Acta Chim. Slov. 2005, 52, 215-223
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Scientific Paper
Influence of Crosslinker and Monomer Ratio on Bead Size
Distribution, Swelling and Polymer Network Flexibility of
4-Nitrophenylacrylate Polymer Supports1
Irena Pulko and Peter Krajnc*
University of Maribor, Faculty of Chemistry and Chemical Engineering, Laboratory for Organic and Polymer Chemistry,
Smetanova 17, 2000 Maribor, Slovenia E-mail: peter.krajnc@uni-mb.si
Received 15-06-2005
Dedicated to the memory ol Prof. Dr. Tatjana Malavašič
Abstract
Sphere shaped polymer supports with styrene and 4-nitrophenylacrylate as monomers and divinylbenzene (DVB) or ethyleneglycoldimethacrylate (EGDMA) as crosslinkers were prepared by free radical polymerization in suspension medium. Ratio of monomers as well as the crosslinking degree varied in order to test the influences on bead size distribution, svvelling and polymer netvvork flexibility. The amount of crosslinker had an effect on bead size, average bead diameters being betvveen 10 /xm and 35 /xm when 5% of crosslinker was applied and betvveen 35 /xm and 55 /xm when 20% was used. The crosslinking degree also affected svvelling in dichloromethane, water, methanol, toluene and acetonitrile, being more intense with lower concentrations of DVB or EGDMA. The flexibility of polymer network was investigated using reactions with 1,8-diaminooctane. High degrees of additional crosslinking was observed, namely betvveen 58 and 95% indicating high flexibility of polymer netvvork.
Key words: polymer supports, 4-nitrophenilacrylate, suspension polymerisation, bead size distribution

Introduction
Polymer supports for anchoring a reactive species, be it a substrate, a reagent, a catalyst, a scavenger or a part of a sensor system, have through the last decades proven to be an invaluable tool facilitating synthesis and separation techniques. Since the first reported tetrapeptide synthesis on solid support,1 the field of polymer supported chemistry is rapidly expanding.2 The most commonly used form of polymer support for batch operations are sphered particles prepared by free radical polymerisation in a suspension medium. The reasons for that are in the relative ease of preparation and in the suitability of the method for producing bead shaped particles between 10 and 500 /im. Such particles can be easily manipulated by the use of standard laboratory equipment for separation. New forms of polymer supports, such as monolithic columns and disks,3 are appearing, especially for separation techniques. On the other hand, beads stili remain the most commonly used form of support, especially for batch type setups. With regards to the chemistry of supports, the majority is prepared from 4-vinylbenzyl chloride (VBC) and crosslinked with DVB. Chloromethyl groups enable further chemical modifications of supports and many derivatives of crosslinked chloromethylated polystyrene
are now commercially available.4 The drawback of VBC/ DVB supports are their mainly non-polar structure, which limits the spectrum of solvents they can be used in. Namely, since the majority of reactive sites are positioned inside the particle, the polymer particle must swell in the reaction medium to enable contacts with the substrate in the solution. Higher degrees of crosslinking partly solve this problem, however for most times the reacting medium is stili the swollen gel. Reactive acrylates have been introduced in to the field of polymer supports in order to address this problem.5 Crosslinked copolymers of 4-nitrophenylacrylate and styrene in the form of beads have been prepared by suspension polymerisation6 and in the form of monoliths by emulsion polymerisation.7 Studies on the functionalisations of these supports have also been published.8'9 The work on 4-nitrophenylacrylate supports so far has proven the suitability of the monomer for inclusion into polymer supports. It has been demonstrated that it can easily be prepared from 4-nitrophenol and acryloyl chloride and polymerizes well with stvrene and divinylbenzene. We therefore wished to study more in detail how the ratio of styrene to 4-nitrophenylacrylate, crosslinker type and degree influence bead size distribution, svvelling of beads and polymer netvvork flexibility. The results are presented in the present paper.
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Experimental section
Materials
Commercially available 4-nitrophenol (Aldrich), acryloylchloride (Merck), triethylamine (Fluka), dichloromethane (DCM, Merck), sodium hydrogen carbonate (Kemika), magnesium sulphate (Kemika), chlorobenzene (Aldrich), Ar,Ar-dimethylformamide (Carlo Erba Reagenti), methanol (Fluka), 1,8-diaminooctane (Aldrich), acetonitrile (Riedel- de Haën), a,a'-azoisobutyronitrile (AIBN; Fluka) and poly(vinylpyrrolidone) (Aldrich, M= 40000) were used as received. Divinylbenzene (DVB; Aldrich, a mixture of 80% ethylstyrene and 20% of a mixture of isometric divinylbenzene), styrene (S; Aldrich) and ethylene glycol dimethacrylate (EGDMA; Aldrich) were washed with NaOH (5%) and water before use. 4-nitrophenylacrylate (A) was prepared by literature methods.10 FTIR spectra were taken on a Perkin-Elmer FT-IR 1720X spectrometer (KBr pellets). Combustion elemental analyses were done on a Perkin-Elmer 2400 CHN analyzer. Optical micrographs were recorded on a Nikon EPIPHOT 300U microscope with a digital camera.
