Photonic and Optoelectronic Polymers - American Chemical Society

Chapter 14

Third-Order Nonlinear Susceptibility of Polydiacetylene-Containing Polymeric Systems 1

2

2

3

M . P. Carreón , L. Fomina , S. Fomine , D. V. G. L. N. Rao , F. J. Aranda , and T. Ogawa Downloaded by UNIV OF GUELPH LIBRARY on October 8, 2012 | http://pubs.acs.org Publication Date: September 1, 1997 | doi: 10.1021/bk-1997-0672.ch014

3

1

2,4

2

Institute de Ciencias Nucleares and Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México, Apartado Postal 70-360, Ciudad Universitaria, Coyoacán, Mexico DF 04510, Mexico Department of Physics, University of Massachusetts, Boston, M A 07160 3

Unconventional polydiacetylenes which include polydiacetylene-containing polymers, amorphous polydiacetylenes and polydiacetylene microcrystal– host polymer composites, are described. These methods give films of high optical quality by simple casting or spin coating. Various new diacetylene molecules were synthesized and their polymerization is discussed. The third-order nonlinear optical susceptibility of these polydiacetylenes, determined by degenerated four wave mixing using 532 nm laser, lies in the range of 10 -10 esu. -11

-9

Since Wegner (7) reported in 1969 the unique polymerization of some diacetylenes in the solid state, much attention has been paid to the subject, and many studies have been reported in the literatures (2). The polydiacetylenes (PDAs) thus obtained are completely crystalline materials having conjugated chains consisting of a triple-singledouble-single bond linkage sequence, and they possess third order nonlinear optical (NLO) susceptibility (3). However, the crystalline PDAs are a group of polymers for which processing into films is not simple, and hence various techniques have been applied to obtain thin films of PDAs, including: (1) Single crystals; (2) LangmuirBlodgett (LB) membranes of aliphatic amphophilic DAs; (3) Vacuum evaporation epitaxy; (4) Solution casting; and (5) Photodeposition from solution (4). The third order susceptibility, χ( ) of the PDAfilmsthus prepared have been reported in literature (5). However, the above mentioned techniques have serious limitations. In general, it is not easy to obtain defectless large single crystals of diacetylenes which undergo topochemical polymerization, although a few have been reported in the literature (6,7). The LB membrane technique has been investigated by many (8,9) and shown to be a useful method to obtain ultra thinfilms of PDAs, and χ values of such PDAfilmshave been reported to be in the range of 10' - 10" esu (5). These values are not satisfactorily high for NLO applications, and the method is rather too 3

(3)

12

11

Corresponding author © 1997 American Chemical Society

In Photonic and Optoelectronic Polymers; Jenekhe, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1997.

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PHOTONIC AND OPTOELECTRONIC P O L Y M E R S

Downloaded by UNIV OF GUELPH LIBRARY on October 8, 2012 | http://pubs.acs.org Publication Date: September 1, 1997 | doi: 10.1021/bk-1997-0672.ch014

