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Monomers for non-bond crosslinking of vinyl polymers
European Polymer Journal 36 (2000) 359±364
Monomers for non-bond crosslinking of vinyl polymers III. Some characteristics of the system Anat Zada, Yair Avny, Albert Zilkha* Department of Organic Chemistry, The Hebrew University of Jerusalem, Jerusalem 91940, Israel Received 30 September 1998; received in revised form 13 January 1999; accepted 29 January 1999
Abstract In the new type of non-bond crosslinking where a vinyl macrocycle acts as the crosslinking agent, it is shown that factors aecting threading of polymer chains into macrocyclic rings which lead to crosslinking, such as ring size, ratio of co-monomer to cyclic monomer and length of the vinyl polymer chain are important factors that govern the extent of the crosslinking. They also aect the swelling of the crosslinked copolymers in various solvents, as well as their thermal properties. Some dierences from conventional crosslinking using difunctional vinyl monomers are pointed out. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Non-bond crosslinking; Cross-linking agent; Vinyl macrocycles; Threading
1. Introduction We have shown previously [1±3] that monomers having one polymerizable double bond besides a large macrocyclic ring through which a polymerizing chain may be threaded in the course of the propagation reaction can act as crosslinking agent for vinyl polymers leading to non-bond physically crosslinked polymers. This we have demonstrated with cyclic octaethylene glycol fumarate [1,2] and cyclic octaethylene glycol-5methacrylamido isophthalate [3]. Since only when threading occurs there would be crosslinking, the factors that govern threading are expected to aect the extent of the crosslinking. We concentrated in the present work on studying some of these factors which were not studied before. The monomers used for the study were cyclic octaethylene glycol fumarate and cyclic nonaethylene glycol fumarate, 29 and 32-membered rings, respectively. Thermal and swelling properties of the non-bond physically
* Corresponding author. Fax: +972-2-658-5345.
crosslinked polymers which are expected to dier from conventional crosslinked polymers were also studied. We have studied before [4] various parameters that aect the threading process and derived a mathematical expression to show the eect of various factors on the amount of threading: N
h i k me mg 1 ÿ eÿne =png ne nbg y V
where N is the number of threadings, me and mg are the number of moles of rings and chains, respectively, ne and ng are the number of atoms in rings and chains, respectively, V is the total volume, and y is the threading angle, which depends on the radius of the ring (r ) and diameter of the chain end (d ) and is determined by cos y d=2r , b and k are constants. Similar results were obtained in later works [5±8]. Out of these, factors of special importance are the ring size and the volume of the system. Polymerization in the presence of solvent that will dilute the system is supposed to lower drastically the amount of threading and consequently the crosslinking. Calculations [9]
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A. Zada et al. / European Polymer Journal 36 (2000) 359±364
have shown that for threading a straight paranic chain into a ring, a 22-membered cyclic structure is required. It can also be shown from molecular models [2] that for threading a polystyrene or a polymethyl methacrylate chain a 27±28 membered ring may be required. Another important factor that comes into play to help in the threading process is molecular recognition, anity between the growing polymer chain end and the ring system [10±13]. Although non-bond crosslinking of vinyl polymers being aected by so many parameters is a much more `delicate' process than the classical crosslinking with monomers of the divinyl benzene type, however, due to the special structure of the non-bond physically crosslinked polymers with more degrees of freedom in the movement of segments between the crosslinks, the resulting crosslinked polymers may have better swelling properties and better mechanical properties as regards impact strength or resisting stresses than the classically cross-linked polymers. These properties may ®nd use in improved ion-exchange resins, gels for electrophoresis, hydrogels, hydrophilic contact lenses, Merri®eld resins for automated synthesis of peptides and nucleic acids, heterogeneous catalysts, etc. 2. Experimental 2.1. Materials
Fumaryl chloride (2.17 ml, 0.02 mol) and dry nonaethylene glycol (8.3 g, 0.02 mol) each dissolved in dry dioxane (20 ml) were added dropwise, simultaneously, from syringes using a Sage metering pump during 24 h. The mixture was stirred under nitrogen for another 7 days and the toluene evaporated in vacuo at 408C. The residue was taken up in a small volume of ethyl acetate and chromatographed on eight times its weight of silica gel using ethyl acetate as eluent, yielding 0.9 g (9%) clear viscous material. Anal. Calcd. for C22H38O12: C, 53.43; H, 7.74. Found: C, 53.43; H, 7.84%. Rf = 0.4. 1 H-NMR (CDCl3): 6.89 (s, 2H, CH1CH), 4.37 (t, 4H, COOCH2), 3.74 (t, 4H, COOCH2CH2), 3.62±3.66 (m, 28H, ±OCH2CH2O±). The mass spectrum gave M+ + 1 at m/e 495.3 as required. 2.4. Homopolymerization of cyclic nonaethylene glycol fumarate Into a 5 ml test tube ¯ushed with N2 was introduced 0.27 g, 5:55 10ÿ4 mol of the cyclic monomer, and to it was added dibenzoyl peroxide (11 mg, 4:55 10ÿ5 mol) and a few drops of THF to help in the solution of the catalyst. The tube was stoppered and heated in an oven at 608C for 3 days followed by 1 day at 908C. The polymer was viscous and was soluble in chloroform. NMR showed the disappearance of the vinyl protons.