Preparation of crosslinked copoly(styrene-4-nitrophenylacrylates) (la, lb, 2a, 2b, 3a, 3b, la’, lb’, 2a’, 2b’, 3a’, 3b’).
Water phase, consisting of 1.67 g of poly (vinylpyrrolidone) dissolved in 250 mL of de-ionized water, which was previously degassed under reduced pressure for 15 min, was put in a 350 mL pofymerisation reactor. Lo this, under stirring with an overhead stirrer at 750 rpm, the oil phase, consisting of a chlorobenzene solution of monomers, crosslinker and radical initiator, was added (5 mL of chlorobenzene, 150 mg of AIBN, 4-nitrophenylacrylate, styrene, divinylbenzene or ethylene glycol dimethacrylate - their amounts are given in Lable 1).
Table 1. Preparation data for polymer supports.
	Acrylate to styrene ratio	Crosslinker	mA (g)	mS (g)	m DVB (g)	m EGDMA (g)
la	1/1	5% DVB	1.737	0.936	0.157	-
lb	1/1	20% DVB	1.737	0.936	0.783	-
2a	2/1	5% DVB	2.316	0.624	0.157	-
2b	2/1	20% DVB	2.316	0.624	0.783	-
3a	1/0	5% DVB	3.474	-	0.157	-
3b	1/0	20% DVB	3.474	-	0.783	-
la'	1/1	5% EGDMA	1.737	0.936	-	0.188
lb'	1/1	20% EGDMA	1.737	0.936	-	0.891
2a'	2/1	5% EGDMA	2.316	0.624	-	0.188
2b'	2/1	20% EGDMA	2.316	0.624	-	0.891
3a'	1/0	5% EGDMA	3.474	-	-	0.188
3b'	1/0	20% EGDMA	3.474	_	_	0.891
Lhe reaction mkture was stirred for 4 hours at 80 °C and for 4 hours at 90 °C. Lhe resulting polymer beads were filtered, purified by Soxhlet extraction with methanol for 24 hours and dried at room temperature for 48 hours. Lhe following amounts of air-dry products were isolated: 2.205 g of la, 2.620 g of lb, 2.297 g of 2a, 2.946 g of 2b, 2.741 g of 3a, 4.028 g of 3b, 1.925 g of la’, 3.061 g of lb’, 2.398 g of 2a’, 3.443 g of 2b’, 2.886 g of 3a’, 3.754 g of 3b’.
Polymer beads were analyzed by FLIR spectroscopy. For combustion analyses air-dry products were further dried for 3 hours at 110 °C in vacuo and dry samples were obtained with the following compositions:
la (crosslinking DVB 5%, A:S = 1:1); Calcd.: C 70.10%, H 5.21%, N 4.43%, found: C 70.25%, H 5.42%, N 5.20%; 3.7 mmol of 4-nitrophenyl groups per gram. FLIR (KBr): 3493, 2926, 1756 (CO), 1614, 1592, 1519, 1490, 1452 (N02), 1347, 1206, 1113, 1012, 863, 747,
700, 495 (cm4).
lb (crosslinking DVB 20%, A:S = 1:1); Calcd.: C 74.04%, H 5.65%, N 3.64%, found: C 72.52%, H 5.70%, N 4.27%; 3.1 mmol of 4-nitrophenyl groups per gram. FLIR (KBr): 3113, 2925, 1760 (CO), 1615, 1592, 1520, 1490, 1452 (N02), 1348, 1206, 1112, 1012, 862, 747, 701 (s), 495 (cm4).
2a (crosslinking DVB 5%, A:S = 2:1); Calcd.: C 65.43%, H 4.69%, N 5.36%, found: C 66.98%, H 5.05%, N 5.43%; 3.9 mmol of 4-nitrophenyl groups per gram. FLIR (KBr): 3494, 2929, 1757 (CO), 1615, 1593, 1520, 1490, 1452 (N02), 1348, 1206, 1115, 1012, 863, 747,
701, 495 (cm4).