laborious for preparation of thick films. The vacuum evaporation-deposition technique is limited as not many diacetylenes are volatile. On the other hand, the solution casting of soluble PDAs tends to give opaquefilmsdue to crystallization into polycrystalline films. Alternative methods to obtain PDA films, which are technically much simpler than the above mentioned, include: (6) Synthesis of processable polymers which contain diacetylenes, and developing PDA networks in the polymer films; (7) Polymer-PDA composites: PDA micro crystals dispersed in amorphous polymers; (8) Polymerization of diacetylenes in the molten state. Studies on the synthesis, characterization, and properties of diacetylene (DA)-containing polymers have been recently reviewed (70, 11). Some of these DA-containing polymers can be cast intofilms,and the DA groups in the polymer films are cross-polymerized to develop PDA networks as shown in Scheme 1. However, similar to the case of DAs, not every DA group incorporated in polymers is topochemically porymerizable, some being highly radiation-sensitive and others being inert to radiation. It is interesting to attempt to disperse microcrystals of light sensitive DAs in transparent amorphous polymers to obtain PDA-containing transparent films. It is possible to prepare films with excellent optical quality by this method (12). If the crystals or particles of PDAs dispersed in the media, are smaller than the wavelength of the light, the materials are transparent. Prasad (73) has attempted to fill the micropores in transparent silica with organic functional materials, and Nakanishi et al have reported a system consisting of microemulsion of PDA crystals in water (14). Some DAs can be readily polymerized by heating them at temperatures above their melting points and the polymerized products are highly transparent, red, glassy materials. It is noteworthy that many light insensitive DAs are readily polymerized in the molten state, whereas the majority of light sensitive DAs are not. An important advantage of polymerization in the molten state is that one can obtain transparent materials with extremely high optical quality and with any desired dimension. In this article, our laboratory's recent results on synthesis, characterization, and third-order NLO susceptibility of PDA-containing systems, are described. EXPERIMENTAL SECTION Synthesis. DA-containing polymers. The polymers which contain DA groups in their main chains can be prepared by (1) direct condensation or addition polymerization of bifunctional monomers which contain DA groups, or (2) oxidative coupling polymerization of terminal bisacetylenic monomers. Because of the reactive DA groups, the first method can be performed only under mild conditions, principally in solutions, and the molecular weights of polymers obtained are generally not very high. Using the second method, polymers with high molecular weights can be obtained with quantitative yields under mild conditions (below 80°C) in solvents in which the polymers are soluble, provided that the monomers and catalysts are highly pure.



C A R R E O N ET A L .

Third-Order Nonlinear Susceptibility of PDAs

A OC-OC sC-CsC oc-oc

OC-OC

OC-OC OC-C-C

c-oc-c-C-C-Ç-

oc-ococ-oc OC-OC OC-OC OC-OC CsC-Cs

-c-oc-c-

fc-OC-C-

c c c c

t c c c -C-OC-C-

-c-oc-c r

-c-oc-c-

-c-

c c c c c-oc-c-

-c-oc-c-

c OC-OC c=c oc-oc

oc

OC-OC OC-OC —OC-OC

OC-OC

OC-OC

oc-oc

oc-oc

OC-OC

Scheme 1. Schematic models of DA-containing polymer chains. A: Before cross-polymerization, B: PDA networks. C: Light insensitive model.


202



However, it is often difficult to remove trace amounts of copper adsorbed by the polymer. The synthesis, characterization, and some properties of the light-sensitive DA-containing polymers, 1 and 2, employed for this study have been reported previously by the authors (15,16,17,18,19). A few novel aromatic DA-containing, light-insensitive polyesters, 3 and 4, were also synthesized. Their structures are shown in Chart 1. A few methacrylates (5) and dimethacrylate (6) which contain aliphatic DA groups were synthesized (20) and are shown in Chart 2. In order to compare the NLO properties of PDA-containing polymers with those of the PDAs obtained by the molten state polymerization, several novel diacetylenes were synthesized, and their chemical structures are shown in Chart 2 for aromatic diesters (7, 8) and aromatic diamide (9). These compounds were synthesized by the Heck reaction of the corresponding bromo compounds with trimethylsirylacetylene, as shown in Chart 3. All of these materials were characterized and verified by thin layer chromatography, IR and NMR spectroscopies, and elemental analysis. Preparation of samples. PDA-Containing polymer films. The DA-containing polymerfilmswere cast from solutions (chloroform for 1 and N-methylpyrrolidone for 2). In the case of 2 the solvent was evaporated at 60°C under reduced pressure. When a heated spin coater was used for 2, highly transparentfilmscould be obtained, but due to their amorphous nature, no crosspolymerization took place on irradiation. In the case of 1, the transparency (crystallinity) of the films depended on the number of methylene units (x may). For example, when χ is 2 and^y is 5, the film is always completely opaque, and in other cases reasonably transparentfilmscan be obtained, although the transparency depends on the casting conditions. PDA-Polymer mixed systems. N,N-m^«-alkylocta-3,5-diynylenediuremane were synthesized by the oxidative coupling reaction of N-n- alkyl- 3 -butynylurethanes obtained by the reaction of corresponding isocyanates with 3-butyn-l-oL n-Octyl, nbutyl and ethyl diurethanes were synthesized, but only the n-octyl diurethane was used because of its higher miscibility with the host polymer. Various polymers such as poly(methylmethacrylate) and polyvinyl acetate) were tested, but it was found that poly(N,N-dimemylaminoethyl methacrylate) gave the best results. Required amounts of the diacetylenic diurethane and the host polymer were dissolved in chloroform, and films were cast or spin coated on quartz panes. The films were then irradiated with UV light or electron beam (12). Amorphous PDAs. The castfilmsof 1 were heated between two quartz panes (1" χ 1", 1 mm thick) at above their melting points, upon which thermal crosspolymerization of DA groups took place and orange to red brown transparent materials were obtained. It seems that simultaneous irradiation with UV light (from a medium pressure Hg lamp) helps the thermal polymerization. Some of the polymers 2 only undergo thermal polymerization in the amorphous state (19).


