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Kinetic study of photoinitiated grafting of diallyl oxydiethylene dicarbonate onto cellulose
.I. Photochem. Photobiol. A: Chem., 59 (1991) 99-104
99
diallyl Kinetic study of photoinitiated grafting of >. oxydiethylene dicarbonate onto celluloseT Kgnazio Renato Bellobono”,
Bruno Marcandallib
and Elena Selli”
‘Department of Physical Chemistry and Electrochemistry,
University of Milan, I-20133 Milan
VtaW bStazione Sperimentale per la Seta, I-20133 Milan (Italy)
(Received December 1, 1990)
Abstract Photochemical grafting of diallyl oxydiethylene dicarbonate onto cellulose (in the form of filter paper) was investigated kinetically at 20 A 2 “C with 1,2-diphenyl-2,2_dimethoxyethanone as photoinitiator. Irradiations were carried out polychromatically with an incident radiation flux of (1.25*O.O7)X1O-7 einstein s-l cm -‘.,The ratio R between the molar concentrations of photoinitiator and monomer was varied in the range 0.032-0.150 and the n/S value (n moles of monomer adsorbed per unit apparent surface S) was varied in the range 8.40-43.8 pm01 cm-*. Two consecutive rate processes were shown by the kinetic curves, with quantum yields differing by about two orders of magnitude. Quantum yields @r for the first, almost constant, rate period were measured as a function of R and n/S. Kinetic features of the grafting process of diallylic monomer are discussed in the light of a general mechanism proposed for grafting of diacrylates and the photochemical reactivity of the allylic vs. the acrylic function.
1. Introduction Thermal polymerization of diallyl oxydiethylene dicarbonate (diethyleneglycol bis(allylcarbonate)) (I), initiated by peroxide radicals, has been known for some time [l, 21. More recently, some detailed studies of the photopolymerization [3, 41 and photocrosslinking [5] of I have been published. Casting by UV irradiation, at wavelengths longer than 280 nm, can be carried out without defects in a much shorter time than thermal curing [3], and is applicable to glazing materials, lenses and other mould products. Furthermore [3], final UV curing of a photochemically produced prepolymer of I, or I blended with other copolymers or comonomers, may give rise to a coating with a very high wiping strength and scratch resistance which can also act as an insulating protective barrier for metals. The physical properties of coatings based on photopolymerized poly(1) are therefore very promising. It was considered to be of interest to investigate the grafting of I onto cellulose (as a model of a reactive support), although the photochemical reactivity of ally1 groups, particularly with unblended monomers, is not expected to be as high as that of diacrylates. The kinetics of the photoinduced grafting of 1 were investigated systematically in order to obtain mechanistic information on the process under +Part 19 of the series “Photosynthetic
lOlO-6030/91/%3.50
Membranes”.
