d h y p e r c o i l e d s t r u c t u r e o f a n t h r a c e n e endt r a p p e d copolymers o f a c r y l i c a c i d a n d NMMA i n d i l u t e a l k a l i n e s o l u t i o n showing F o r s t e r r a d i i f o r v a r i o u s e n e r g y - t r a n s f e r p r o cesses .



416


shown below. The composition and molecular weights of the poly mers synthesized are shown i n Table I [9]. As can be seen, Polymer A contains about two porphyrin groups per chain, but no anthraquinone, while the second sample, B, contains one porphy r i n and about three anthraquinone groups per molecule. Comparison of the emission fluorescence emission spectra of these two poly mers on e x c i t a t i o n of the porphyrin r i n g by i r r a d i a t i o n with l i g h t i n the range of 500 to 650 nm showed some quenching of the emis sion i n aqueous solutions of polymer Β with respect to the control sample A. In a c i d conditions the aqueous solution i s a bright green color, while i n base i t i s a brownish purple. Ο

Measurements of the ESR spectra of these solutions were also made at various temperatures. In a frozen aqueous glass at -40°C the ESR signals were observed i n sample B, but not i n sample A. The G factor of the photo-driven signal at pH 11 was 2.0037 ± 0.0002, i n excellent agreement with the assumption that i t i s a spin exchange average of Ρ * G = 2.0025 an AQ" G = 2.0047. The y i e l d of P t Q" r a d i c a l p a i r s was about 3% based on double i n t e gration of the signals. Under a c i d conditions the resonance i s symmetrical with a G factor of 2.0025 which suggests that the AQ r a d i c a l ion has been protonated and that only the s i g n a l f o r P* i s observed. When the l i g h t i s turned o f f , the signal decays slowly over a period of about 15 minutes at -40°C.

Table I.

Polymer

Properties of A c r y l i c Acid Copolymers

w

a

E

mol % Ρ

5

n

v

mol % Q°

V A Β

17.5 χ 10

4

5.5 χ 10

4

2430

0.11

2.7

764

0.14

1.0

0 3.0

0.40

a . From i n t r i n s i c v i s c o s i t y i n dioxane using Κ = 76 χ 10— and a = 0.50 (see: Sandler, S. R.; Karo, W. "Polymer Syntheses", Academic Press: New York, 1977; V o l . I I , Chapter 9. b. From UV absorbance i n methanol s o l u t i o n at 412 nm based on e f o r model compound tetratolylporphine = 4.2 χ 10 . c. From UV absorbance i n MeOH at 255 nm based on e f o r model compound 2-methylanthraquinone = 5.0 χ 10 . 5

s

4



30.

G U I L L E T ET AL.


From t h e s e r e s u l t s , i t was c o n c l u d e d t h a t i n c l u s i o n o f p o r p h y r i n - q u i n o n e moitiés i n a p o l y e l e c t r o l y t e such as p o l y ( a c r y l i c a c i d ) p r o v i d e s a u s e f u l model t o s t u d y p h o t o e l e c t r o n t r a n s f e r p r o cesses observed i n a r t i f i c i a l photosynthesis. The advantage o f t h e use o f such polymers i s t h a t e x p e r i m e n t s may be c a r r i e d o u t i n aqueous, r a t h e r t h a n o r g a n i c media and t h a t t h e p r e s e n c e o f t h e s e p o l y e l e c t r o l y t e s may s t a b i l i z e some o f t h e r e s u l t a n t s e p a r a t e d i o n p a i r s . Furthermore, v a r i a t i o n s i n the e f f i c i e n c y of e l e c t r o n t r a n s f e r and o t h e r i m p o r t a n t parameters i n t h e p r o c e s s can be made by c h a n g i n g t h e pH o r i o n i c s t r e n g t h o f t h e medium. Having d e v e l o p e d a s u c c e s s f u l model f o r t h e a c t i v e c e n t e r i t was a l s o o f i n t e r e s t t o see i f one c o u l d produce a n t e n n a s t r u c tures capable of e x c i t o n t r a n s f e r t o the porphyrin moiety. The p h o t o p h y s i c a l p r o p e r t i e s o f a s u i t a b l e polymer have a l r e a d y been r e p o r t e d by Hargreaves and Webber [ 1 0 ] . The p h e n y l a n t h r a c e n e group has the n e c e s s a r y o v e r l a p o f i t s e m i s s i o n spectrum w i t h t h e a b s o r p t i o n spectrum o f t h e p o r p h y r i n r i n g . Thus i t w o u l d be expected to accept s i n g l e t e x c i t o n s migrating i n a c h a i n c o n t a i n i n g r e p e a t i n g phenyl a n t h r a c e n e groups. A c c o r d i n g l y , we s y n t h e s i z e d copolymers o f 10-phenyl 9 - a n t h r y l m e t h y l m e t h a c r y l a t e c o n t a i n i n g minor amounts o f monomer I [ 1 1 ] . E v i d e n c e f o r e f f i c i e n t energy m i g r a t i o n and t r a n s f e r t o t h e p o r p h y r i n r i n g was o b s e r v e d by measurement o f t h e e x c i t a t i o n spectrum o f t h e p o r p h y r i n f l o r e s c e n c e e m i s s i o n w i t h and w i t h o u t t h e antenna m o l e c u l e . The r e s u l t s a r e shown i n F i g u r e 4. I t i s c l e a r from t h i s f i g u r e t h a t a s u b s t a n t i a l increase i n the absorption c r o s s - s e c t i o n f o r the porphyrin has been o b t a i n e d by t h e use o f t h e p h e n y l a n t h r a c e n e antenna. v