14.


CARREON E T A L .


Chart 1

- j - 0 ~ ( C H ) r - C = C - C = C - ( C H f e - C O — (CHgJy-CO-]^ 2

-

O—CH

2

CH2-O-CO—(CH )4-CO-]-

2

2

4


203






Chart 3


XT

Si(CH ) 3

H

=

Si(CH ) 3

3

3

χ

X = COOCH (10) 3

= CHO(11)

15


206


The DA groups of polymers 3 and 4 are not light sensitive and they polymerize only in the molten state. They were melted between two quartz windows and irradiated with UV light. The DAs 3-9 were polymerized between two quartz windows at temperatures above their melting points with simultaneous irradiation of UV light. χ(3) Measurements. The third order NLO susceptibility was determined by degenerate four wave mixing using a pico second laser consisting of a mode-locked Quantel Nd: Yag laser that was frequency doubled to 532 nm. It should be noted that some χ values thus observed are resonance enhanced as the most of PDAs absorb the 532 nm light.


(3)

RESULTS AND DISCUSSION DA-Containing Polymers. The crystallinity of DA-containing polymers differs tremendously depending on the chemical structure of spacer groups. In the cases of polydianilides 2 (18, 19), the polymer with χ = 3 and a copolymer were totally amorphous and not photosensitive, and they underwent crosspolymerization only by heating at 180°C, while the polymers with χ = 2, 4 and 8, and some copolymers gave crystalline films in which microcrystallites of about 30 Â were dispersed, and the films become blue or bluish purple on irradiation. The χ values of these films were determined and they were found to he between 10 - 10' esu (21). The irradiated microcrystalline films of these DA-containing polyamides have absorption maxima at around 560 and 610 nm, and the amorphous thermally treated films have no absorption maximum, but the absorption tails from shorter wavelength region up to 650nm. This means that the measured χ values are resonant enhanced. It is worth mentioning that the difference in χ values between the crystalline and amorphous films is about an order of magnitude, and that the amorphous films have much better transparency than the crystalline films. (3)

10

9

(3)

(3)