Toluene was dried by azeotropic distillation and standing over sodium. Dioxane was dried over sodium. Octaethylene and nonaethylene glycols were prepared [14] and dried by heating in vacuo at 1308C. Fumaryl chloride (Fluka) was used as received. Silica gel 60 (Merck) was used for column chromatography. TLC was carried out on aluminum oxide 60 F254 neutral (Type E) plates (Merck) eluted with ethyl acetate and visualized with iodine. Cyclic octaethylene glycol fumarate was prepared as before [2].
2.5. Copolymerization of cyclic nonaethylene glycol fumarate with styrene or methyl methacrylate
2.2. Instrumentation
3. Results and discussion
NMR spectra were recorded on a Bruker AMX-300 instrument. Mass spectra were carried out on a TSQ 70 Finnigan Mat triple quadruple spectrometer using chemical ionization. A Sage metering pump model 365 was used. Thermal properties were measured on a Mettler TA 4000 DSC.
To ®nd out the eect of ring size on the non-bond crosslinking of vinyl monomers, we have synthesized cyclic nonaethylene glycol fumarate having a 32-membered ring. This is in addition to cyclic octaethylene glycol fumarate having a 29-membered ring, which we have synthesized previously [2]. High dilution technique was used for the synthesis. Fumaryl chloride and nonaethylene glycol diluted with dioxane were added dropwise, simultaneously, from a Sage metering pump into stirred toluene at room temperature over a period of 24 h, and stirred for another 7 days. The macrocycle was isolated by column chromatography using silica
2.3. Cyclic nonaethylene glycol fumarate Into a 500 ml three-necked ¯ask ®tted with a condenser, a CaCl2 guard tube, a magnetic stirrer and a nitrogen atmosphere, toluene (200 ml) was introduced.
The polymerization was carried out in 5 ml test tubes as before. Dibenzoyl peroxide (2% molar) was dissolved in styrene or methyl methacrylate, dried on MgSO4, and the required amount was taken for copolymerization with the cyclic monomer. The polymerization was carried out for 3 days at 608C followed by 1 day at 908C. Solid copolymers were obtained.
A. Zada et al. / European Polymer Journal 36 (2000) 359±364
361
Table 1 Copolymerization of styrene and methyl methacrylate (MMA) with cyclic octaethylene glycol fumaratea Co-monomer
Molar ratio of cyclic monomer to co-monomer
Benzoyl peroxide (mole%)
Volume of solvent absorbed by 1 g copolyer (ml) CHCl3 Toluene
Styrene Styrene Styrene Styrene Stryene Stryene MMA MMA MMA MMA MMA Styrene Styrene MMA
1 1 1 1 1 1 1 1 1 1 1 1 1 1
2 2 2 2 2 2 2 2 2 2 2 9 9 9
10.42 13.63 ± 15.69 4.96b
5.96 6.13 7.42 8.47 5.22b
9.29 10.25 15.23 4.1b
5.63 7.28 7.18 1.55b
13.6 6.0b 11.3
6.26 7.9 3.07
: : : : : : : : : : : : : :
3 7 10 12 15 20 3 5 7 10 20 20 22 10
c
c
c
c
a Copolymerization was carried out for 3 days at 608C and 1 day at 908C. Swelling of copolymers was carried out for 2 days at 258C. b Partially dissolved. c Completely dissolved.
gel. Its structure was veri®ed by NMR, MS and elemental analysis.
Since as mentioned in Section 1 so many factors come into play during the threading process, we studied the free radical copolymerization of each of these monomers with styrene and methyl methacrylate using dierent molar ratios of vinyl monomer to cyclic
monomer, and also using dierent dibenzoyl peroxide initiator concentrations to change the length of the chain of the polymer that has to be threaded inside the macrocyclic ring. The results obtained with the cyclic octaethylene glycol fumarate are given in Table 1 and those with cyclic nonaethylene glycol fumarate in Table 2. The results recorded include swelling of the crosslinked polymers in toluene and chloroform. From Table 1, it can be seen that there is no strong eect of the type of the co-monomer, and similar swelling results are obtained with both methyl methacrylate and styrene. There is an eect of the swelling solvent on the extent of swelling, the crosslinked copolymers swelling is better in CHCl3 than in toluene.