0 Elsevier Sequoia/Printed
in The Netherlands
100
experimental conditions similar to those used in the photochemical characterization of membrane systems. 2. Experimental
preparation
and
details
Cellulose, in the form of filter paper (“black band”, Schleicher and Schiill, F.R.G.), was employed as a macroporous support. Diallyl oxydiethylene dicarbonate (diethyleneglycol bis(allylcarbonate)) (I) (Enimont, Italy) was purified by double vacuum distillation (2 mbar) in a helium atmosphere (boiling point, 159-161 “C; refractive index n,(589 nm), 1.4504 at 20 “C). 1,2-Diphenyl2,2-dimethoxyethanone (Ciba Geigy, Switzerland) was a commercial product (Irgacure 651) and was used as received. Reagent grade acetone was purified by refluxing with sodium hydroxide and potassium permanganate; it was then dried by refluxing, followed by distillation over sodium under a nitrogen stream. A 1 ml aliquot of a solution of I in dry acetone (0.2375-1.558 M) (containing a quantity of photoinitiator to yield a molar ratio R between photoinitiator and I in the range 0.032-0.150) was uniformly deposited by standard procedures using a microsyringe dispenser on cellulose discs (diameter, 6 cm). Using this method, 8.4043.8 pmol of I per square centimetre of apparent cellulose surface was adsorbed. The solvent was evaporated at room temperature in a nitrogen stream. Irradiations were performed using a 500 W high pressure mercury arc lamp (Italquartz, Italy) at a constant impinging radiation flux of (1.25 f 0.07) x lo-’ einstein s-l cmp2. The samples were irradiated through a quartz window in a water-cooled cylindrical reactor; the temperature was maintained at 20 f 2 “C and oxygen was excluded by passing a stream of nitrogen. After the desired irradiation time, the samples were Soxhlet extracted with acetone, which efficiently removed non-reacted monomer, residual photoinitiator and ungrafted homopolymers. Graft yields as a function of time were obtained as the ratio between the amount of grafted monomer I and that initially deposited onto the surface. The amount of grafted I was calculated by subtracting the amount of non-reacted monomer and ungrafted homopolymer (determined by gravimetric analysis of the Soxhlet-extracted acetone solution) from the amount of monomer initially adsorbed onto the cellulose. Gravimetric determination of non-reacted I and its ungrafted homopolymer was carried out after complete evaporation of an appropriate aliquot of extracted solution on a small aluminium disc. The total solute content was corrected for the residual presence of photoinitiator, determined by spectrophotometric analysis; this varied according to irradiation time. The presence of photoproducts derived from the photoinitiator was found to affect the final result of the gravimetric analysis only slightly (no more than 2%). The irradiation flux was measured by potassium ferrioxalate actinometry. The mean polychromatic quantum yields were calculated from the absorbed energy, which was evaluated by integration over the emission spectrum of the lamp and the absorption spectrum of the photoreactive mixture (determined by the concentration of the photoinitiator). 3. Results
and discussion
Photochemical dimethoxyethanone
grafting of I onto cellulose, in the presence of 1,2-diphenyl-2,2as photoinitiator, was investigated kinetically at 20&2 “C. The
101
impinging polychromatic radiation flux was kept constant at (1.25 f 0.07) x lo-’ einstein s -1 m-2. The ratio R between the molar concentrations of photoinitiator and diallylic monomer was varied in the range 0.032-0.150. The moles of monomer n adsorbed per unit apparent surface S of cellulose varied in the range 8.40-43.8 pmol I cmW2. A typical graft yield curve is given in Fig. 1. All graft yield curves have a similar shape, independent of n/S. A brief induction period is initially observed (usually about 5 min), followed by an almost linear graft yield VS. time; this corresponds to the first constant rate period, the quantum yield (CD,) of which can be easily measured. A progressive decrease in rate leads successively to a slight inflexion point, after which a second constant rate period of grafting, with a quantum yield much lower than G1 (about two orders of magnitude lower), is observed up to nearly quantitative yields. The grafting yields at the inflexion point (which marks the passage from the first to the second rate period) vary as a function of the ratio R of the photoinitiator and n/S (although less markedly), as can be seen from the plots in Fig. 2. The variation
0
0
20
40
60
60
100
120
lRRAOlATlON TIME (mid
Fig. 1. Graft yield of I on cellulose VS.time r at n/S=39.8 PmoI I cm-’ and R=0.054. Impinging polychromatic photoenergy: 1.25 X lo-’ einstein s-l cm- *. Uncertainties are reported graphically as the maximum deviation from the mean.
3 $ 80 mk =c E Y 60 w-d 41;; g:v 40
0 9.4 - 9.5 pmol cm+ l
l&l- 17.2 A 23.0-23.4 L 32.1- 34.3 0 39.8-43.8
se, 5 5
20 0
0
0.05
0.10
pmol pmol pmol pmol
cm-* crnm2 cms2 cm+?
0.15
R
Fig. 2. Percentage graft yield of I on cellulose at the inflexion point of the graft yield curves (Fig. 1) (end of the first, almost constant, rate period) as a function of R at various values of n/S. Impinging polychromatic photoenergy: USX lo-’ einstein s-’ cm-*.