2b (crosslinking DVB 20%, A:S = 2:1); Calcd.: C 69.76%, H 5.17%, N 4.50%, found: C 69.85%, H 5.35%, N 4.78%; 3.4 mmol of 4-nitrophenyl groups per gram. FLIR (KBr): 3492, 2927, 1757 (CO), 1615, 1592, 1524, 1490, 1452 (N02), 1347, 1206, 1113, 1011, 862, 747,
702, 494 (cm4).
3a (crosslinking DVB 5%, A:S = 1:0); Calcd.: C 57.64%, H 3.81%, N 6.92%, found: C 57.40%, H 4.09%, N 6.78%; 4.8 mmol of 4-nitrophenyl groups per gram. FLIR (KBr): 3491, 2935, 1759 (CO), 1615, 1593, 1522, 1490, 1449 (N02), 1348, 1206, 1128, 1012, 862, 747, 690, 495 (cm4).
3b (crosslinking DVB 20%, A:S = 1:0); Calcd.: C 62.68%, H 4.38%, N 5.91%, found: C 63.09%, H 4.09%, N 6.00%; 4.3 mmol of 4-nitrophenyl groups per gram. FLIR (KBr): 3486, 2933, 1759 (CO), 1615, 1592, 1520, 1490, 1448 (N02), 1348, 1291, 1206, 1161, 1124, 1012, 863, 747, 710, 495 (cm4).
la’ (crosslinking EGDMA 5%, A:S = 1:1); Calcd.: C 68.20%, H 5.19%, N 4.40%, found: C 69.01%, H 5.51%, N 4.72%; 3.4 mmol of 4-nitrophenyl groups per gram. FLIR (KBr): 3646, 2931, 2858, 1760 (CO), 1683, 1634, 1615, 1593, 1520, 1490, 1453 (N02), 1348, 1206, 1113, 863, 748, 701, 495 (cm4).
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lb’ (crosslinking EGDMA 20%, A:S = 1:1); Calcd.: C 66.68%, H 5.56%, N 3.53%, found: C 66.51%, H 5.68%, N 3.64%; 2.6 mmol of 4-nitrophenyl groups per gram. FTIR (KBr): 3646, 3083, 2942, 1760 (CO), 1732 (CO), 1615, 1593, 1525, 1491, 1453 (N02), 1347, 1206, 1110, 862, 747, 700, 493 (cm4).
2a’ (crosslinking EGDMA 5%, A:S = 2:1); Calcd.: C 63.50%, H 4.65%, N 5.37%, found: C 64.67%, H 5.01%, N 5.44%; 3.9 mmol of 4-nitrophenyl groups per gram. FTIR (KBr): 3649, 2936, 2859, 1759 (CO), 1684, 1616, 1593, 1524, 1490, 1453 (N02), 1348, 1205, 1111, 862, 747, 701, 494 (cm4).
2b’ (crosslinking EGDMA 20%, A:S = 2:1); Calcd.: C 62.99%, H 5.09%, N 4.38%, found: C 62.90%, H 5.29%, N 4.58%; 3.3 mmol of 4-nitrophenyl groups per gram. FTIR (KBr): 2940, 1760 (CO), 1731 (CO), 1616, 1593, 1525, 1490, 1453 (N02), 1348, 1206, 1112, 862, 747, 700, 494 (cm4).
3a’ (crosslinking EGDMA 5%, A:S = 1:0); Calcd.: C 56.20%, H 3.81%, N 6.87%, found: C 56.19%, H 4.04%, N 6.89%; 4.9 mmol of 4-nitrophenyl groups per gram. FTIR (KBr): 3647, 2938, 2860, 1760 (CO), 1616, 1593, 1520, 1489, 1449 (N02), 1348, 1202, 1124, 862, 746, 689, 494 (cm4).
3b’ (crosslinking EGDMA 20%, A:S = 1:0); Calcd.: C 56.91%, H 4.34%, N 5.76%, found: C 56.81%, H 4.53%, N 5.88%; 4.2 mmol of 4-nitrophenyl groups per gram. FTIR (KBr): 3646, 2945, 1760 (CO), 1731 (CO), 1616, 1593, 1519, 1490, 1450 (N02), 1348, 1206, 1117, 1012, 885, 862, 746, 690, 494 (cm4).
Reactions of crosslinked copoly(styrene-4-nitrophenylacrylates) (la, lb, 2a, 2b, 3a, 3b, la’, lb’, 2a’, 2b’, 3a’, 3b’) with 1,8-diaminooctane.