Table 2 Copolymerization of styrene and methyl methacrylate (MMA) with cyclic nonaethylene glycol fumaratea Co-monomer Styrene Styrene Styrene Styrene Styrene MMA MMA
Molar ratio of cyclic monomer to co-monomer 1:5 1 : 10 1 : 20 1 : 30 1 : 40 1 : 10 1 : 20
Volume of toluene absorbed by 1g copolymer (ml) 1.85 3.58 5.66 6.99 b
4.74 b
a Copolymerization was carried out for 3 days at 608C and 1 day at 908C using 2 mole% of dibenzoyl peroxide as initiator. Swelling of copolymers was carried out for 2 days at 258C. b Completely dissolved.
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Table 3 Crosslinking of styrene and methyl methacrylate (MMA) with divinyl benzenea Co-monomer
Divinylbenzene (wt%)
Styrene Styrene Styrene MMA MMA MMA
0.25 0.50 5.0 0.25 0.50 5.0
Volume of solvent absorbed by 1 g copolymer (ml) CHCl3 Toluene 8.75 ± ± 7.25 ± ±
6.1 4.6 1.45 3.65 2.65 0.80
a The copolymerization was carried out using 2% benzoyl peroxide as initiator for 3 days at 608C and 1 day at 908C. Swelling of copolymers was carried out for 2 days at 258C.
Furthermore, it is observed that there is a constant increase in the degree of swelling either in CHCl3 or in toluene, as the molar ratio of the co-monomer to the cyclic crosslinking monomer is increased but up to a certain degree. This means that on using less crosslinking agent, there is a lower extent of crosslinking and that is why the distance between the crosslinks increases, with a consequent increase of the extent of swelling. On further increasing the ratio of vinyl monomer to cyclic monomer, the high dilution brought about by the increased amount of vinyl monomer lowers the extent of threading, and as found before increasing the solvent volume in a system lowers the extent of threading [4]. That is why the copolymers obtained at relatively high ratios of vinyl monomer to cyclic monomer are either partially or completely soluble for lack of sucient crosslinks. Now from the threading equation mentioned in Ref. [4], lowering the chain length that has to be threaded is supposed to increase the extent of threading. So we took the highest ratios of vinyl monomer to cyclic monomer where already only soluble copolymer was obtained, and we increased for these ratios the amount of benzoyl peroxide from 2 to 9% in order to decrease the chain length of the vinyl polymer obtained, and indeed where with the low concentration of benzoyl peroxide a soluble polymer was obtained, here with the higher catalyst concentration insoluble crosslinked polymers were formed (Table 1). It was interesting to compare the non-bond crosslinking system with that of the conventional divinyl benzene system. The results (Table 3) indicate that while with the latter system increasing the percentage of crosslinking agent lowers strongly the swelling of the polymer to very low levels (see results obtained (Table 3) with 5% divinyl benzene where the swelling was only 0.8±1.45 ml toluene/1 g crosslinked polymer), with the non-bond crosslinking system even at 33% vinyl macrocycle crosslinker the swelling was about 6 ml toluene for 1 g crosslinked polymer, and with the new type of crosslinking even still higher ratios will
lead to crosslinked polymers with a relatively high swelling capability contrary to conventional crosslinking with difunctional crosslinking agents. On comparing the results obtained with the two macrocyclic monomers, it can be seen that there are some eects due to the size of the ring. When the ring size is increased from 29 to 32 atoms, the extent of swelling of the crosslinked polymer is de®nitely lowered. This can be explained by the fact that threading is made easier and its extent, therefore, increases with increasing the ring size. More threading means that there are more `crosslinks' which limits the extent of swelling as in conventional crosslinking systems. Another observation is that even at higher molar ratios of vinyl monomer to cyclic monomer it was possible to obtain threading and crosslinking. With the smaller ring at a ratio of 1 : 20 of cyclic octaethylene fumarate to styrene already the copolymer was soluble due to insucient threading, caused by the dilution eect of the increased monomer concentration, but with the larger ring cyclic nonaethylene glycol fumarate, only at a ratio of cyclic monomer to styrene of 1 : 40, the copolymer obtained was not enough crosslinked and was soluble. This means that the eect of dilution of the system which lowers the extent of threading is less eective with larger ring systems, which make the threading through them easier. In Fig. 1, the rate of swelling of a copolymer of cyc-
Fig. 1. Swelling rate in toluene of copolymer of styrene and cyclic nonaethyleneglycol fumarate at a molar ratio of 30 : 1.
A. Zada et al. / European Polymer Journal 36 (2000) 359±364
363
Table 4 DSC analysis of crosslinked copolymers obtained with cyclic octaethylene glycol fumaratea Polymer
Molar ratio of cyclic monomer to co-monomer
Tg (8C)
Polycyclic octaethylene glycol fumarate Polystyrene Polymethyl methacrylate Copolymer styrene-cyclic monomer Copolymer MMA-cyclic monomer
± ± ± 1:7 1:7
ÿ30.8 92.2 106.8 49.6 50.7
a Polymerizations carried out as in Table 1 using 2 mol% benzoyl peroxide (8 mol% for homopoly cyclic monomer). In the thermal analysis, the temperature was increased at the rate of 108C/min.
