J. MATER. CHEM., 1994,4(8), 1159-1165 A Convergent Synthesis of Extended Aryl Ester Dendrimers W. James Feast and Neil M. Stainton IRC in Polymer Science and Technology, University of Durham, Durham, UK DH 1 3LE A convergent synthesis of aryl ester dendrimers with flexible spacer units via a series of ‘dendron wedges’ of increasing size is described. The development of a high yield iterative protection/deprotection scheme for phenolic groups as acetates, in the presence of aryl esters, enables dendrimers to be assembled using this approach. In recent years there has been significant interest in well- defined, highly branched polymeric species, and the unusual characteristics that arise in these materials as a consequence of their novel topologies and molecular structures.’ The synthesis of macromolecular, three-dimensional species with hyperbranched architecture was pioneered by Tomalia et aL2 and Newkome et d3in the early eighties.Since then, there have been many reports of the stepwise synthesis of families of monodisperse polymeric species. Such materials have been termed ‘starburst’ polymers or dendrimers (from the Greek, dendritic =treelike), on account of their highly branched topology. Two distinct techniques have emerged for the stepwise synthesis of dendritic polymers. The first of these, the so-called divergent route, starts at the centre of the molecule and proceeds outwards by means of an appropriate iterative scheme; however, with increasing dendrimer size, or gener- ation, the number of repeat units in the formation of each successive layer increases rapidly, following a geometric pro- gression and ultimately leads to practical difficulties involving isolation and purification of products due to the problems associated with incomplete reaction at increasingly congested reaction sites.More recently, convergent techniques have been developed, notably by Hawker and FrC~het,~ and used by a number of groups working in the field. In this approach, dendrimer construction commences at what will ultimately be the periph- ery of the molecule and progresses inwards uia a series of ‘dendron wedges’ of increasing size that can be attached to the core molecule in the final step. It is this technique that has been utilised by Miller et aL5 in their synthesis of a series of aryl ester dendrimers based on 5-hydroxyisophthalic acid.Unlike their linear analogue, poly( p-hydroxybenzoic acid), which is essentially infusible and insoluble and consequently extremely difficult to process, such dendrimers have been found to be highly soluble in a wide range of common organic solvents. Here we report the synthesis of polyester dendrimers uia a convergent route incorporating linear aromatic spacer or extension units between successive branch junctions in order to assess their thermal, chemical and mechanical properties with a view to exploring the potential of such materials for novel applications. In this way, we combine the concept of dendritic layer-block copolymers of Hawker and Frechet6 with aryl ester dendrimer synthesis similar to that of Miller et aL5 Our novel approach to dendrimer synthesis utilizes acetates as protecting groups in the preparation of polyaryl esters as opposed to the silyl ether protection used by Miller et al.’ Results and Discussion We have prepared a series of three aryl ester dendrimers, having 7, 16 and 19 aromatic rings via a convergent route involving the development of an iterative protection/deprotec- tion scheme for phenolic groups as acetates in the presence of aryl esters.The synthesis is based on the reaction of 5-acetoxyisophthaloy1 dichloride, 2 (the branching urtit) and 4-acetoxybenzoyl chloride, 1 (the spacer unit) with a series of phenolic ‘dendron wedges’ and subsequent selectik e ester cleavage to yield wedges of increasing size (see Scheme 1).The acid chlorides were prepared