102
with R is significant up to a threshold value of R = 0.075. At greater values of R, the graft yield remains constant up to the highest value of R investigated, and ranges from 73% to 82% for n/S values in the range 8.4-43.8 prnol cm-* (see Fig. 2). The influence of R at various n/S values, and of n/S at various R values, on the quantum yields Q1 is shown in Figs. 3 and 4. It can be seen (Fig. 3) that the dependence of Q1 on R is almost linear up to a value of R of approximately 0.05. The slope of the Q1 VS. R curve is about 1.2 for n/S =9 pmol cme2 and progressively increases to about 12 for n/S=42 Fmol cm-*. At R-0.07 a plateau is reached and Q1 depends only on the n/S values. This is best illustrated in Fig. 4 at R -0.075 and R =0.150. The same type of dependence is also shown at R =0.0.54 and, with a lower slope, at R = 0.032. Some type of threshold effect of n/S is indicated (n/S=2-7 pmol cme2), and below a certain amount of I adsorbed per unit surface of cellulose, no grafting is observed.
I
1
_ 1.0 -
0
8.4 - 9.5 l 16.1-17.2 A 23.0-23.4 * 32.1 -34.3 0 39.8-43.6
pmol pmol pm01 pmol pm01
1
1
I
!
cm-2 cm-* CI?-~ cm+ cm+
0
A--
0.10
0.05
cl
0.15
R
Fig. 3. Influence of R, at various n/S values, on quantum yields @,.
“0
10
20
30
40
50
60
n/S (pm01cm+) Fig. 4. Influence
of nlS, at various R values, on quantum yields @,.
103
The behaviour of I in these grafting studies can be discussed in the light of a general mechanism proposed for the grafting of monoacrylates and diacrylates [6-g]. Qualitatively, some features are common to both types of difunctional molecule, e.g. two constant rate periods separated by a brief or very brief induction period, which sometimes cannot be detected experimentally. However, some specific aspects are shown by I: (i) the dependence of @i on n/S and R (a dependence on R only is detected with acrylates); (ii) the small, but definite, initial induction period in the graft yield kinetic curves (Fig. 1) (this is completely absent or unobservable with acrylates); (iii) the threshold effect of n/S on @r shown in Fig. 4. These results seem to indicate that the overall mechanism [9] is the same for the two types of molecule, but some differences occur due to the different photochemical reactivity of the allylic function. RIRz z
(R1R2)*
(RIRz)* R,‘+ Cell -
(1)
R,‘+R;
(2)
RIH + Cell
(R1R2)* + Cell -
(3)
RIRz + Cell
(4)
Cell, cellulose; R,, C6H,C=O; CH=CH,+R,‘(R,‘)-
R2, C6H,C(OMe)2; R1R2, photoinitiator.
R,(R$-CH+H
A
k
(5)
CHz=cHA
-
RI(Rz)-(CH,-FH),-CH+H
k
A x, integer; A, monomer backbone, Cell’+ R1(R2)-CH2-
YH -
Cell- YH- CH,- R,(Rr)
A
A
Cell.+Rr(R,)-(CH,-yHX_CH2-FHA
(6)
Cell-(YH-CH,X.,-R,(Rz) A
(7)
A
The induction and threshold effects ((ii) and (iii) above) may indicate that grafting does not take place unless a critical concentration of reacting radicals (cellulose surface (Cen’) and radicals of I) is formed. In order that this may happen, a certain limiting amount of photoinitiator is necessary on the cellulose surface, or must reach it by diffusion. Roth the threshold effect (iii) and the induction time (ii) may thus be explained. In addition, a rate-limiting step for the grafting of I must be represented by reaction (5), so that a mass effect is observed (influence of n/S on Gj; see Fig. 4) as well as a photoinitiator effect (influence of R on @,, in a certain range of R values; see Fig. 3). Finally, the slope of the ?D1vs. R relationship for the grafting of I, in its approximately linear region (see Fig. 3), is two orders of magnitude lower than that of the corresponding relationship for diacrylates [9]. This reflects the slow photochemical reactivity of I, in which the carbonyl group of the acrylates is absent and the double bond is not activated. Nevertheless the photografting of I may be carried out satisfactorily and in a reasonable period of time provided that the radiation intensity is appropriate and the reaction is stopped at the end of the first constant rate period.