Copoly(styrene-4-nitrophenylacrylate) la (0.1504 g) or lb (0.1510 g) or 2a (0.1524 g) or 2b (0.1508 g) or 3a (0.1517 g) or 3b (0.1518 g) or la’ (0.1515 g) or lb’ (0.1509 g) or 2a’ (0.1509 g) or 2b’ (0.1505 g) or 3a’ (0.1509 g) or 3b’ (0.1530 g) was suspended in 5 mL of AyV-dimethylformamide and the appropriate amount of 1,8-diaminooctane was added (0.3430 g for la, 0.2818 g for lb, 0.4160 g for 2a, 0.3483 g for 2b, 0.5354 g for 3a, 0.4601 g for 3b, 0.3396 g for la’, 0.2729 g for lb’, 0.4181 g for 2a’, 0.3386 g for 2b’, 0.5323 g for 3a’, 0.4453 g for 3b’, at a molar ratio of 4-nitrophenylacrylate to 1,8-diaminooctane 1 to 5). The reaction mkture was heated
at 50 °C under stirring for 24 hours. The solid product was filtered, washed with Ar,Ar-dimethylformamide (3x5 mL), a mixture of Ar^V-dimethylformamide and triethylamine (1:1; 3x5 mL) and methanol (3x5 mL) and dried at room temperature for 20 hours and in vacuo at 90 °C for 3 hours. FTIR and elemental analyses data are as follows:
4a (derived from la); found 5 mmol of amido and amino groups/g. FTIR (KBr): 3299, 2927, 2854, 1652 (CO), 1602, 1540, 1494, 1452, 1385, 1248, 1095, 1029, 761, 701, 668, 544 (cm4).
4b (derived from lb); found 4.1 mmol of amido and amino groups/g. FTIR (KBr): 3626, 3314, 2926, 2853, 1652 (CO), 1520, 1452, 701 (cm4).
5a (derived from 2a); found 5.5 mmol of amido and amino groups/g. FTIR (KBr): 3306, 2927, 2854, 1652 (CO), 1543, 1452, 702 (cm4).
5b (derived from 2b); found 4.6 mmol of amido and amino groups/g. FTIR (KBr): 3317, 2926, 2852,
1651  (CO), 1521, 1451, 702 (cm4).
6a (derived from 3a); found 7 mmol of amido and amino groups/g. FTIR (KBr): 3305, 2929, 2855, 1652 (CO), 1549, 1445, 720 (cm4).
6b (derived from 3b); found 5.8 mmol of amido and amino groups/g. FTIR (KBr): 3439, 2929, 2855,
1652 (CO), 1540, 711 (cm4).
4a’ (derived from la’); found 4 mmol of amido and amino groups/g. FTIR (KBr): 3304, 2928, 1729 (CO), 1654 (CO), 1541, 1453, 1386, 1180, 1106, 762, 701 (cm4).
4b’ (derived from lb’); found 3.7 mmol of amido and amino groups/g. FTIR (KBr): 3417, 2928, 1732 (CO), 1661 (CO), 1539, 1453, 761, 701 (cm4).
5a’ (derived from 2a’); found 5.6 mmol of amido and amino groups/g. FTIR (KBr): 3303, 2928, 1729, 1652, 1453, 1385, 1250, 1192, 1097, 762, 702 (cm4).
5b’ (derived from 2b’); found 4.1 mmol of amido and amino groups/g. FTIR (KBr): 3413, 3062, 3027, 2929, 2855, 1732 (CO), 1661 (CO), 1540, 1495, 1454, 1386, 1109, 762, 702, 668, 467 (cm4).
6a’ (derived from 3a’); found 7.2 mmol of amido and amino groups/g. FTIR (KBr): 3308, 2929, 1730 (CO), 1654 (CO), 1547, 1457, 1385, 722 (cm4).
6b’ (derived from 3b’); found 5.8 mmol of amido and amino groups/g. FTIR (KBr): 2930, 1732 (CO), 1661 (CO), 1548, 1456, 1166, 668 (cm4).
Table 2. Swelling capacities of polymer supports (mL/g).