The h y p e r c o i l e d c o n f o r m a t i o n assumed by p o l y e l e c t r o l y t e s c o n t a i n i n g l a r g e a r o m a t i c groups has a f u r t h e r i n t e r e s t i n g a p p l i c a t i o n . I t was f o u n d t h a t aqueous s o l u t i o n s o f such p o l y m e r s , when exposed t o s m a l l amounts o f w a t e r - i n s o l u b l e a r o m a t i c s d i s s o l v e d i n ether, w i l l concentrate the l a r g e hydrophobic molecules i n the center o f the h y p e r c o i l e d conformation. This i s equivalent t o h a v i n g a r e v e r s i b l e t r a p . When t h e a n t e n n a m o l e c u l e s i n t h e copolymer a r e i r r a d i a t e d w i t h l i g h t , t h e e x c i t a t i o n energy moves around on t h e i n s i d e o f t h e c o i l and i f t h e s p e c t r o s c o p i c p r o p e r t i e s o f t h e antenna and t r a p a r e p r o p e r l y chosen, most o f t h e energy w i l l be t r a n s f e r r e d t o t h e t r a p . F o r example, F i g u r e 5 shows t h e e m i s s i o n from an aqueous s o l u t i o n o f a copolymer o f s u l f o n a t e d p o l y ( v i n y l n a p h t h a l e n e ) c o n t a i n i n g s m a l l amounts o f p e r y l e n e [ 6 ] . A n a l y s i s i n d i c a t e s t h a t t h e r e i s a p p r o x i m a t e l y oe p e r y l e n e t r a p p e r polymer m o l e c u l e . I f the i r r a d i a t i o n i s c a r r i e d out i n t h e p r e s e n c e o f a i r t h e r e i s a v e r y r a p i d removal o f t h e p e r y l e n e f l u o r e s c e n c e by t h e c o n v e r s i o n o f t h e p e r y l e n e t o i t s endo p e r o x i d e v i a t h e i n t e r m e d i a c y o f s i n g l e t oxygen. The r e a c t i o n i n shown s c h e m a t i c a l l y below.




418

F i g u r e 3. (a,b) EPR s i g n a l o f poly(AA-POMA-AQ) ; (c) EPR s i g n a l o f poly(AA-POMA). EPR o p e r a t i n g c o n d i t i o n s were 0.5-mT m o d u l a t i o n a m p l i t u d e a t 100 kHz a n d 5-mW microwave power a t 9.05 Hz; t h e temp e r a t u r e was 235 K. The s p e c t r a l range d i s p l a y e d i s 10 mT. (Rep r i n t e d from Ref. 9. C o p y r i g h t 1985 American Chemical S o c i e t y . )

30

c « « 4.

Wave length (nm) F i g u r e 4. M o d i f i e d e x c i t a t i o n s p e c t r a o f copolymer I . c a l c u l a t e d spectrum b y s u b t r a c t i o n ; e x c i t a t i o n spectrum o f t e t r a t o l y l p o r p h i n e measured under t h e same e x p e r i m e n t a l c o n d i t i o n s ( m o n i t o r e d a t 650 nm). H



30. GUILLET ET AL.


430

460 490 Wavelength

520 (nm)

419

550

Figure 5. Perylene emission from an aqueous solution of s u l fonated poly(2-vinylnaphthalene) (SP2VN) before and a f t e r i r r a diation i n a i r .

10

20

Irradiation time (min) Figure 6. Rate of conversion of perylene to endoperoxide as e s t i mated from decrease i n fluorescence i n t e n s i t y at 450 nm.