104 Acknowledgments
This work was financially supported by the Italian National Research Council (C.N.R.) through Progetto Finalizzato per la Chimica Fine II. Technical assistance by Miss Cristina Crippa is gratefully acknowledged.
References 1 M. Lasch, F. Rudolph, L. Schreiner and H. Schulze, J. Appl. Polym. Sci., 11 (1967) 369. 2 C. Hoffrnann and F. Rudolph, Pluste Kuutsch., 29 (1972) 41. 3 I. R. Bellobono and M. Zeni, Makromol. Chem., Rapid Commun., 7 (1986) 733. 4 I. R. Bellobono, E. Selli, L. Righetto, P. Rafellini and L. Trevisan, Makromol. Chem., 190 (1989) 1945. 5 I. R. Bellobono, in hoc. Rudtech Europe ‘89 Conf, Radtech Europe, Basel, 1989, p. 243. 6 I. R. Bellobono, F. Tolusso, E. Selli, S. Calgari and A. Berlin, J. A@. Po[ym. Sci., 26 (1981) 619. 7 I. R. Bellobono, S. Calgari and E. Selli, in J. E. Kresta (ed.), Polymer Additives, Plenum, New York, 1984, p. 83. 8 I. R. Bellobono and E. Selli, in N. S. Allen (ed.), PhotopoIymetiation and Photoimaging Science and Technology, Elsevier, London, 1989, Chapter 4. 9 I. R. Bellobono, E. Selli, B. Marcandalli, D. Comi and E. Rastelli, I. Photochem., 35 (1986) 99.
99
diallyl Kinetic study of photoinitiated grafting of >. oxydiethylene dicarbonate onto celluloseT Kgnazio Renato Bellobono”,
Bruno Marcandallib
and Elena Selli”
‘Department of Physical Chemistry and Electrochemistry,
University of Milan, I-20133 Milan
VtaW bStazione Sperimentale per la Seta, I-20133 Milan (Italy)
(Received December 1, 1990)
Abstract Photochemical grafting of diallyl oxydiethylene dicarbonate onto cellulose (in the form of filter paper) was investigated kinetically at 20 A 2 “C with 1,2-diphenyl-2,2_dimethoxyethanone as photoinitiator. Irradiations were carried out polychromatically with an incident radiation flux of (1.25*O.O7)X1O-7 einstein s-l cm -‘.,The ratio R between the molar concentrations of photoinitiator and monomer was varied in the range 0.032-0.150 and the n/S value (n moles of monomer adsorbed per unit apparent surface S) was varied in the range 8.40-43.8 pm01 cm-*. Two consecutive rate processes were shown by the kinetic curves, with quantum yields differing by about two orders of magnitude. Quantum yields @r for the first, almost constant, rate period were measured as a function of R and n/S. Kinetic features of the grafting process of diallylic monomer are discussed in the light of a general mechanism proposed for grafting of diacrylates and the photochemical reactivity of the allylic vs. the acrylic function.