Solvent\Polymer	la	lb	2a	2b	3a	3b	la'	lb'	2a'	2b'	3a'	3b'
Methanol	2.6	2.0	2.0	1.8	2.4	2.5	4.0	1.7	2.7	1.9	2.5	1.5
Water	2.8	1.7	2.0	1.7	2.7	2.6	3.9	2.0	3.2	1.7	2.3	1.4
Acetonitrile	4.2	3.6	3.4	3.3	5.6	3.2	3.9	3.2	2.7	3.8	4.4	2.8
Dichloromethane	17.4	3.9	7.3	3.5	8.6	4.7	5.8	4.2	11.6	4.5	5.8	3.4
Toluene	4.1	3.3	3.0	3.0	3.3	3.1	3.1	3.6	3.8	3.3	2.6	2.2
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Table 3. Elemental		analysis	data of polymer supports.						
Polymer	Acrylate to styrene ratio		Crosslinker	Elemental	analysis-calculated (%)		Elemental	analysis- found	(%)
support				C	H	N	C	H	N
la	1/1		5% DVB	70.10	5.21	4.43	70.25	5.42	5.20
lb	1/1		20% DVB	74.04	5.65	3.64	72.52	5.70	4.27
2a	2/1		5% DVB	65.43	4.69	5.36	66.98	5.05	5.43
2b	2/1		20% DVB	69.76	5.17	4.50	69.85	5.35	4.78
3a	1/0		5% DVB	57.64	3.81	6.92	57.40	4.09	6.78
3b	1/0		20% DVB	62.68	4.38	5.91	63.09	4.09	6.00
la'	1/1		5% EGDMA	68.20	5.19	4.40	69.01	5.51	4.72
lb'	1/1		20% EGDMA	66.68	5.56	3.53	66.51	5.68	3.64
2a'	2/1		5% EGDMA	63.50	4.65	5.37	64.67	5.01	5.44
2b'	2/1		20% EGDMA	62.99	5.09	4.38	62.90	5.29	4.58
3a'	1/0		5% EGDMA	56.20	3.81	6.87	56.19	4.04	6.89
3b'	1/0		20% EGDMA	56.91	4.34	5.76	56.81	4.53	5.88
Determination of the swelling capacities of polymer resins.
10 mL of solvent (methanol or water or acetonitrile or dichloromethane or toluene) was poured over 1 mL of air-dry polvmer resin, which was weighed and placed in a graduated cvlinder. After 24 hours the volume of swollen beads was measured. Lhe swelling capacities per gram of polvmer are presented in the Lable 2.
Results and discussion
Preparation of beads
4-Nitrophenylacrylate was chosen as a monomer due to its proven reactivity both in terms of polvmerisation (together with DVB) and in terms of possible further functionalisations (reactive ester due to its good leaving group).8'9 Furthermore, 4-nitrophenylacrylate can be easilv prepared from 4-nitrophenol and acryloyl chloride. Lhe method of suspension in conjunction with free radical polvmerisation (by using azobisizobutironitrile as the initiator) has already been successfully applied for 4-nitrophenylacrylate.6 However, the effects of the reactor size and shape, stirrer shape and stirrer speed have not been studied in detail. Previously, we have set the overhead stirrer speed to 285 rpm to obtain the beads of approximately 200 /im to 500 /im in diameter. Lhis time we wished to make them smaller; around 100 jim. By using a 350 mL three neck reactor with a blade type PLFE overhead stirrer at 750 rpm (stirrer type IKA Euro - SL PCV) we were able to obtain beads with the size distribution around the desired value. Lhus for aH our investigations and bead preparations the overhead stirrer with the PLFE blade set at a speed of 750 rpm was used. Further modification of the beads preparation procedure in comparison with our previously reported procedures8 was in the purification
of pofymer beads. Lhere were some indications that traces of the stabilising agent (polyvinylpirrolidone) and unreacted monomers were stili present inside the pofymer beads after washing. We therefore used Soxhlet extraction (24 hours with methanol) to remove aH the remaining stabiliser and monomers. Lhe combustion elemental analysis (see Lable 3) proved that aH samples were purified satisfactorily as the values of elemental percentage were very close to the calculated ones. At the same time, this presented one of the evidence for the successful pofymerisation of both the monomer and the crosslinker. Additionally, FLIR spectroscopy was used to determine the polymer structure. Peaks at 1760 cm4 (4-nitrophenylacrylate carbonyl groups), 1730 cm4 (EGDMA carbonyl groups), 1345 cm4 (nitro groups) were ali strong, proving the presence of the groups in the polymer structure.