Table 5 DSC analysis of crosslinked copolymers obtained with cyclic nonaethylene glycol fumaratea Polymer
Molar ratio of cyclic monomer to co-monomer
Polycyclic nonaethylene glycol fumarate Polystyrene Polymethyl methacrylate Copolymer styrene-cyclic monomer
± ± ± 1 1 1 1 1
Copolymer MMA-cyclic monomer
: : : : :
5 10 20 10 20
Tg (8C) ÿ39.4 87.4 109.3 38.4 50.1 58.6 39.7 54.7
a Polymerizations carried out as in Table 2, using 2 mol% benzoyl peroxide (8 mol% for homopoly cyclic monomer). In the thermal analysis, the temperature was increased at the rate of 108C/min.
lic nonaethylene glycol fumarate with styrene at a 1 : 30 ratio, is shown. As seen most of the swelling occurs after a few hours. For further characterization of the non-bond crosslinked polymers, we studied some of their thermal properties. The results of the DSC analysis conducted with crosslinked copolymers of cyclic octaethylene glycol fumarate are given in Table 4, and those with cyclic nonaethylene glycol fumarate in Table 5. It can be seen that the Tg's of the copolymers were in between those of the homopolymers from which they were composed. This ®ts the behavior of a random copolymer where there are mixed phases, and the copolymer's properties are an average between those of the homopolymers and not that of a crosslinked polymer, where Tg increases usually due to the crosslinking. However, on comparing the Tg's obtained by calculation from the equation used for calculating Tg of a copolymer, 1=Tg w1 =Tg1 w2 =Tg2 , where w1 and w2 are the fractional weights of each monomer component in the copolymer and Tg1 and Tg2 are the Tg's of the respective homopolymers, it became clear that the experimental Tg's found from DSC were generally higher than those calculated for soluble copolymers (Table 6) showing that non-bond physical crosslinking does
exert some restrictions on the movement of the polymer chains. In addition, it was found that the slope of the DSC experimental curve from which Tg was measured became more moderate on increasing the concentration of the macrocylic crosslinking agent, until it became dicult to discern the in¯ection point. For example, compare the DSC curves obtained with copolymers of styrene and cyclic nonaethylene glycol fumarate at a 5 : 1 ratio (Fig. 2) and 10 : 1 ratio (Fig. 3). Similar
Fig. 2. DSC analysis of copolymer of styrene and cyclic nonaethyleneglycol fumarate at a molar ratio of 5 : 1.
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Table 6 Experimental and calculated Tg's of non-bond crosslinked polymersa Co-monomer
Molar ratio of cyclic monomer to co-monomer
Cyclic octaethylene glycol and: Styrene 1: MMA 1: Cyclic nonaethylene glycol and: Styrene 1: Styrene 1: Styrene 1: MMA 1: MMA 1:
Experimental
Tg (8C)
Calculated
7 7
49.6 50.7
32.9 37.7
5 10 20 10 20
38.4 50.1 58.6 39.7 54.7
12.0 33.7 53.4 42.8 66.5
a Experimental conditions as in Tables 4 and 5, respectively. The Tg's were calculated from the equation below, where w1 and w2 are the weight fractions of the two homopolymers and Tg1 and Tg2 are their respective Tg's.
1 w1 w2 Tg Tg1 Tg2 .
References
Fig. 3. DSC analysis of copolymer of styrene and cyclic nonaethyleneglycol fumarate at a molar ratio of 10 : 1.
results were observed on comparing the DSC curves of the copolymers of methyl methacrylate with cyclic nonaethylene glycol fumarate at a ratio of 10 : 1 and 20 : 1. This can be due to the increased crosslinking obtained at the higher ratios of the cyclic crosslinking agent to vinyl monomer with subsequent more hindering of motion of the polymer segments.
[1] Zada A, Avny Y, Zilkha A. Polym Preprints 1997;38(1):145. [2] Zada A, Avny Y, Zilkha A. Eur Polym J 1999;35:1159. [3] Zada A, Avny Y, Zilkha A. Eur Polym J 2000;36:351. [4] Agam G, Graiver D, Zilkha A. J Am Chem Soc 1976;98:5206. [5] Clarson SJ, Mark JE, Semlyen JA. Polym Commun 1987;27:244. [6] Shen YS, Gibson HW. Macromolecules 1992;25:2058. [7] Shen YX, Xie D, Gibson HW. J Am Chem Soc 1994;116:537. [8] Loveday D, Wilkes GL, Bheda MC, Shen YX, Gibson HW. J Macromol Sci Chem 1995;A32(1):1. [9] Harrison IT. J Chem Soc Perkins Trans I 1974:301. [10] Hayes W, Stoddart JF. In: Semlyen JA, editor. Large ring molecules. New York: Wiley, 1996. p. 433. [11] Davis AP. In: Semlyen JA, editor. Large ring molecules. New York: Wiley, 1996. p. 473. [12] Gibson HW. In: Semlyen JA, editor. Large ring molecules. New York: Wiley, 1996. p. 191. [13] Raymo FM, Stoddart JF. Trends Polym Sci 1996;4:208. [14] Coudert G, Mpassi M, Gillaumet G, Selve C. Synth Comm 1986;16:19.