1. Introduction Thermal polymerization of diallyl oxydiethylene dicarbonate (diethyleneglycol bis(allylcarbonate)) (I), initiated by peroxide radicals, has been known for some time [l, 21. More recently, some detailed studies of the photopolymerization [3, 41 and photocrosslinking [5] of I have been published. Casting by UV irradiation, at wavelengths longer than 280 nm, can be carried out without defects in a much shorter time than thermal curing [3], and is applicable to glazing materials, lenses and other mould products. Furthermore [3], final UV curing of a photochemically produced prepolymer of I, or I blended with other copolymers or comonomers, may give rise to a coating with a very high wiping strength and scratch resistance which can also act as an insulating protective barrier for metals. The physical properties of coatings based on photopolymerized poly(1) are therefore very promising. It was considered to be of interest to investigate the grafting of I onto cellulose (as a model of a reactive support), although the photochemical reactivity of ally1 groups, particularly with unblended monomers, is not expected to be as high as that of diacrylates. The kinetics of the photoinduced grafting of 1 were investigated systematically in order to obtain mechanistic information on the process under +Part 19 of the series “Photosynthetic
lOlO-6030/91/%3.50
Membranes”.
0 Elsevier Sequoia/Printed
in The Netherlands
100
experimental conditions similar to those used in the photochemical characterization of membrane systems. 2. Experimental
preparation
and
details
Cellulose, in the form of filter paper (“black band”, Schleicher and Schiill, F.R.G.), was employed as a macroporous support. Diallyl oxydiethylene dicarbonate (diethyleneglycol bis(allylcarbonate)) (I) (Enimont, Italy) was purified by double vacuum distillation (2 mbar) in a helium atmosphere (boiling point, 159-161 “C; refractive index n,(589 nm), 1.4504 at 20 “C). 1,2-Diphenyl2,2-dimethoxyethanone (Ciba Geigy, Switzerland) was a commercial product (Irgacure 651) and was used as received. Reagent grade acetone was purified by refluxing with sodium hydroxide and potassium permanganate; it was then dried by refluxing, followed by distillation over sodium under a nitrogen stream. A 1 ml aliquot of a solution of I in dry acetone (0.2375-1.558 M) (containing a quantity of photoinitiator to yield a molar ratio R between photoinitiator and I in the range 0.032-0.150) was uniformly deposited by standard procedures using a microsyringe dispenser on cellulose discs (diameter, 6 cm). Using this method, 8.4043.8 pmol of I per square centimetre of apparent cellulose surface was adsorbed. The solvent was evaporated at room temperature in a nitrogen stream. Irradiations were performed using a 500 W high pressure mercury arc lamp (Italquartz, Italy) at a constant impinging radiation flux of (1.25 f 0.07) x lo-’ einstein s-l cmp2. The samples were irradiated through a quartz window in a water-cooled cylindrical reactor; the temperature was maintained at 20 f 2 “C and oxygen was excluded by passing a stream of nitrogen. After the desired irradiation time, the samples were Soxhlet extracted with acetone, which efficiently removed non-reacted monomer, residual photoinitiator and ungrafted homopolymers. Graft yields as a function of time were obtained as the ratio between the amount of grafted monomer I and that initially deposited onto the surface. The amount of grafted I was calculated by subtracting the amount of non-reacted monomer and ungrafted homopolymer (determined by gravimetric analysis of the Soxhlet-extracted acetone solution) from the amount of monomer initially adsorbed onto the cellulose. Gravimetric determination of non-reacted I and its ungrafted homopolymer was carried out after complete evaporation of an appropriate aliquot of extracted solution on a small aluminium disc. The total solute content was corrected for the residual presence of photoinitiator, determined by spectrophotometric analysis; this varied according to irradiation time. The presence of photoproducts derived from the photoinitiator was found to affect the final result of the gravimetric analysis only slightly (no more than 2%). The irradiation flux was measured by potassium ferrioxalate actinometry. The mean polychromatic quantum yields were calculated from the absorbed energy, which was evaluated by integration over the emission spectrum of the lamp and the absorption spectrum of the photoreactive mixture (determined by the concentration of the photoinitiator). 3. Results