in a two-step process from the appropriate hydroxyacids, which are commercially avail- able. Initial acetylation of the phenol groups with acetic anhydride in basic aqueous solution was followed by conver- sion to the acid chloride by reaction with phosphorus penta- chloride. Overall yields of typically 86-89% were obtained. To overcome the increasingly cumbersome nomenclature for the larger aromatic molecules, these are named with reference to the number of aromatic rings in the structure followed by an abbreviation indicating the reactive functional group (see Scheme 1). Reaction of 4-acetoxybenzoyl chloride, 1, with phenol in a solution of dichloromethane and pyridine with 4-dimethylaminopyridine (DMAP) catalysis5 yielded phenyl 4-acetoxybenzoate, [21-OAc (see Scheme 1).The conversion of [21-OAc to phenyl 4-hydroxybenzoate, [2]-OH, was first attempted by selective reduction with sodium borohydride under mild condition^,^ a technique successfully employed by ourselves in related work involving alkyl benzoates.Despite prolonged reaction times, no change was detected and the phenyl acetate remained intact. On reaction of [2]-OAc with ethanolic potassium hydroxide on a small scale at 0 “C, yields of the desired product were obtained in excess of 70%; however, on scaling up this process, significant phenol con- tamination was observed, indicating the lack of specificity of the reaction.Following purification by vacuum sublimation, [2]-OH was obtained in low yield. Reaction of the trifunc- tional core molecule, benzene-l,3,5tricarbonyltrichloride, 3, with the deprotected product, [2]-OH, in dichloromethane- pyridine solution with DMAP catalysis yielded 171, the extended first-generation dendrimer, in high yield and without the need for purification by column chromatography. In a similar manner, [5]-OAc, the protected second-generation wedge was prepared by reaction of [2]-OH with 5-acetoxyisophthaloy1 dichloride, 2, in excellent yield. The deacetylation of [51-OAc was initially attempted using borohydride reductive hydrolysis. Analysis of the products revealed a significant proportion of [2]-OH, indicating aryl benzoate hydrolysis.Other basic hydrolysis techniques were equally unsuccessful and resulted in undesirable fragmen- tation. Investigation of acid-catalysed deacetylation showed that a 10% v/v aqueous hydrochloric acid solution and tetrahydrofuran mixture was suitably regiospecific and gave [5]-OH cleanly and in high yield. Adaptation of this pro- cedure for [2)-OH synthesis was only partially successful and an appreciable quantity of phenol was produced. The yield of J. MATER. CHEM., 1994, VOL. 4 Q0.pO 0..Q.o [71 [2]-OH did not justify changing from the preferred mode of synthesis as previously described. From the experience of other re~earchers,~ who concluded that the coupling of large branched aryl ester wedges to an aromatic core using either pyridine or DMAP resulted in significant ester exchange, the weaker base, N,N-dimethylani- line, was used to catalyse the reaction of [5]-OH with benzene-1,3,5-tricarbonyltrichloride 3 to produce [161, the second-generation dendrimer, after purification by column chromatography. The extended second-generation wedge, [6]-OH, was pre- pared by the reaction of 4-acetoxybenzoyl chloride 1 with [5]-OH in dichloromethane-pyridine solution with DMAP followed by acid-catalysed deacetylation.This wedge was attached to the core molecule 3 using the method described above, to yield [191, the extended second-generation dendrimer. We have tried unsuccessfully to prepare larger dendrimers using this route.The synthesis of [13]-OAc, the protected third-generation wedge, from [6]-OH with 5-acetoxy-isophthaloyl dichloride 2 has been carried out without prob- lem; however, attempts to deprotect this in aqueous acid-THF solution have failed. IR spectroscopy of the product