Monomers for non-bond crosslinking of vinyl polymers III. Some characteristics of the system Anat Zada, Yair Avny, Albert Zilkha* Department of Organic Chemistry, The Hebrew University of Jerusalem, Jerusalem 91940, Israel Received 30 September 1998; received in revised form 13 January 1999; accepted 29 January 1999
Abstract In the new type of non-bond crosslinking where a vinyl macrocycle acts as the crosslinking agent, it is shown that factors aecting threading of polymer chains into macrocyclic rings which lead to crosslinking, such as ring size, ratio of co-monomer to cyclic monomer and length of the vinyl polymer chain are important factors that govern the extent of the crosslinking. They also aect the swelling of the crosslinked copolymers in various solvents, as well as their thermal properties. Some dierences from conventional crosslinking using difunctional vinyl monomers are pointed out. # 1999 Elsevier Science Ltd. All rights reserved. Keywords: Non-bond crosslinking; Cross-linking agent; Vinyl macrocycles; Threading
1. Introduction We have shown previously [1±3] that monomers having one polymerizable double bond besides a large macrocyclic ring through which a polymerizing chain may be threaded in the course of the propagation reaction can act as crosslinking agent for vinyl polymers leading to non-bond physically crosslinked polymers. This we have demonstrated with cyclic octaethylene glycol fumarate [1,2] and cyclic octaethylene glycol-5methacrylamido isophthalate [3]. Since only when threading occurs there would be crosslinking, the factors that govern threading are expected to aect the extent of the crosslinking. We concentrated in the present work on studying some of these factors which were not studied before. The monomers used for the study were cyclic octaethylene glycol fumarate and cyclic nonaethylene glycol fumarate, 29 and 32-membered rings, respectively. Thermal and swelling properties of the non-bond physically
* Corresponding author. Fax: +972-2-658-5345.
crosslinked polymers which are expected to dier from conventional crosslinked polymers were also studied. We have studied before [4] various parameters that aect the threading process and derived a mathematical expression to show the eect of various factors on the amount of threading: N
h i k me mg 1 ÿ eÿne =png ne nbg y V
where N is the number of threadings, me and mg are the number of moles of rings and chains, respectively, ne and ng are the number of atoms in rings and chains, respectively, V is the total volume, and y is the threading angle, which depends on the radius of the ring (r ) and diameter of the chain end (d ) and is determined by cos y d=2r , b and k are constants. Similar results were obtained in later works [5±8]. Out of these, factors of special importance are the ring size and the volume of the system. Polymerization in the presence of solvent that will dilute the system is supposed to lower drastically the amount of threading and consequently the crosslinking. Calculations [9]
0014-3057/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 9 9 ) 0 0 0 7 5 - 0
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A. Zada et al. / European Polymer Journal 36 (2000) 359±364
have shown that for threading a straight paranic chain into a ring, a 22-membered cyclic structure is required. It can also be shown from molecular models [2] that for threading a polystyrene or a polymethyl methacrylate chain a 27±28 membered ring may be required. Another important factor that comes into play to help in the threading process is molecular recognition, anity between the growing polymer chain end and the ring system [10±13]. Although non-bond crosslinking of vinyl polymers being aected by so many parameters is a much more `delicate' process than the classical crosslinking with monomers of the divinyl benzene type, however, due to the special structure of the non-bond physically crosslinked polymers with more degrees of freedom in the movement of segments between the crosslinks, the resulting crosslinked polymers may have better swelling properties and better mechanical properties as regards impact strength or resisting stresses than the classically cross-linked polymers. These properties may ®nd use in improved ion-exchange resins, gels for electrophoresis, hydrogels, hydrophilic contact lenses, Merri®eld resins for automated synthesis of peptides and nucleic acids, heterogeneous catalysts, etc. 2. Experimental 2.1. Materials
Fumaryl chloride (2.17 ml, 0.02 mol) and dry nonaethylene glycol (8.3 g, 0.02 mol) each dissolved in dry dioxane (20 ml) were added dropwise, simultaneously, from syringes using a Sage metering pump during 24 h. The mixture was stirred under nitrogen for another 7 days and the toluene evaporated in vacuo at 408C. The residue was taken up in a small volume of ethyl acetate and chromatographed on eight times its weight of silica gel using ethyl acetate as eluent, yielding 0.9 g (9%) clear viscous material. Anal. Calcd. for C22H38O12: C, 53.43; H, 7.74. Found: C, 53.43; H, 7.84%. Rf = 0.4. 1 H-NMR (CDCl3): 6.89 (s, 2H, CH1CH), 4.37 (t, 4H, COOCH2), 3.74 (t, 4H, COOCH2CH2), 3.62±3.66 (m, 28H, ±OCH2CH2O±). The mass spectrum gave M+ + 1 at m/e 495.3 as required. 2.4. Homopolymerization of cyclic nonaethylene glycol fumarate Into a 5 ml test tube ¯ushed with N2 was introduced 0.27 g, 5:55 10ÿ4 mol of the cyclic monomer, and to it was added dibenzoyl peroxide (11 mg, 4:55 10ÿ5 mol) and a few drops of THF to help in the solution of the catalyst. The tube was stoppered and heated in an oven at 608C for 3 days followed by 1 day at 908C. The polymer was viscous and was soluble in chloroform. NMR showed the disappearance of the vinyl protons.