and discussion
Photochemical dimethoxyethanone
grafting of I onto cellulose, in the presence of 1,2-diphenyl-2,2as photoinitiator, was investigated kinetically at 20&2 “C. The
101
impinging polychromatic radiation flux was kept constant at (1.25 f 0.07) x lo-’ einstein s -1 m-2. The ratio R between the molar concentrations of photoinitiator and diallylic monomer was varied in the range 0.032-0.150. The moles of monomer n adsorbed per unit apparent surface S of cellulose varied in the range 8.40-43.8 pmol I cmW2. A typical graft yield curve is given in Fig. 1. All graft yield curves have a similar shape, independent of n/S. A brief induction period is initially observed (usually about 5 min), followed by an almost linear graft yield VS. time; this corresponds to the first constant rate period, the quantum yield (CD,) of which can be easily measured. A progressive decrease in rate leads successively to a slight inflexion point, after which a second constant rate period of grafting, with a quantum yield much lower than G1 (about two orders of magnitude lower), is observed up to nearly quantitative yields. The grafting yields at the inflexion point (which marks the passage from the first to the second rate period) vary as a function of the ratio R of the photoinitiator and n/S (although less markedly), as can be seen from the plots in Fig. 2. The variation
0
0
20
40
60
60
100
120
lRRAOlATlON TIME (mid
Fig. 1. Graft yield of I on cellulose VS.time r at n/S=39.8 PmoI I cm-’ and R=0.054. Impinging polychromatic photoenergy: 1.25 X lo-’ einstein s-l cm- *. Uncertainties are reported graphically as the maximum deviation from the mean.
3 $ 80 mk =c E Y 60 w-d 41;; g:v 40
0 9.4 - 9.5 pmol cm+ l
l&l- 17.2 A 23.0-23.4 L 32.1- 34.3 0 39.8-43.8
se, 5 5
20 0
0
0.05
0.10
pmol pmol pmol pmol
cm-* crnm2 cms2 cm+?
0.15
R
Fig. 2. Percentage graft yield of I on cellulose at the inflexion point of the graft yield curves (Fig. 1) (end of the first, almost constant, rate period) as a function of R at various values of n/S. Impinging polychromatic photoenergy: USX lo-’ einstein s-’ cm-*.
102
with R is significant up to a threshold value of R = 0.075. At greater values of R, the graft yield remains constant up to the highest value of R investigated, and ranges from 73% to 82% for n/S values in the range 8.4-43.8 prnol cm-* (see Fig. 2). The influence of R at various n/S values, and of n/S at various R values, on the quantum yields Q1 is shown in Figs. 3 and 4. It can be seen (Fig. 3) that the dependence of Q1 on R is almost linear up to a value of R of approximately 0.05. The slope of the Q1 VS. R curve is about 1.2 for n/S =9 pmol cme2 and progressively increases to about 12 for n/S=42 Fmol cm-*. At R-0.07 a plateau is reached and Q1 depends only on the n/S values. This is best illustrated in Fig. 4 at R -0.075 and R =0.150. The same type of dependence is also shown at R =0.0.54 and, with a lower slope, at R = 0.032. Some type of threshold effect of n/S is indicated (n/S=2-7 pmol cme2), and below a certain amount of I adsorbed per unit surface of cellulose, no grafting is observed.
I
1
_ 1.0 -
0
8.4 - 9.5 l 16.1-17.2 A 23.0-23.4 * 32.1 -34.3 0 39.8-43.6
pmol pmol pm01 pmol pm01
1
1
I
!
cm-2 cm-* CI?-~ cm+ cm+
0
A--
0.10
0.05
cl
0.15
R
Fig. 3. Influence of R, at various n/S values, on quantum yields @,.
“0
10
20
30
40
50
60
n/S (pm01cm+) Fig. 4. Influence
of nlS, at various R values, on quantum yields @,.