indicated that appreciable aryl ester hydrolysis had taken place. It is significant to note that similar problems have been experienced in related systems for wedges of a comparable size.5 It is possible that the increasing extent of aryl ester hydrolysis with increasing size of wedge, as compared to removal of the protecting group, may be attributed at least in part to the folding back of the large dendrimer branches towards the focal point of the wedge, thus congesting the reactive site and compounding the already serious problem of undesirable aryl ester hydrolysis, resulting simply as a result of the increasing aryl ester: acetate ratio.Characterisation of the Dendrimers On account of the high symmetry of the dendrimers, both 'H and I3C NMR spectroscopy have proved invaluable in the characterisation of these materials. Two-dimensional hetero- nuclear correlation NMR spectroscopy has been utilised for the unambiguous assignment of individual proton and carbon resonances within the structures. The assignments follow in a straightforward way from the spectra of the wedge compo- nents in the synthetic route, bearing in mind relative peak intensities and correlation tables of chemical shifts of related compounds (see Fig.1 and 2). The dendrimers have also been characterised by elemental analysis and gel permeation chromatography (GPC) (see r191 Fig. 3). For [7] and [16], elemental analysis shows close agreement with the theoretical values whilst GPC illustrates their monodisperse nature. Elemental analysis and GPC of [19] indicate the presence of a small quantity of impurity that could not be removed by column chromatography. GPC is consistent with the hypothesis that this trace impurity is unreacted wedge material. Characterisation by mass spec-trometry has also been attempted using conventional CI and EI techniques; however, both of these have resulted in frag- mentation to species containing only single aromatic units and no molecular ions have been detected.The dendrimers are highly soluble in organic solvents such as chloroform, dichloromethane and THF, contrasting mark- edly with their linear analogue poly (p-hydroxybenzoic acid), which is virtually insoluble in common organic solvents. Thermogravimetric analysis has shown that at a constant heating rate of 10°C min-' in a nitrogen atmosphere, the dendrimers retain 98% of their mass up to around 400°C. Wide-angle X-ray powder diffraction indicates that [7] is crystalline whereas both [16] and [19] are less well ordered compounds (see Fig.4). Heat of fusion measurements by differential scanning calorimetry support this view (Table 1 ). Experimental All organic reagents were obtained from Aldrich and used without further purification.Melting points were obtained on an Electrothermal digital melting point apparatus unless otherwise stated and reported without correction. IR spectra were recorded on a Perkin-Elmer 1600 series FTIR. Thermogravimetric analyses were carried out using a Shimadzu TGA-50. Differential scanning calorimetry was performed on a Perkin-Elmer DSC-7 at a scanning rate of 10°C min-l. 'H and 13C NMR spectra were recorded on a Varian 200 MHz Gemini or Varian 400 MHz spectrometer, as indicated, and were referenced to TMS. Gel permeation chromatography was carried out in chlorFform ?sing thre? 5 pm columns of PL gel with pore size 100 A, lo3 A and lo5A and a Waters differential refractometer detector.X-Ray diffraction was carried out on a Siemens Diffraktometer D5000. 