420


As shown i n F i g u r e 6 t h e r a t e appears t o be f i r s t o r d e r , up t o r e l a t i v e l y h i g h degrees o f c o n v e r s i o n , and i s s e v e r a l orders o f magnitude f a s t e r t h a n i n t h e absence o f t h e polymer e l e c t r o l y t e . A f t e r t h e p e r y l e n e has been r e a c t e d i n t h i s way i t c a n be removed from t h e aqueous s o l u t i o n b y e x t r a c t i o n w i t h e t h e r , more p e r y l e n e can t h e n be added, and t h e r e a c t i o n c o n t i n u e d . We b e l i e v e t h a t t h e s e new p h o t o c a t a l y s t s may have i m p o r t a n t a p p l i c a t i o n s t o s y n t h e t i c c h e m i s t r y . Because o f t h e i r h i g h c a t a l y t i c e f f i c i e n c y a n d t h e i r a n a l o g y t o b i o l o g i c a l c a t a l y s t s which u s u s a l l y c o n t a i n a h y d r o p h o b i c p o c k e t i n which c h e m i c a l r e a c t i o n can t a k e p l a c e , we c a l l t h e s e u n u s u a l m o l e c u l e s "photozymes". The c o n f o r m a t i o n o f t h e s e p o l y m e r s can be changed r a d i c a l l y b y chang i n g e i t h e r t h e i o n i c s t r e n g t h o r t h e pH o f t h e medium. T h i s g i v e s c o n s i d e r a b l e f l e x i b l i t y i n d e s i g n i n g a system w i t h b o t h a h i g h e f f i c i e n c y f o r t r a p p i n g aromatic molecules, and f o r producing t h e d e s i r e d p r o d u c t . However, p o t e n t i a l a p p l i c a t i o n s f o r t h e s e new systems have y e t t o be d e v e l o p e d . In c o n c l u s i o n , i t i s now p o s s i b l e t o s t u d y t h e p h o t o c h e m i s t r y o f a l a r g e number o f i n t e r e s t i n g o r g a n i c m a t e r i a l s i n aqueous s y s tems u s i n g a s p h o t o c a t a l y s t s p o l y e l e c t r o l y t e s c o n t a i n i n g s e n s i t i z i n g antenna groups. The aqueous medium p r o v i d e s many p o t e n t i a l advantages. I n a d d i t i o n t o b e i n g a v e r y cheap s o l v e n t , i m p o r t a n t v a r i a t i o n s i n t h e p h o t o c h e m i s t r y can be o b s e r v e d b y c o n t r o l l i n g t h e pH a n d i o n i c s t r e n g t h o f t h e s o l u t i o n . Acknowledgments We acknowledge t h e f i n a n c i a l s u p p o r t o f t h i s r e s e a r c h b y t h e Natu r a l S c i e n c e s a n d E n g i n e e r i n g R e s e a r c h C o u n c i l o f Canada. Y.T. i s g r a t e f u l t o t h e O j i Paper Co. L t d . f o r f i n a n c i a l s u p p o r t d u r i n g h i s s t u d i e s a t the U n i v e r s i t y o f Toronto. Literature Cited 1. 2. 3. 4. 5. 6. 7.

Guillet, J. E. Polymer Photophysics and Photochemistry: Cambridge University Press: Cambridge, 1985; Chapter 9. Ren, X.-X.; Guillet, J. E. Macromolecules 1985, 18, 2012. Holden, D. Α.; Rendall, W. Α.; Guillet, J. E. Ann. Ν. Y. Acad Sci. 1981, 366, 11. Guillet, J. E.; Rendall, W. A. Macromolecules 1986, 19, 224. Guillet, J. E.; Wang, J.; Gu, L. Macromolecules 1986, 19, 2793. Guillet, J. E.; Gu, L., submitted for publication. (a) Kong, J. L. Y.; Loach, P. A. In Frontiers of Biological Energetics -- Electrons to Tissues; Dutton, P. L.; Leigh, J. S.; Scarpa, Α., Eds.; Academic Press: New York, 1978; Vol. 1, pp 73-82. (b) Kong, J. L. Y.; Loach, P. A. J. Heterocycl. Chem. 1980, 17, 737. (c) Kong, J. L. Y.; Spears, K. G.; Loach, P. A. Photochem. Photobiol. 1982, 35, 545.


30. GUILLET ET AL.


(a) McIntosh, A. R.; Siemiarczuk, Α.; Bolton, J. R.; Stillman, M. J.; Ho, T.-F.; Weedon, A. C. J. Am. Chem. Soc. 1983,105, 7215. (b) Ho, T. F.; McIntosh, A. R.; Bolton, J. R. Nature (London) 1980, 286, 254. 9. Guillet, J. E., Takahashi, Υ., McIntosh, A. R.; Bolton, J. R. Macromolecules, 1985, 18, 1788. 10 Hargreaves, J. S.; Webber, S. E. Macromolecules 1984, 17, 235. 11. Takahashi, Y.; Guillet, J. E., unpublished work. Downloaded by QUEENSLAND UNIV OF TECHNOLOGY on October 31, 2014 | http://pubs.acs.org Publication Date: November 30, 1987 | doi: 10.1021/bk-1987-0358.ch030

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RECEIVED March 13, 1987


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