Toluene was dried by azeotropic distillation and standing over sodium. Dioxane was dried over sodium. Octaethylene and nonaethylene glycols were prepared [14] and dried by heating in vacuo at 1308C. Fumaryl chloride (Fluka) was used as received. Silica gel 60 (Merck) was used for column chromatography. TLC was carried out on aluminum oxide 60 F254 neutral (Type E) plates (Merck) eluted with ethyl acetate and visualized with iodine. Cyclic octaethylene glycol fumarate was prepared as before [2].
2.5. Copolymerization of cyclic nonaethylene glycol fumarate with styrene or methyl methacrylate
2.2. Instrumentation
3. Results and discussion
NMR spectra were recorded on a Bruker AMX-300 instrument. Mass spectra were carried out on a TSQ 70 Finnigan Mat triple quadruple spectrometer using chemical ionization. A Sage metering pump model 365 was used. Thermal properties were measured on a Mettler TA 4000 DSC.
To ®nd out the eect of ring size on the non-bond crosslinking of vinyl monomers, we have synthesized cyclic nonaethylene glycol fumarate having a 32-membered ring. This is in addition to cyclic octaethylene glycol fumarate having a 29-membered ring, which we have synthesized previously [2]. High dilution technique was used for the synthesis. Fumaryl chloride and nonaethylene glycol diluted with dioxane were added dropwise, simultaneously, from a Sage metering pump into stirred toluene at room temperature over a period of 24 h, and stirred for another 7 days. The macrocycle was isolated by column chromatography using silica
2.3. Cyclic nonaethylene glycol fumarate Into a 500 ml three-necked ¯ask ®tted with a condenser, a CaCl2 guard tube, a magnetic stirrer and a nitrogen atmosphere, toluene (200 ml) was introduced.
The polymerization was carried out in 5 ml test tubes as before. Dibenzoyl peroxide (2% molar) was dissolved in styrene or methyl methacrylate, dried on MgSO4, and the required amount was taken for copolymerization with the cyclic monomer. The polymerization was carried out for 3 days at 608C followed by 1 day at 908C. Solid copolymers were obtained.
A. Zada et al. / European Polymer Journal 36 (2000) 359±364
361
Table 1 Copolymerization of styrene and methyl methacrylate (MMA) with cyclic octaethylene glycol fumaratea Co-monomer
Molar ratio of cyclic monomer to co-monomer
Benzoyl peroxide (mole%)
Volume of solvent absorbed by 1 g copolyer (ml) CHCl3 Toluene
Styrene Styrene Styrene Styrene Stryene Stryene MMA MMA MMA MMA MMA Styrene Styrene MMA
1 1 1 1 1 1 1 1 1 1 1 1 1 1
2 2 2 2 2 2 2 2 2 2 2 9 9 9
10.42 13.63 ± 15.69 4.96b
5.96 6.13 7.42 8.47 5.22b
9.29 10.25 15.23 4.1b
5.63 7.28 7.18 1.55b
13.6 6.0b 11.3
6.26 7.9 3.07
: : : : : : : : : : : : : :
3 7 10 12 15 20 3 5 7 10 20 20 22 10
c
c
c
c
a Copolymerization was carried out for 3 days at 608C and 1 day at 908C. Swelling of copolymers was carried out for 2 days at 258C. b Partially dissolved. c Completely dissolved.
gel. Its structure was veri®ed by NMR, MS and elemental analysis.