4Acetoxybenzoic Acid To a stirred solution of 4-hydroxybenzoic acid (225.00g, 1.63mol) in aqueous sodium hydroxide solution (2.7 1,2.5 mol 1-l) cooled in an ice-water bath, acetic anhydride (750 ml) was added slowly. Stirring was continued until the mixture solidified, upon which the flask was shaken for 10min and J. MATER. CHEM., 1994, VOL. 4 [2]-OAc [2]-0H o\ ?00 t I oQg-c, [6]-OAc 3, 8 H', THF pyridie, DMAP Cocl b KOH.E!OH pyridine, DMAP f 3, N,N -dimethylaniline d c'Tfrn'2, OCOCH3 pyridine,DMAP Scheme 1 Outline routes to dendrimers [7], [16] and [19] the contents acidified to pH 1 using concentrated hydro-chloric acid.The white slurry was filtered and the filtrate extracted with ethyl acetate (4 x 300 ml). A further portion of ethyl acetate (900 ml) was added to this and the filtered off residue was dissolved in it. The resulting solution was washed with water (5 x 1 1) and the combined aqueous washings extracted with ethyl acetate (500 ml). The organic portions were combined, dried (MgS04), the solvent removed by evaporation and the residue dried under vacuum at 80 "C for 4h. to yield a white powdery solid (272.04 g, 1.51 mol, 92.7%). Mp 188.0-189.5"C (lit. 191-192°C). 'H NMR (CDCl,, 200 MHz), 6 2.34 (s, 3H, CH,), 7.21 (d, 8.96 Hz, 2H, ArH), 8.15 (d, 8.90 Hz, 2H, ArH). I3C NMR (CDCI,, 200 MHz) 6 21.66 (CH,), 122.26 (aromatic C-H), 127.31 (aromatic C-R), 132.37 (aromatic C-H), 155.48 (aromatic C-0), 169.35 (ArCO,H), 171.83 (COCH3).v,,Jcm-': 2996.0, 1754.5, 1681.1. CAcetoxybenzoyl Chloride An intimate mixture of 4-acetoxybenzoic acid (304.88 g, 1.69 mol) and phosphorus pentachloride (358.01 g, 1.72 mol) in a flask fitted with a reflux condenser and gas absorption device was warmed gently with a heat gun to initiate the reaction and shaken occasionally until the vigorous evolution J. MATER. CHEM., 1994, VOL. 4 f a I I oi ari91 I b Fig. 1 13C NMR spectra of dendrimers in CDCl, of hydrogen chloride had ceased. The reaction mixture was stirred for a further 30min at room temperature to form a pale yellow homogeneous oil. After removal of phosphorus oxychloride by distillation at atmospheric pressure, the residue was distilled at reduced k 22.50 24.30 26.10 t /min Fig.3 GPC of dendrimers [71, [161 and [191 pressure (107 "C, 0.4 mbar) to produce a clear colourless oil that yielded a white crystalline solid (317.51 g, 1.60 mol, 94.5%) on cooling. Mp 29.5-30.5 "C(lit. 29-30 "C). 'H NMR [2H,]acetone, 200 MHz) 6 2.33 (s, 3H, CH,), 7.41 (d, 9.08 Hz, 2H, ArH), 8.20 (d, 9.00 Hz, 2H, ArH). 13C NMR ([2H,]acetone, 200 MHz) 6 21.42 (CH,), 124.05 (aromatic C-H), 131.35 (aromatic C-R), 134.26 (aromatic C-H), 158.11 (aromatic C-0), 168.00 (COCl), 169.42 (COCH,). v,,Jcm-': 1773.8, 1597.4, 1499.3, 1370.3, 1199.4, 1162.1. Phenyl 4-Acetoxybenzoate, [21-OAc To a suspension of 4-acetoxybenzoyl chloride (315.75 g, 1.59 mol) in pyridine (1350 ml) were added phenol (179.47 g, 1.91 mol), 4-dimethylaminopyridine (9.74 g, 0.080 mol) and dichloromethane (650 ml).After it had been stirred at room temperature for 48 h, the solution was washed with aqueous hydrochloric acid (15 x 400 ml, 10% v/v), aqueous sodium hydroxide (15 x 400 ml, 1.0 mol 1-I) and aqueous potassium carbonate (2 x 300 ml, 1.0 mol-l), dried (MgS04), the solvent removed by evaporation and the residue dried under vacuum to yield a light tan solid (376.79 g, 1.47 mol, 92.5%). Mp 83.5-85.5 "C. 'H NMR (CDC13, 200 MHz) 6 2.33 (s, 3H, CH3), 7.22 (m, 5H, ArH), 7.43 (m, 2H, ArH), 8.23 (d, 8.58Hz, 2H, f n 136 1 32 128 124 120 W2) Fig. 2 2D Heteronuclear correlation spectrum of [16] in CDC1, J.MATER. CHEM., 1994, VOL. 4 1 I I' 10 20 30 40 50 60 70 80 90 2Hdegrees Fig. 4 Wide-angle X-ray diffraction traces of [71, [161 and [191 ArH). I3C NMR (CDCl,, 200 MHz) 6 21.67 (CH,), 122.19, 122.38, 126.47 (all aromatic C-H), 127.59 (aromatic C-R), 130.03, 132.30 (both aromatic C-H), 151.36, 155.32 (both aromatic C-0), 164.89 (ArC=O), 169.32 (CH,C=O). vmax/cm-l: 3068.0, 1755.2, 1728.9, 1601.5, 1501.9, 1485.2. Phenyl4Hydroxybenzoate, [2]-OH To an ice-cold solution of sodium hydroxide (31.16 g, 0.779 mol) in ethanol (1.3 1) was added phenyl 4-acetoxy- benzoate (199.44 g, 0.779 mol) and the mixture stirred at 0 "C for 30 min. The mixture was filtered and water (6 