In the case of the aliphatic polyesters 1, the cast films irradiated by electron beam develop yellow to red orange colors (15-17) and their χ( ) values rangefrom0.5 to 7 x 10" esu (21). The crystallinity of the polyester 1 with x=l is less (32%) than that of the polymer with x=2 (64%). This difference causes the difference in the χ( ) values, the former being 0.5 x 10' esu and the latter being 1.2 x 10" esu, with both films irradiated with 50 Mrads electron beam The polyesters with y = 7 and χ = 1, 2, 3, 4 and 9 were recently synthesized (22). When χ is 2 the cast polymer film is completely opaque and it became orange on irradiation, and when χ is 3, the films became blue on irradiation with UV light. Figure 1 shows the absorption spectra of polyester films irradiated or heated. The absorption in the region of the wave length shorter than 400 nm is mainly due to scattering, as the films are not completely transparent but contain large crystallites. The absorption spectra A, B, C, and D are of a copolyester obtained from the following two monomers; dipropargyldecamemylenedicarboxylate (40 mol %) and dipropargylterephthalate (60 mol %). The copolyester forms a liquid crystal phase and the DA groups of the 3

10

3

10

10




300

400


500 WAVELENGTH

600

700

(nm)

Figure 1. UV-Visible absorption spectra of some cross-polymerized DA-containing polyesters. A: Copolymer before cross-polymerization. B : Copolymer irradiated with electron beam (50 Mrads) at 20°C. C: Copolymer irradiated at 60°C (Liquid crystalline state) with UV light. D: Copolymer heated at 100°C (molten state). E: Polyester 1 (x=3, y=7) irradiated with UV light.


207

900

208


copolymer are photosensitive and the film becomes orange by irradiation. The spectra show that the PDA structures formed in the solid state and liquid crystalline state polymerizations are same. (3)

The blue film of polyester from l-pentyn-3-ol and azelaic acid had a χ value of 3 x 10" esu, which is somewhat higher than that of the red orange films (22). ΊΚ spectra of these irradiated polyester films indicate that almost all the DA groups in the polymers had been consumed for cross-polymerization by the irradiation with 50 Mrads electron beam. The topochemical polymerization is supposed to take place only in the crystalline state, and therefore it seems that both 1,4- and 1,2-crosspolymerizations could take place also in the amorphous region for these DAcontaining polymers when irradiated with high energy radiation or heated at elevated temperatures. The ΊΚ spectra of the amorphous state cross-polymerized films and those of solid state polymerized films are often identical indicating the structure of the former consists mainly of the 1,4-pofydiacetylene structure ( Figure 2).


10

The crystallinity and transparency of these films depend on the film preparation conditions. In the case of polymers 2, rapid evaporation of the solvent on a heated spin coater gives completely amorphous, transparent films, but no topochemical polymerization of the DA groups takes place. In the case of polymers 1 it is also possible to obtain completely transparent films by rapid evaporation of the solvent, however, such amorphous films do not form appreciable amounts of PDA networks on irradiation and thus their χ are negligible. This is a significant disadvantage of the polymer films which contain topochemically polymerizable DA groups, when optical applications are desired. Recently, it was found in our laboratory that solvent impregnation into the cross-polymerized, semi-crystalline PDA films, improve considerably optical quality of the films. (3)

(3)

From these results, it can be concluded that the χ values of PDA containing polymer thinfilmsmeasured by the degenerate four wave mixing method at 532 nm, are in the order of 10' esu and do not vary significantly depending on the crystallinity and chemical structures. These factors can influence only within the same order of magnitude, and the values of 10" esu seem to be the characteristic value for the PDA networks. 10

10

Amorphous PDAs. Yu et al.(25) prepared poly(hexa-2,4-diynylene terephthalate), which is not photosensitive, but does polymerize by heating at 150°C. Α χ value of 3.2 x 10" esu (determined the degenerate four wave mixing technique at 532 nm.) has been reported for this material. The polymers 3 and 4 (Chart 4) are not photosensitive, but underwent cross-polymerization when heated at 180°C (in the molten state) for 2.5 hours with simultaneous UV irradiation, giving red transparent materials. The χ values for these materials were found to be 1.9 - 3.5 x 10' esu for polymers 3 and 2.7 -2.9 x 10" esu for polymers 4. Absorption spectra of one of the polymers 4 are shown in Figure 3. The films have an absorption maximum at 400 nm and a trough at 340-350 nm, but absorption tails down towards 700 nm due to their amorphous nature. (3)

10

(3)

10

10



14.