Lhe yields of bead shaped particles from suspension polymerisations were very high, ranging from 80% upwards. In order to study the effects of the crosslinker type and degree and monomer ratio on the bead parameters, we prepared pofymer supports crosslinked with DVB or EGDMA (both types with either 5% or 20% of crosslinker) and with the ratios of acrylate (A) to styrene (S; styrene was added to the monomer mixture to alter the overall polarity of the polymer matrix and to »dilute« the reactive sites on the backbone) varying from 1:1 to the supports with no styrene present (Lable 1). In aH cases except in the čase of the pofymer supports 3a (S/A = 0/1, 5% DVB), la’ (S/A = 1/1, 5% EGDMA) and 3a’ (S/A = 0/1, 5% EGDMA) sphered polymer supports were obtained. In the čase of la’, 3a and 3a’ particles were mostly of eliptic shape (see Figure 1). We can thus conclude that this modified procedure of suspension polymerisation is very suitable for the preparation of 4-nitrophenylacrylate based pofymer supports with a narrower bead size distribution.
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Figure 1. Optical micrograph of polymer beads la, lb, 2a, 2b, 3a, 3b, la’, lb’, 2a’, 2b’, 3a’, 3b’.
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Bead size distribution
Bead size distribution of spherical polymer supports can be determined by the use of a sieve and weighing the fractions obtained. While this is stili the most common method, it requires a lot of equipment and is time consuming. Another method is to use laser diffraction. Recenth/, in parallel with the advent of digital photography, optical microscopy in conjunction with appropriate softvvare has been used to measure the bead size and evaluate the data.11 We used a Nikon EPIPHOT 300U microscope with a digital camera to measure the bead diameters. The data of diameters were processed by Microsoft Excel and expressed as bead size distribution columns (see Figure 2). Bead size distribution is very important since the size of the beads influence the reaction kinetics while when beads are used as a separation medium in a chromatography column, the characteristics of the column much depend on the bead size.
With an intent of testing the effect of crosslinking degree on the average bead diameter we prepared polymer supports with 5% and 20% of crosslinker (molar percentage), both DVB and EGDMA crosslinked. The measurements of bead diameters showed that regardless of the type of crosslinker, beads with 5% of crosslinker were smaller compared to beads with 20% of crosslinker (5% crosslinked la 20.6 /im, 20% crosslinked lb 35.4 /im; see Table 4). While the type of the crosslinker didn’t have a profound effect, the styrene to acrylate ratio influenced the bead size distribution, being broader for samples with more styrene incorporated into the polymer matrix (see Figure 2). Generalh/, the bead size distributions columns reveal the most of the beads to be betvveen 25 and 40 /jim (Figure 2), which is a relativeh/ narrow bead size distribution considering that the particles were prepared by suspension polymerisation technique, which usually gives broader size distribution.
Table 4. Average bead diameters.
Polvmer	r   (um)
la	20.6
lb	35.4
2a	35.0
2b	42.2
3a	-
3b	41.5
la'	-
lb'	52.5
2a'	11.7
2b'	55.2
3a'	-
3b'	28.9

¦ lb B2b P3b
< 20              20 - 40             40 - 60             60 - 80              > 80
Diameter (jim)

llb' B2b'
:3b'
< 20               20 - 40             40 - 60             60 - 80               > 80
Diameter (jim)

 20 -;
 I
< 10              10 - 20            20 - 40            40 - 60              > 60
Diameter (jim)
Figure 2. Bead size distribution of polvmer supports.
Polymer network flexibility
Polymer network flexibility can be of great importance when a polymer support is used in a synthesis setup. Site-site interactions can have substantial influence on the reaction pathway.12 By varying the loading of the reactive group on the polymer (the amount of reactive groups in the polymer, usually given in mmol per gram of polymer) these interactions can be changed. With the introduction of multifunctional reagents, polymers can be additionally crosslinked. The amount of additional crosslinking taking plače gives us the information about polymer netvvork flexibility. The phenomenon of subsequent crosslinking the already formed polymer supports has been described before. Booth and Hodges prepared an electrophilic scavenger resin by reacting chloromethylated polystyrene with tris(2-aminoethyl)amine and approximately 25% of free amine groups reacted with another chloromethyl group
80


20
0
60

40

20
0
60
40

2a'
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Table 5. Structure of products after the reactions with 1,8-diaminooctane.
Polymer support
Acrylate to styrene ratio
Crosslinker
Calculated %N for structure A
Calculated %N for stracture B
* calculated from mathematically constructed curves for each polymer.