1) was added to the filtrate and the solution acidified to pH 1 with concen- trated hydrochloric acid.The resultant precipitate was col- lected by filtration, washed with aqueous hydrochloric acid (10% v,/v) then hexane and dried under vacuum at room temperature overnight. The filtrate was extracted with ethyl acetate, the organic layers combined, dried (MgS04) and the solvent removed by evaporation to yield a white residue that was dried under vacuum. The crude product portions were combined and purified by vacuum sublimation (ca. 210 "C, 0.3 mbar) followed by recrys- tallisation from hot toluene and washed with cold toluene and hexane to yield a white crystalline solid (72.57g, 0.339 mol, 43.5%) Mp 184.0-185.0 "C (lit. 170-174 "C). 'H NMR C2H,]acetone, 400 MHz) 6 7.01 (d, 9.2 Hz, 2H, ArH), 7.26 (m, 3H, ArH), 7.45 (m, 2H, ArH), 8.06 (d, 8.8 Hz, 2H, ArH), 9.36 (s, lH, OH).13C NMR ([2H6];icetone, 400 MHz) 6 115.56 (aromatic C-H), 120.88 (aromatic C-R), 122.06, 125.63, 129.42, 132.36 (all aromatic C-H). 151.52 (aromatic C-0), 162.50 (aromatic C-OH), 164.45 (C=O). v,,Jcm-': 3397.6, 3055.9, 1698.8, 1603.4, 1585.3, 1509.6,851.8. Extended First-generation Dendrimer, [71 To a solution of benzene-1,3,5-tricarbonyltrichloride (5.31 g, 0.0200 mol) in pyridine (220 ml) were added phenyl 4-hydroxybenzoate ( 15.00 g, 0.070 1 mol), 4-dimethj lamino- pyridine (0.40 g, 0.033 mol) and dichloromethane (80 ml) and the solution stirred at room temperature for 4 days. After addition of a further portion of dichloromethane (250 ml), the solution was washed with aqueous hydrochloric acid (4 x 250 ml, 10% v/v), aqueous sodium hydroxide (3 x 250 ml, 1.0 mol 1-I) and brine (2 x 250 ml I, dried (MgS04) and the solvent removed by evaporation.The residue was recrystallised from a mixture of ethyl acetate and ethanol (3: 1). On drying under vacuum at 100 "C overnight, this yielded a white powdery solid (12.10 g, 0.0152 mol, 75.8%). (Found: C, 72.36; H, 3.77%. C48H30012 requires (2, 72.18; H, 3.76%). 'H NMR (CDCl,, 400 MHz) 6 7.23 (d, 8.4 Hz, 6H, ArH), 7.29 (m, 3H, ArH), 7.44 (m, 12H, ArH), 8.33 (d, 8.8 Hz, 6H, ArH), 9.29 (s, 3H, ArH). 13C NMR (CDCl,, 400MHz) 6 121.61, 121.82, 126.01 (all aromatic C-H), 127.75 (aromatic C-R), 129.52 (aromatic C-H), 130.94 (aromatic C-R), 131.99, 136.37 (both aromatic C-H), 150.76, 154.45 (both aromatic C-0), 162.59, 164.19 (both C=O).The spectrum and assignments are shown in Fig. 1. v,,Jcm-' 3065.9, 1734.7, 1591.8, 1502.8. 5-Acetoxyisophthalic Acid To a stirred solution of 5-hydroxyisophthalic acid I 100.00 g, 0.59 mol) in aqueous sodium hydroxide (11, 2.5 moll- ') cooled in an ice-water bath, was added acetic anhydride (200 ml) slowly. Stirring was continued for 2 h, upon which the flask was shaken for 10min and the contents acidified to pH 1 using concentrated hydrochloric acid. The white slurry was filtered off and the filtrate extracted with ethyl acetate (6 x 300 ml). A further portion of ethyl acetate (900 ml) was added to this and the filtered residue was dissolved in it.The resultant solution was washed with water (5 x 500 ml) and the combined aqueous washings extracted with ethyl acetate (500 ml). The organic portions were combined, dried (MgS04), the solvent removed by evaporation and the residue dried under vacuum at 80 "C for 4 h. to yield a white powdery solid Table 1 Physical properties of dendrimers [7], [16] and [19] relative molecular dendrimer mass 798c71 c 161 1878 c 191 2238 solubility(chloroform, 25 "C)/ g 1-' 297 156 174 2% wt. loss mp (DSC)/OC A€usHIJ g-' 4 TG)/"C 190 71 278 152 20 389 136 13 405 (122.56 g, 0.547 mol, 99.7%). Mp 246-247 "C (lit. 238-240 "C). 'H NMR (C2H6]acetone, 200 MHz) 6 2.35 (s, 3H, CH,), 8.00 (d, 1.52 Hz, 2H, ArH), 8.56 (t, 1.52 Hz, lH, ArH).I3C NMR (CZH6]acetone, 200 MHz) 6 21.30 (CH,), 128.44 (aromatic C-H), 128.89 (aromatic C-H), 133.73 (aromatic C-R), 152.46 (aromatic C-0), 166.50 (ArCO,H), 170.06 (COCH,). v,,Jcm-': 3100-2600, 1770.4, 1693.8. 