Since as mentioned in Section 1 so many factors come into play during the threading process, we studied the free radical copolymerization of each of these monomers with styrene and methyl methacrylate using dierent molar ratios of vinyl monomer to cyclic
monomer, and also using dierent dibenzoyl peroxide initiator concentrations to change the length of the chain of the polymer that has to be threaded inside the macrocyclic ring. The results obtained with the cyclic octaethylene glycol fumarate are given in Table 1 and those with cyclic nonaethylene glycol fumarate in Table 2. The results recorded include swelling of the crosslinked polymers in toluene and chloroform. From Table 1, it can be seen that there is no strong eect of the type of the co-monomer, and similar swelling results are obtained with both methyl methacrylate and styrene. There is an eect of the swelling solvent on the extent of swelling, the crosslinked copolymers swelling is better in CHCl3 than in toluene.
Table 2 Copolymerization of styrene and methyl methacrylate (MMA) with cyclic nonaethylene glycol fumaratea Co-monomer Styrene Styrene Styrene Styrene Styrene MMA MMA
Molar ratio of cyclic monomer to co-monomer 1:5 1 : 10 1 : 20 1 : 30 1 : 40 1 : 10 1 : 20
Volume of toluene absorbed by 1g copolymer (ml) 1.85 3.58 5.66 6.99 b
4.74 b
a Copolymerization was carried out for 3 days at 608C and 1 day at 908C using 2 mole% of dibenzoyl peroxide as initiator. Swelling of copolymers was carried out for 2 days at 258C. b Completely dissolved.
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Table 3 Crosslinking of styrene and methyl methacrylate (MMA) with divinyl benzenea Co-monomer
Divinylbenzene (wt%)
Styrene Styrene Styrene MMA MMA MMA
0.25 0.50 5.0 0.25 0.50 5.0
Volume of solvent absorbed by 1 g copolymer (ml) CHCl3 Toluene 8.75 ± ± 7.25 ± ±
6.1 4.6 1.45 3.65 2.65 0.80
a The copolymerization was carried out using 2% benzoyl peroxide as initiator for 3 days at 608C and 1 day at 908C. Swelling of copolymers was carried out for 2 days at 258C.
Furthermore, it is observed that there is a constant increase in the degree of swelling either in CHCl3 or in toluene, as the molar ratio of the co-monomer to the cyclic crosslinking monomer is increased but up to a certain degree. This means that on using less crosslinking agent, there is a lower extent of crosslinking and that is why the distance between the crosslinks increases, with a consequent increase of the extent of swelling. On further increasing the ratio of vinyl monomer to cyclic monomer, the high dilution brought about by the increased amount of vinyl monomer lowers the extent of threading, and as found before increasing the solvent volume in a system lowers the extent of threading [4]. That is why the copolymers obtained at relatively high ratios of vinyl monomer to cyclic monomer are either partially or completely soluble for lack of sucient crosslinks. Now from the threading equation mentioned in Ref. [4], lowering the chain length that has to be threaded is supposed to increase the extent of threading. So we took the highest ratios of vinyl monomer to cyclic monomer where already only soluble copolymer was obtained, and we increased for these ratios the amount of benzoyl peroxide from 2 to 9% in order to decrease the chain length of the vinyl polymer obtained, and indeed where with the low concentration of benzoyl peroxide a soluble polymer was obtained, here with the higher catalyst concentration insoluble crosslinked polymers were formed (Table 1). It was interesting to compare the non-bond crosslinking system with that of the conventional divinyl benzene system. The results (Table 3) indicate that while with the latter system increasing the percentage of crosslinking agent lowers strongly the swelling of the polymer to very low levels (see results obtained (Table 3) with 5% divinyl benzene where the swelling was only 0.8±1.45 ml toluene/1 g crosslinked polymer), with the non-bond crosslinking system even at 33% vinyl macrocycle crosslinker the swelling was about 6 ml toluene for 1 g crosslinked polymer, and with the new type of crosslinking even still higher ratios will
lead to crosslinked polymers with a relatively high swelling capability contrary to conventional crosslinking with difunctional crosslinking agents. On comparing the results obtained with the two macrocyclic monomers, it can be seen that there are some eects due to the size of the ring. When the ring size is increased from 29 to 32 atoms, the extent of swelling of the crosslinked polymer is de®nitely lowered. This can be explained by the fact that threading is made easier and its extent, therefore, increases with increasing the ring size. More threading means that there are more `crosslinks' which limits the extent of swelling as in conventional crosslinking systems. Another observation is that even at higher molar ratios of vinyl monomer to cyclic monomer it was possible to obtain threading and crosslinking. With the smaller ring at a ratio of 1 : 20 of cyclic octaethylene fumarate to styrene already the copolymer was soluble due to insucient threading, caused by the dilution eect of the increased monomer concentration, but with the larger ring cyclic nonaethylene glycol fumarate, only at a ratio of cyclic monomer to styrene of 1 : 40, the copolymer obtained was not enough crosslinked and was soluble. This means that the eect of dilution of the system which lowers the extent of threading is less eective with larger ring systems, which make the threading through them easier. In Fig. 1, the rate of swelling of a copolymer of cyc-
Fig. 1. Swelling rate in toluene of copolymer of styrene and cyclic nonaethyleneglycol fumarate at a molar ratio of 30 : 1.