Found % N
% additional crosslinking*
la	1/1	5% DVB	8.73	5.62	7.02	61
lb	1/1	20% DVB	7.2	4.41	5.74	58
2a	2/1	5% DVB	10.53	7.22	7.78	87
2b	2/1	20% DVB	8.86	5.73	6.48	81
3a	1/0	5% DVB	13.5	10.34	9.91	over 95
3b	1/0	20% DVB	11.58	8.25	8.18	over 95
la'	1/1	5% EGDMA	8.66	5.57	5.53	over 95
lb'	1/1	20% EGDMA	6.98	4.25	5.42	68
2a'	2/1	5% EGDMA	10.53	7.22	7.82	86
2b'	2/1	20% EGDMA	8.62	5.54	5.69	over 95
3a'	1/0	5% EGDMA	13.42	10.25	10.11	over 95
3b'	1/0	20% EGDMA	11.29	7.96	8.06	over 95
on the polymer thus crosslinking it.13 Similar behaviour was observed when preparing a monolithic polymer scavenger.14 The method of using 1,8-diaminooctane to test poh/mer netvvork flexibility has alreadv been used on copoly(styrene-4-nitrophenylacrylate; 1:1 monomer ratio), crosslinked with 2% or 4% DVB.8 By determining the amount of nitrogen in the polymer, a good estimate of reaction product is possible providing the ester groups are completeh/ removed.
To evaluate the polymer netvvork flexibility of polymer supports (various amounts of crosslinker and various monomer ratio; see Table 1), we reacted them with 1,8-diaminooctane (see Scheme 1) at 50 °C in AyV-dimethylformamide for 24 hours. Proceedings of the reactions were monitored by FTIR spectroscopy in order to check the presence of the signal for nitro group at 1345 cm4.
\   /
NO2
HN-(-CH2)-NH2 A
CH2)
Scheme 1
For aH the samples spectroscopic data suggested the complete removal of the phenoxy groups and ali polymers were subjected to combustion elemental analysis. The results are summarized in Table 5.
Since one amine molecule replaces two phenoxy groups in the čase of additionally crosslinked product B (Scheme 1), the percentage of nitrogen in the product is lower than in the čase of replacing one phenoxy group (product type A). We calculated the theoretičal values of nitrogen for both types of product and for products
having partially structure A and partially structure B. For every sample of starting beads (la, lb, 2a, 2b, 3a, 3b, la’, lb’, 2a’, 2b’, 3a’, 3b’), a curve of nitrogen content dependency on the structure was constructed and a mathematical relation calculated (see Table 6). From these relations, we could estimate the ratio of A to B type product for aH the samples (4a, 4b, 5a, 5b, 6a, 6b, 4a’, 4b’, 5a’, 5b’, 6a’, 6b’).
Table 6. Coeficients for the relation: AC = ax2 = additional crosslinking in %; x= percentage of by elemental analysis).
+ bx + c (AC nitrogen found
Polymer
a
b
c
la lb 2a 2b 3a 3b la' lb' 2a' 2b' 3a' 3b'
-2.64 -2.70 -2.82 -2.67 -4.25 -3.14 -2.54 -2.72 -2.84 -2.66 -4.14 -3.04
5.79 ^t.45 19.99
7.04 69.97 32.57 3.92 -5.95 20.46 5.39 66.72 28.80
150.59 172.11 102.24 147.01
-170.14 44.72 156.82 174.4 100.12 151.68
-149.77 63.07
The first finding was surprising, namely we found no severe effect of the crosslinking degree on the structure of the product (61% of additionally crossinked product derived from la, 58% additionally crosslinked product derived from lb; 87% of additionally crossinked product derived from 2a, 81% additionally crosslinked product derived from 2b; over 95% of additionalh/ crossinked product derived from 3a, over 95% additionalh/ crosslinked product derived from 3b). As expected, samples with higher content of acrylate gave products with more additional crosslinking (samples
o
O
B
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 3

Polymer
Polymer




Polymer
Polymer
Polymer Figure 3. Swelling degrees of polymer supports.
3a, 3b, la’, 2b’, 3a’, 3b’ yielded only product type B indicating that ali amine groups reacted, while samples 2a, 2b, 2a’ yielded products with mostly B type structure, as approx. 85% of additional crosslinking took plače). Ali polymer supports proved to have a highly flexible polymer matrix as with none of them the ratio of A to B type product was lower than 1 to 1 (at least 58% additional crosslinking) and with half of the polymer supports almost ali amine groups reacted.