5-Acetoxyisophthaloy1 Dichloride An intimate mixture of 5-acetoxyisophthalic acid (98.93 g, 0.492 mol) and phosphorus pentachloride (213.2 g, 1.024 mol) in a flask fitted with a reflux condenser and gas absorption device was warmed gently with a heat gun to initiate the reaction and shaken occasionally until the vigorous evolution of hydrogen chloride had ceased. To ensure complete reaction, the reaction mixture was stirred for a further 1h at room temperature to yield a pale yellow homogeneous oil.After removal of phosphorus oxychloride by distillation at atmospheric pressure, the residue was distilled at reduced pressure (132-134 "C, 0.3 mmHg) to produce a clear oil that yielded a white crystalline solid (1 13.75 g, 0.436 mol, 88.6%) on cooling. Mp 51.5-52.5 "C. 'H NMR (c2H6]acetone, 200 MHz) 6 2.37 (s, 3H, CH,), 8.28 (s, 2H, ArH), 8.65 (s, lH, ArH). 13C NMR ([2H6]acetone, 200 MHz) 6 21.29 (CH,), 131.33 (aromatic C-H), 132.03 (aromatic C-H), 136.44 (aromatic C- R), 153.07 (aromatic C-0), 167.46 (COCl), 169.78 (COCH,). v,aJcm-l; 3092.6, 3028.7, 1774.7. Protected Second-generation Wedge, [51-OAc To a suspension of 5-acetoxyisophthaoyl dichloride (68.02 g, 0.261 mol) in pyridine (1 1) were added phenyl 4-hydroxy- benzoate ( 114.00 g, 0.533 mol), 4-dimethylaminopyridine (1.50 g, 0.012 mol) and dichloromethane (330 ml).After the mixture had been stirred at room temperature for 36 h, dichloromethane (1.6 1) was added and the solution washed with aqueous hydrochloric acid (12 x 600 ml, 10% v/v) and aqueous sodium hydroxide (12 x 600 ml, 1.0 mol 1-'), dried (MgSO,), the solvent removed by evaporation and the residue dried under vacuum to yield a white powdery solid (149.52 g, 0.243 mol, 93.0%). Mp 194.5-196.5 "C. 'H NMR (CDCI,, 400 MHz) 6 2.40 (s, 3H, CH,), 7.23 (m, 4H, ArH), 7.29 (m, 2H, ArH), 7.43 (m, 8H, ArH), 8.24 (d, 1.6 Hz, 2H, ArH), 8.32 (d, 8.4 Hz, 4H, ArH), 8.91 (t, 1.6 Hz, lH, ArH). 13C NMR (CDCl,, 400 MHz) 6 20.95 (CH,), 121.62, 121.83, 125.97 (all aromatic C-H), 127.58 (aromatic C-R), 128.71, 129.09, 129.50 (all aromatic C-H), 131.27 (aromatic C-R), 131.93 (aromatic C-H), 150.78, 151.07, 154.56 (all aromatic C-0), 162.72, 164.24 (both aromatic C=O), 168.89 (OCOCH,).v,,Jcm-': 3078.6, 1773.3, 1743.8, 1590.5, 1503.1, 746.4. Second-generation Wedge, [5]-OH To a mixture of aqueous hydrochloric acid (700 ml, 10% v/v) and THF (2.1 1) was added [5]-OAc (147.73 g, 0.240 mol) and the solution refluxed for 18 h. On cooling, the THF was removed by evaporation and the residue extracted with ethyl acetate (4 x 500 ml), dried (MgSO,) and the solvent removed by evaporation. The residue was recrystallised from hot toluene, washed with hexane and dried under vacuum to yield a fine white powder (119.34 g, 0.208 mol, 86.6%).Mp 202.0-205.5 "C. 'H NMR (C2H6] acetone, 400 MHz) 6 7.33 (m, 6H, ArH), 7.49 (m, 4H, ArH), 7.59 (d, 8.8 Hz, 4H, ArH), 7.98 (d, 1.6 Hz, 2H, ArH), 8.31 (d, 9.2 Hz, 4H, ArH), 8.49 (t, 1.6 Hz, lH, ArH), 9.50 (broad, IH, OH). I3C NMR (C2H6]acetone, 400 MHz) 6 122.57, 122.77, 123.28, 123.36, 126.75 (all aromatic C-H), 128.38 (aromatic C-R), 130.32 (aromatic C-H), J. MATER. CHEM., 1994, VOL. 4 132.22 (aromatic C-R), 132.47 (aromatic C-H), 152.1 1, 156.08, 159.03 (all aromatic C-0), 164.22, 164.79 (both aromatic C=O). v,,Jcm-': 3386.1, 3071.1. 1736.7, 1708.8, 1601.2, 1498.0, 744.7. Second-generation Dendrimer, [161 To a stirred solution of [Sl-OH (4.50 g, 0.00784 mol) in dichloromethane (30 ml) and N,N-dimethylaniline ( 1.5 ml) was added benzene-1,3,5-tricarbonyl trichloride (0.58 g, 0.002 18 mol) and the mixture stirred at room temperature for 40 h.On addition of dichloromethane (60 ml), the solution was washed with aqueous hydrochloric acid solution (3 x 50 ml, 10% v/v) and brine (2 x 50 ml). After the mixture had been dried (MgSO,), the solvent was removed by evapor- ation and the residue purified by column chromatography [dichloromethane-1 % ethyl acetate/silica (Merck silica gel 60)] followed by recrystallisation (1: 1 ethanol-ethyl acetate) to yield a white powder (1.01 g, 0.000 538 mol, 24.7%). (Found: C, 70.61; H, 3.46%. ClllH66030 requires C, 70.93; H, 3.51%). 