\

/

··'/ /

-η ΨΙ / •/ h / r

' i

f/ï t

'//

A

\

\ 2300

' \ •

;

\ \\

r ~, /

f

\ 2250

WAVE NUMBER

'

\

D

2200

Γ

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-1

(cm )

Figure 2. FT-IR Spectra of DA groups of polyester 1 (x=2, y=8). A: Before cross-polymerization. B: Irradiated with UV light for 30 minutes at room temperature. C: Heated at 90°C for 96 hours (solid state thermal cross-polymerization). D: Heated at 180°C for 4 hours (amorphous state cross-polymerization).


209

210


Chart 4


ο

3

12

+

HO-(CH2) -OH x

4



14.

CARREON E T A L .


Figure 3. UV-Visible absorption spectra of polyester 4 (x=10) heated at 180°C. A: 1 hr, B: 2 hrs, C: 3 hrs. Film thickness: 0.011 mm


211

212


The aliphatic DAs linked to methacrylate groups 5 and 6 undergo rapid polymerization; the vinyl polymerization is followed by the diacetylenic polymerization, resulting in yellow, highly transparent, glassy materials. The χ values for these materials obtained by heating at 140°C for 60 minutes (almost total consumption of DA groups in the molecules) (20) were found to be on the order of 5 -6.5 x 10~ esu, independently of the types: Polymerfrom5 R= n-Bu: 6.5, R= nHex: 6.2, R= n-Oct: 5.7; Polymerfrom6 : 5.2 x 10" esu. The visible absorption spectra of these materials do not have maxima, but the spectra decrease and show almost no absorption at the wavelengths greater than 400 nm. It seems that the materials consist of mixtures of short conjugated units. (3)

12


12

Since the DA groups incorporated in polymer chains are not topochemically crosspolymerizable, it is worth studying the bulk, molten state polymerization of these DAs for comparison. Therefore, aromatic diacetylenic esters 7 and 8 and an amide 9 were prepared, and they were polymerized at 180°C for 2.5 hours with simultaneous UV irradiation. These amorphous materials are highly transparent to the naked eyes and they are deep red when the thickness of the film is about 0.05 mm. They have absorption maxima around 420 nm but the spectra continuously decease towards 600 nm. The χ values of films of 8 with thickness of 0.040 mm were found to be 4.2 x 10" and 3.5 x 10' esu for R - CH and phenyl, respectively. The filmsfrom7 with thicknesses of 0.027-0.030 mm were deep red and their χ values could not be measured due to excessive absorption. The DA 9 underwent polymerization when heated at 180°C for 2.5 hrs giving a red film (thickness: 0.038 mm) which had an absorption maximum around 420 nm, with the peak tailing down to 700 nm. It has a χ value of 1.1 x 10" esu and an excellent transparency to the naked eyes. An aromatic, highly conjugated DA 14 (Chart 4), was synthesized (24) and it was polymerized in the molten state at 225°C under nitrogen for 3 min. to obtain a red film on glass. This material showed a χ value of 5.6 x 10" esu. These aromatic amorphous PDAs generally have absorption in the region 600-700 nm, and therefore their χ values are resonance enhanced. However, it can be said that the polymerized materials have excellent optical quality due to their completely amorphous nature, and that their values are comparable with those of PDA-containing polymer films. (3)

10

10

3

(3)

(3)

9

(3)

10

(3)