A. Zada et al. / European Polymer Journal 36 (2000) 359±364
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Table 4 DSC analysis of crosslinked copolymers obtained with cyclic octaethylene glycol fumaratea Polymer
Molar ratio of cyclic monomer to co-monomer
Tg (8C)
Polycyclic octaethylene glycol fumarate Polystyrene Polymethyl methacrylate Copolymer styrene-cyclic monomer Copolymer MMA-cyclic monomer
± ± ± 1:7 1:7
ÿ30.8 92.2 106.8 49.6 50.7
a Polymerizations carried out as in Table 1 using 2 mol% benzoyl peroxide (8 mol% for homopoly cyclic monomer). In the thermal analysis, the temperature was increased at the rate of 108C/min.
Table 5 DSC analysis of crosslinked copolymers obtained with cyclic nonaethylene glycol fumaratea Polymer
Molar ratio of cyclic monomer to co-monomer
Polycyclic nonaethylene glycol fumarate Polystyrene Polymethyl methacrylate Copolymer styrene-cyclic monomer
± ± ± 1 1 1 1 1
Copolymer MMA-cyclic monomer
: : : : :
5 10 20 10 20
Tg (8C) ÿ39.4 87.4 109.3 38.4 50.1 58.6 39.7 54.7
a Polymerizations carried out as in Table 2, using 2 mol% benzoyl peroxide (8 mol% for homopoly cyclic monomer). In the thermal analysis, the temperature was increased at the rate of 108C/min.
lic nonaethylene glycol fumarate with styrene at a 1 : 30 ratio, is shown. As seen most of the swelling occurs after a few hours. For further characterization of the non-bond crosslinked polymers, we studied some of their thermal properties. The results of the DSC analysis conducted with crosslinked copolymers of cyclic octaethylene glycol fumarate are given in Table 4, and those with cyclic nonaethylene glycol fumarate in Table 5. It can be seen that the Tg's of the copolymers were in between those of the homopolymers from which they were composed. This ®ts the behavior of a random copolymer where there are mixed phases, and the copolymer's properties are an average between those of the homopolymers and not that of a crosslinked polymer, where Tg increases usually due to the crosslinking. However, on comparing the Tg's obtained by calculation from the equation used for calculating Tg of a copolymer, 1=Tg w1 =Tg1 w2 =Tg2 , where w1 and w2 are the fractional weights of each monomer component in the copolymer and Tg1 and Tg2 are the Tg's of the respective homopolymers, it became clear that the experimental Tg's found from DSC were generally higher than those calculated for soluble copolymers (Table 6) showing that non-bond physical crosslinking does
exert some restrictions on the movement of the polymer chains. In addition, it was found that the slope of the DSC experimental curve from which Tg was measured became more moderate on increasing the concentration of the macrocylic crosslinking agent, until it became dicult to discern the in¯ection point. For example, compare the DSC curves obtained with copolymers of styrene and cyclic nonaethylene glycol fumarate at a 5 : 1 ratio (Fig. 2) and 10 : 1 ratio (Fig. 3). Similar
Fig. 2. DSC analysis of copolymer of styrene and cyclic nonaethyleneglycol fumarate at a molar ratio of 5 : 1.
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A. Zada et al. / European Polymer Journal 36 (2000) 359±364
Table 6 Experimental and calculated Tg's of non-bond crosslinked polymersa Co-monomer
Molar ratio of cyclic monomer to co-monomer
Cyclic octaethylene glycol and: Styrene 1: MMA 1: Cyclic nonaethylene glycol and: Styrene 1: Styrene 1: Styrene 1: MMA 1: MMA 1:
Experimental
Tg (8C)
Calculated
7 7
49.6 50.7
32.9 37.7
5 10 20 10 20
38.4 50.1 58.6 39.7 54.7
12.0 33.7 53.4 42.8 66.5
a Experimental conditions as in Tables 4 and 5, respectively. The Tg's were calculated from the equation below, where w1 and w2 are the weight fractions of the two homopolymers and Tg1 and Tg2 are their respective Tg's.
1 w1 w2 Tg Tg1 Tg2 .
References
Fig. 3. DSC analysis of copolymer of styrene and cyclic nonaethyleneglycol fumarate at a molar ratio of 10 : 1.
results were observed on comparing the DSC curves of the copolymers of methyl methacrylate with cyclic nonaethylene glycol fumarate at a ratio of 10 : 1 and 20 : 1. This can be due to the increased crosslinking obtained at the higher ratios of the cyclic crosslinking agent to vinyl monomer with subsequent more hindering of motion of the polymer segments.
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