'H NMR (CDCl,, 400 MHz) 6 7.22 (d, 8.8 Hz, 12H, ArH), 7.29 (m, 6H, ArH), 7.44 (m, 24H, ArH), 8.32 (d, 8.8 Hz, 12H, ArH), 8.45 (d, 1.6 Hz, 6H, ArH), 9.01 (t, 1.2 Hz, 3H, ArH), 9.37 (s, 3H, ArH).13C NMR (CDCl,, 400 MHz) 6 121.63, 121.84, 126.05 (all aromatic C-H), 127.74 (aromatic C-H), 128.60, 129.55, 129.73 (all aromatic C-H), 130.74, 131.70 (both aromatic C-R), 132.01, 136.72 (both aromatic C-H), 150.78, 150.84, 154.51 (all aromatic C-0), 162.60 (2 x aromatic C=O), 164.23 (aromatic C=O). The spectrum and assignments are shown in Fig. 1 and the hetcor spectrum in Fig. 2. v,Jcm-': 3073.6, 1739.3, 1599.9, 1503.4, 742.0, Protected Extended Second-generation Wedge, [61-OAc To a stirred solution of 4-acetoxybenzoyl chloride (5.42 g, 0.0273 mol) in pyridine (250 ml) were added [Sl-OH (12.50 g, 0.0218 mol), dichloromethane (80 ml) and 4-dimethyl-aminopyridine (0.18 g, 0.0015 mol). After this mixture had been stirred at room temperature for 48 h, dichloromethane (250 ml) was added and the solution washed with aqueous hydrochloric acid (6 x 300 ml, 10% v/v) and aqueous sodium hydroxide (9x300m1, 1.0mol 1-I).After the mixture had been dried (MgSO,), the solvent was removed by evaporation and the residual oil dried under vacuum to yield a fine white powder (13.34 g, 0.0181 mol, 83.1%). Mp 75.0-76.0 "C. 'H NMR (CDCl,, 400 MHz) 6 2.36 (s, 3H, CH,), 7.22 (m, 4H, ArH), 7.30 (A,4H, ArH), 7.43 (m, 8H, ArH), 8.28 (d, 8.8 Hz, 2H, ArH), 8.32 (d, 8.8 Hz, 4H, ArH), 8.36 (d, 1.6 Hz, 2H, ArH), 8.96 (t, 1.6 Hz, lH, ArH). I3CNMR (CDCl,, 400 MHz) 6 21.18 (CH,), 121.66, 121.88, 122.14 (all aromatic C-H), 125.91 (aromatic C-R), 126.01 (aromatic C-H), 127.62 (aromatic C-R), 128.84, 129.29, 129.54 (all aromatic C-H), 131.41 (aromatic C-R), 131.98, 132.00 (both aromatic C-H), 150.82, 151.35, 154.62, 155.33 (all aromatic C-0), 162.77, 163.94, 164.29 (all aromatic C=O), 168.69 (OCOCH,).vmaX/cm-': 3074.3, 1738.3, 1601.2, 1503.7, 742.1. Extended Second-generation Wedge, [6]-OH To a solution of [6]-OAc (97.51 g, 0.132 mol) in THF (1.5 1) was added aqueous hydrochloric acid (0.5 1, 10% v/v) and the mixture refluxed for 24 h. When the mixture had been cooled water (300 ml) was added and the THF removed by evapor- ation. The residue was extracted with ethyl acetate (3 x 400 ml) and the combined organic layers washed with aqueous hydro- chloric acid (3 x 300 ml, 10% v/v), dried (MgSO,) and the solvent removed by evaporation.The residual oil was dried under vacuum to give a white solid that was recrystallised J. MATER. CHEM., 1994, VOL. 4 from toluene, washed with hexane and dried under vacuum (120 "C) overnight to yield a white powdery solid (79.44 g, 0.114 mol, 86.7%). Mp 215.5-216.0 "C. 'H NMR (C2H6]acetone, 400 MHz) 6 7.04 (d, 8.8 Hz, 2H, ArH), 7.32 (m, 6H, ArH), 7.49 (m, 4H, ArH), 7.62 (d, 8.8 Hz, 4H, ArH), 8.14 (d, 8.8 Hz, 2H, ArH), 8.31 (d, 8.8 Hz, 4H, ArH), 8.44 (d, 1.6 Hz, 2H, ArH), 8.86 (t, 1.6 Hz, lH, ArH), 9.54 (broad, lH, OH). 