Z-scan experiment was carried out for some amorphous PDAs using wavelength of 1064 nm A typical example is shown in Figure 4. Two photon absorption coefficient, β, was found to be 2.7 cm/GW. PDA-Polymer Composite Systems. If microcrystals of PDAs can be homogeneously dispersed in an amorphous, transparent polymer, and if the crystal size is smaller than the wavelength of the applied laser, the materials can probably have applications. An example of this was previously mentioned (12) and the χ values were measured (25) for the system consisting of N^-dioctylocta-S^diynylenediurethane dispersed in poly(N,N-dimemylammoemyi methacrylate). Up to (3)





Χ *

213

χ ο Χο

ο ο ο ο

-8

-6

-4

-2

0 Ζ

2

4

6

8

(cm)

Figure 4. Ζ-Scan experiment of 4,4'-butadiynylenedi-«nonyldibenzoate polymerized at 190°C for 1 hr between 2 quartz windows. χ : calculated. Ο : observed.


10


214


values of 2.0 - 2.9 χ 10" esu, almost independent of the applied dose (5 - 100 Mrads), indicating that about 20 Mrads are sufficient to polymerize the DA crystals dispersed in the host polymer (25). Amounts of the DA higher than 20% causes formation of larger crystals and the films have poor transparency. The irradiated films have an absorption maximum around 550 nm. The optical quality of these films, however, was not satisfactory and their absorption spectra were rather broad over 400 - 700 nm with a maxima at 550 nm. This broadness seems to be due to wide distribution of crystal size. By selecting suitable combinations of DAs and host polymers, it is possible to obtain high optical quality films which contain PDA microcrystals. Further studies are being conducted in our laboratory, and an excellent nanocomposite system has recently been found (26). Its absorption spectrum shown in Figure 5 indicates high optical quality of the film, as the spectrum is as defined as those of single crystal PDAs. An advantage of this method over the microemulsion of PDA crystals in water (14) is that the mixed systems are more stable than the emulsion in aqueous media where coagulation of dispersed particles may take place over a prolonged period. CONCLUSIONS (3)

10

As described these PDA systems have χ values on the order of 10" esu. Some of the values are resonant enhanced due to absorption of 532 nm radiation. Measurements at longer wavelengths such as 1064 nm, are needed in order to further evaluate the NLO properties of these PDAs. The topochemicalry porymerizable DAcontaining polymers tend to form crystalline polymers, and it is difficult to obtain thick films with high optical transparency without sacrificing the crystallinity. A new technique for obtaining amorphous films which involves topochemicalry formed PDA networks is currently being developed in our laboratory. The polymers containing DA groups which do not undergo topochemical polymerization, such as 3 and 4, can give orange to red, glassy materials with excellent optical transparency. Their absorption covers wavelengths shorter than 700 nm, although the absorption spectra have local minima around 330 - 350 nm. The DAs which do not undergo topochemical polymerization, but polymerize in the molten state, give similar results to the above materials. It should be mentioned that the DAs which polymerize at high temperatures are not useful because thermal decomposition often accompanies polymerization, thus decreasing the optical quality of materials. Therefore, it is desirable to polymerize the DAs rapidly below 200°C. These systems, which consists of microcrystals of photosensitive DAs in host polymers, are interesting materials. Very little has yet been studied on such systems. DA microcrystals dispersed in an amorphous polymer can be converted to PDA microcrystals by irradiation, and when the crystal size is small compared with the wavelength of the laser, the materials may be useful for NLO applications. Systems with new combinations of DAs and host polymers are expected to appear in the fixture.



14.



200

400

600

800

215

900

WAVELENGTH (nm)

Figure 5. UV-Visible absorption spectra of a composite film of N^'-m^Ai-butylocta-S^-diynylene diurethane dispersed in polyvinyl acetate-co-viny^yrroMone). 20% by weight DA/host polymer. Film thickness: 10 μιη. UV-Irradiated for 2.5 hrs.