Swelling capabilities
Ali prepared polymer supports were subjected to svvelling tests, namely in solvents: water, methanol, dichloromethane, acetonitrile and toluene. Tests were done by measuring the volume of solvent uptake by the beads; volume of swollen beads taken after suspending 1 mL of beads in 10 mL of solvent for 24 hours. We noticed that ali svvellings took plače rapidly as there was
no change in the volume of the swollen beads after 15 minutes and 24 hours. As expected, crosslinking degree affected swelling in ali solvents, being more intense for the beads with 5% of crosslinker. The difference in swelling of 5% crosslinked beads in comparison with 20% crosslinked is betvveen 2 and 4 fold (17,4 mL/g in dichloromethane for 5% crosslinked resin la, 3.9 mL/g in dichloromethane for 20% crosslinked resin lb; 7.3 mL/g in dichloromethane for 5% crosslinked resin 2a, 3.5 mL/g in dichloromethane for 20% crosslinked resin 2b; see Table 2 and Figure 3). The type of crosslinker (DVB vs. EGDMA) had a smaller effect than the degree of crosslinking. Generally, polymers crosslinked with DVB swell to a larger extent than polymers crosslinked with EGDMA (3a (5%, DVB crosslinked) 5.6 mL/g in acetonitrile, polymer 3a’ (5%, EGDMA crosslinked ) 4.4 mL/g in acetonitrile). Swollen volumes of beads are betvveen 1.4 mL/g (polymer 3b’

20

4
4

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223
20% EGDMA , in water) and 17 mL/g (polymer la 5% DVB, in dichloromethane). For most polymers swelling capabilities dependence on solvent was as follows: DCM, acetonitrile, toluene, methanol and water, being highest in DCM and lowest in water.
Conclusions
With an appropriate choice of reaction vessel, stirrer type and speed we were able to produce 4-nitrophenylacrylate based polymer supports crosslinked with either DVB or EGDMA. High yield of spherical particles and relatively narrow bead size distribution were achieved for the polymers with different amount of crosslinker and ratios of styrene to acrylate. Ali crosslinked polymers proved to have a very flexible backbone as high levels of additional crosslinking occured after the reactions with a bifunctional amine although excess amine concentrations were used and further studies will be needed to confirm this nature of the polymers.
Acknowledgement
The financial support of the Ministry of Science and Higher Education of the Republic of Slovenia and the British Council Slovenia is acknowledged. We are grateful to Prof. B. Stanovnik and T. Stipanovič for elemental analyses.
References
1.   R. B. Merrifield, /. Am. Chem. Soc. 1963, 85, 2149-2154.
2.   S. V. Fey, I. R. Baxendale, R. N. Bream, P. S. Jackson, A. G. Leach, D. A. Longbottom, M. Nesi, J. S. Scott, R. I. Storer, S. J. Tavlor, /. Chem. Soc. Perkin Trans. 1 2000, 3815-4195.
3.   F. Švec, F. B. Fennikova, Z. Deyl (Eds.), Monolithic Materials: Preparation, Properties and Applications, Elsevier, Amsterdam, 2003.
4.   http://argonaut.biotage.com/products/index.html.
5.   B. S. R. Reddy, R. Arshadv, M. H. George, Macromolecules 1983, 16, 1813-1817.
6.   M. Zupan, P. Krajnc, S. Stavber, Polymer 1996, 37, 5477-5481.
7.   P. Krajnc, D. Štefanec, J. F. Brown, N. R. Cameron, /. Polym. Sci. Pt. A: Polym. Chem. 2005, 43, 296-303.
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10.   F. Narasimhaswamy, N. F. Murthy, S. C. Sumathi, B. S. Reddy, Angew. Makromol. Chem. 1993, 213, 21-32.
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Povzetek

Stirenske in 4-nitrofenilakrilatne nosilce zamrežene z divinilbenzenom ali etilen glikol dimetakrilatom smo pripravili s suspenzijsko polimerizacijo. Pri različnem razmerju monomerov in stopnji zamreženja smo analizirali porazdelitev velikosti delcev, nabrekanje v različnih topilih in fleksibilnost polimerne matrike. Ugotovili smo, da stopnja zamreženja vpliva tako na velikost delcev kot tudi na njihovo nabrekanje v različnih topilih. Povprečmi premer 5% zamreženih nosilcev je v intervalu med 10 in 35 µm, premer 20% zamreženih nosilcev pa v intervalu med 35 in 55 µm. 5% zamreženi nosilci nabrekajo bolj kot 20% zamreženi, kar velja za obe zamreževali. Fleksibilnost polimerne matrike smo določili z modelno reakcijo z 1,8-diaminooktanom. Visoka stopnja dodatnega zamreženja (48 do več kot 95%) kaže na fleksibilnost polimerne matrike.
Pulko and Krajnc     4-Nitrophenylacrylate Polymer Supports