13C NMR (C2H6]acetone, 400 MHz) 6 116.38 (aromatic C-H), 116.47 (aromatic C-R), 122.76, 123.26, 126.74 (all aromatic C-H), 128.45 (aromatic C-R), 129.20, 129.63, 130.31 (all aromatic C-H), 132.30 (aromatic C-R), 132.47, 133.53 (both aromatic C-H), 152.07, 152.78, 155.93 (all aromatic C-0), 163.64 (aromatic C=O), 163.75 (aromatic C-OH), 164.76, 165.02 (both aromatic C=O).vmaX/cm-': 3392.1, 3072.5, 1739.3, 1591.0, 1503.5, 744.0. Extended Second-generation Dendrimer, [ 191 To a stirred solution of [6]-OH (1.55 g, 0.00223 mol) in dichloromethane ( 12 ml) and N,N-dimethylaniline (0.4 ml) was added benzene-1,3,5-tricarbonyl trichloride (0.169 g, 0.000 636 mol) and the mixture stirred at room temperature for 40 h. On addition of dichloromethane (30 ml), the solution was washed with aqueous hydrochloric acid solution (5 x 50 ml, 10% v/v) and brine (2 x 50 ml).The organic layer was dried (MgS04) and the solvent was removed by evapor- ation. The residue was purified by column chromatography ([dichloromethane-1 %ethyl acetate/silica (Merck silica gel 60)] to produce a white solid that was recrystallised from 1: 1 ethanol-ethyl acetate then dried under vacuum (120 "C, over- night) to yield a white powder (0.24 g, 0.000 107 mol, 16.9%). (Found: C, 68.94; H, 3.40%. C13&17@36 requires: C, 70.78; H, 3.49%). 'H NMR (CDCl,, 400MHz) 6 7.24 (d, 8.4 Hz, 12H, ArH), 7.30 (m, 6H, ArH), 7.45 (m, 24H, ArH), 7.53 (d, 8.4 Hz, 6H, ArH), 8.34 (d, 8.4 Hz, 12H, ArH), 8.40 (m, 12H, ArH), 9.00 (s, 3H, ArH), 9.33 (s, 3H, ArH). 13C NMR (CDCl,, 400 MHz) 6 121.76, 121.98, 122.25, 126.14 (all aromatic C-H), 126.82, 127.78 (both aromatic C-R), 128.92, 129.47, 129.66 (all aromatic C-H), 131.07, 131.59 (both aromatic C-R), 132.10, 132.38, 136.61 (all aromatic C-H), 150.92, 151.40, 154.71, 155.10 (all aromatic C-0), 162.62, 162.86, 163.91, 164.38 (all aromatic C=O).The spectrum and assignments are shown in Fig. 1. vmax/cm-l: 3071.9, 1738.7, 1599.8, 1504.1. Protected Third-generation Wedge, [ 131-OAc To a suspension of 5-acetoxyisophthaoyl dichloride (2.68 g, 0.0103 mol) in pyridine (250 ml) were added [6]-OH (15.00 g, 0.0216 mol), 4-dimethylaminopyridine (0.15 g, 0.0012 mol) and dichloromethane (80ml). After the mixture had been stirred at room temperature for 4 days, dichloromethane (250 ml) was added and the solution washed with aqueous hydrochloric acid (15 x 200 ml, 10% v/v) and aqueous sodium hydroxide (15 x 200 ml, 1.0 mol 1-'), dried (MgSO,), the solvent removed by evaporation and the residue drietl under vacuum (90°C, 4 h) to yield a white powdery solid (7.11 g, 0.00451 mol, 41.8%).'H NMR (CDCl,, 400 MHz) 6 2.40 (s, 3H, CH,), 7.24 (m, 8H, ArH), 7.29 (m, 4H, ArH), ".44 (m, 20H, ArH), 8.25 (d, 1.6 Hz, 2H, ArH), 8.32 (d, 8.4 Hz, 8H, ArH), 8.36 (d, 8.8 Hz, 4H, ArH), 8.39 (d, 1.6 Hz, 4H, ArH), 8.92 (t, 1.6 Hz, lH, ArH), 8.97 (t, 1.6 Hz, 2H, ArH). 13(: NMR (CDCI,, 400 MHz) 6 20.97 (CH,), 121.62, 121.84, 122.13, 125.99 (all aromatic C-H), 126.46, 127.60 (both aromatic C-R), 128.80 (2 x aromatic C-H), 129.17, 129.37, 129.51 (all aromatic C-H), 131.18, 131.42 (both aromatic C-R) 131.95, 132.16 (both aromatic C-H), 150.76, 151.12, 151.28.154.57, 155.08 (all aromatic C-0), 162.61, 162.72, 163.81, 164.25 (all aromatic C=O), 168.91 (OCOCH,). vmax/cm-': 3075.9, 1740.4, 1600.5, 1503.4, 742.8. Conclusion The synthesis of aryl ester dendrimers with flexible. spacer units has been achieved via a convergent process. All niaterials are soluble in conventional organic solvents. The der idrimers form miscible blends with PET, the properties of wliich will form the basis of future publications. We gratefully acknowledge the provision of an SER(I Quota award (N.M.S.) and the help of Miss S. C. E. Backson, Mrs. J. M. Say and Dr. A. M. Kenwright in some aspects of the characterisation of these materials. References D. A. Tomalia, A. M. Naylor and W. A. Goddard 111, Angefii. Chem., Int. Ed. Engl., 1990,29, 138. D. A. Tomalia, H. Baker, J. R. Dewald, M. Hall, C. Kallos, S. Martin, J. Roeck, J. Ryder and P. Smith, Polym. J., 1985, 17, 117. G. R. Newkome, Z. Yao, G. R. Baker and V. K. Gupt.1, J. Org. Chem., 1985, 50, 2003. C. J. Hawker and J. M. J. Frechet, J. Am. Chem. Soc., 990, 112, 7638. T. M. Miller, E. W. Kwock and T. X. Neenan, Macronolecules, 1992,25,3143. C. J. Hawker and J. M. J. Frechet. J. Am. Clzem. Soc., 992, 114, 8405. J. Quick and J. K. Crelling, J. Org. Chem., 1978,43, 155. Paper 4/01367K; Received 8th Mu *ch,1994