Safety considerations The DAs are high energy compounds and their polymerization is highly exothermic. Therefore, heating a large amount at elevated temperatures may cause an explosive reaction. The topochemicalry porymerizable DAs are also sensitive to pressure. We tried to preparefilamentsof polyester 1 (x=2, y=$) in an extruder, and when about 4 g of the polymer was melted in the extruder it suddenly exploded. Temperature (probably overheating) and pressure caused an explosion. Acknowledgement This study was supported by GrantsfromCONACyT (Consejo Nacional de Ciencia y Tecnologia) of Mexican Government, Grant Nos. 3595-0540E and 4264-E9406, and from DGAPA (Direccion General de Asuntos del Personal Académico) of our imiversity, Grant Nos. IN-101492 and IN-101793.


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Literature Cited (1) Wegner, G. Z.Naturforsch. 1969, 24B, 824. (2) Cantow, H.-J. Ed. Advances in Polymer Science,vol.63, "Polydiacetylenes", Springer-Verlag: Berlin, 1984. (3) (a) Bloor, D; Chance, R.R. Eds. Polydiacetylenes, Martinus Nijihoff: Dordrecht 1985. (b) Chamla, D.S; Zyss, J., Eds. Nonlinear Optical Properties of Organic Molecules and Crystals," vol.2, Academic Press: Orlando, Florida, 1987. (4) Paley, M.S.; Frazier, D.O.; Abdeldeyem, H.; Armstrong, S.; McManus, S.P. J.Am.Chem.Soc., 1995, 117, 4775. (5) Kajzar, F.; Messier, J.; in Conjugated Polymers, Brédas, J.L.; Silbey, R., Eds. Kluwer Academic Publishers: Dordrecht, 1991; pp. 517-541. (6) Wegner, G., Macromol. Chem., 1971, 145, 85. (7) Yee, K.C.; Chance, R.R, J.Polym.Sci., Polym.Phys.Ed., 1978, 16, 431. (8) Ulman, Α., An Introduction to Ultrathin Organic FilmsfromLangmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991. (9) Bader, H.; Dorn, K.; Hupfer, B.; Ringdorf; H., Adv.Polym.Sci., 1985, 61, 1. (10) Ogawa, T.; Fomine, S., Trrends Polym. Sci., 1994, 2, 308. (11) Ogawa, T., Prog. Polym. Sci., 1995, 20, 943. (12) Alexandrova, L.; Chavarin, C.; Ogawa, T., Paper presented at POLYMEX'93 (An Intnat. Symp. Polym.), Cancún, Mexico. Nov. 1993. Preprints, pp 215 (1993). (13) Prasad, P.N., Chapter in this Book. (14) Nakanishi, H., Chapter in this Book. (15) Fomine, S.; Neyra, R.; Ogawa, T., Polym. J., 1994, 26, 845. (16) Fomine, S.; Maciel, A.; Ogawa, T., Polym. J., 1994, 26, 1270. (17) Fomine, S.; Sabchez, S.; Ogawa, T., Polym. J., 1995, 27, 165. (18) Fomine, S.; Ogawa, T., Polym. J., 1994, 26, 93. (19) Fomine, S.; Marin, M.; Ogawa, T., Macromol. Symp., 1994, 84, 91. (20) Fomina, L.; Fomine, S.; Ogawa, T., Polym. Bull., 1995, 34, 547. (21) Fomine, S.; Adem, E.; Rao, D.; Ogawa, T., Polym. Bull., 1995, 34, 169. (22) Pedrosa, G.; Fomine, S.; Ogawa, T., unpublished results. (23) Yu, L.; Chen, M.; Dalton, L.R., J.Polym.Sci., Polym.Chem.Ed., 1991, 29, 127. (24) Fomine, S.; Fomina, L.; Quiroz-Florentino, H.; Mendez, J.M.; Ogawa, T., Polym. J., 1995, 27, 1085. (25) Valverde, C.; Alexandrova, L.; Adem, E.; Rao, D.; Ogawa, T., Polym. Adv. Tech., 1996, 7, 27. (26) Ogawa, T., Unpublished results.


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