103
The behaviour of I in these grafting studies can be discussed in the light of a general mechanism proposed for the grafting of monoacrylates and diacrylates [6-g]. Qualitatively, some features are common to both types of difunctional molecule, e.g. two constant rate periods separated by a brief or very brief induction period, which sometimes cannot be detected experimentally. However, some specific aspects are shown by I: (i) the dependence of @i on n/S and R (a dependence on R only is detected with acrylates); (ii) the small, but definite, initial induction period in the graft yield kinetic curves (Fig. 1) (this is completely absent or unobservable with acrylates); (iii) the threshold effect of n/S on @r shown in Fig. 4. These results seem to indicate that the overall mechanism [9] is the same for the two types of molecule, but some differences occur due to the different photochemical reactivity of the allylic function. RIRz z
(R1R2)*
(RIRz)* R,‘+ Cell -
(1)
R,‘+R;
(2)
RIH + Cell
(R1R2)* + Cell -
(3)
RIRz + Cell
(4)
Cell, cellulose; R,, C6H,C=O; CH=CH,+R,‘(R,‘)-
R2, C6H,C(OMe)2; R1R2, photoinitiator.
R,(R$-CH+H
A
k
(5)
CHz=cHA
-
RI(Rz)-(CH,-FH),-CH+H
k
A x, integer; A, monomer backbone, Cell’+ R1(R2)-CH2-
YH -
Cell- YH- CH,- R,(Rr)
A
A
Cell.+Rr(R,)-(CH,-yHX_CH2-FHA
(6)
Cell-(YH-CH,X.,-R,(Rz) A
(7)
A
The induction and threshold effects ((ii) and (iii) above) may indicate that grafting does not take place unless a critical concentration of reacting radicals (cellulose surface (Cen’) and radicals of I) is formed. In order that this may happen, a certain limiting amount of photoinitiator is necessary on the cellulose surface, or must reach it by diffusion. Roth the threshold effect (iii) and the induction time (ii) may thus be explained. In addition, a rate-limiting step for the grafting of I must be represented by reaction (5), so that a mass effect is observed (influence of n/S on Gj; see Fig. 4) as well as a photoinitiator effect (influence of R on @,, in a certain range of R values; see Fig. 3). Finally, the slope of the ?D1vs. R relationship for the grafting of I, in its approximately linear region (see Fig. 3), is two orders of magnitude lower than that of the corresponding relationship for diacrylates [9]. This reflects the slow photochemical reactivity of I, in which the carbonyl group of the acrylates is absent and the double bond is not activated. Nevertheless the photografting of I may be carried out satisfactorily and in a reasonable period of time provided that the radiation intensity is appropriate and the reaction is stopped at the end of the first constant rate period.
104 Acknowledgments
This work was financially supported by the Italian National Research Council (C.N.R.) through Progetto Finalizzato per la Chimica Fine II. Technical assistance by Miss Cristina Crippa is gratefully acknowledged.
References 1 M. Lasch, F. Rudolph, L. Schreiner and H. Schulze, J. Appl. Polym. Sci., 11 (1967) 369. 2 C. Hoffrnann and F. Rudolph, Pluste Kuutsch., 29 (1972) 41. 3 I. R. Bellobono and M. Zeni, Makromol. Chem., Rapid Commun., 7 (1986) 733. 4 I. R. Bellobono, E. Selli, L. Righetto, P. Rafellini and L. Trevisan, Makromol. Chem., 190 (1989) 1945. 5 I. R. Bellobono, in hoc. Rudtech Europe ‘89 Conf, Radtech Europe, Basel, 1989, p. 243. 6 I. R. Bellobono, F. Tolusso, E. Selli, S. Calgari and A. Berlin, J. A@. Po[ym. Sci., 26 (1981) 619. 7 I. R. Bellobono, S. Calgari and E. Selli, in J. E. Kresta (ed.), Polymer Additives, Plenum, New York, 1984, p. 83. 8 I. R. Bellobono and E. Selli, in N. S. Allen (ed.), PhotopoIymetiation and Photoimaging Science and Technology, Elsevier, London, 1989, Chapter 4. 9 I. R. Bellobono, E. Selli, B. Marcandalli, D. Comi and E. Rastelli, I. Photochem., 35 (1986) 99.