CHAPTER IV HOFMANN REARRANGEMENT IN CROSSLINKED POLYMERIC MATRICES

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Berenice Andrews
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1 CHAPTER IV HOFMANN REARRANGEMENT IN CROSSLINKED POLYMERIC MATRICES The Hofmann degradation reaction has been used as a synthetic route for the preparation of amines Tanaka and Senju reported the Hofmann degradation of p~lyacrylamides~~' Sodium hypochlorite was used as the reagent and polyvinyl amine hydrochloride was isolated almost quantitatively. The effects of reaction conditions on the degradation reaction and the yield of amino compounds were demonstrated in these studies. Eldridge has reported the preparation of graft polyvinyl amine by the Hofmann degradation of polyaory1ami.de grafted to crosslinked polyvinyl alcohol particles containing magnetic iron oxide 89, It was observed that the conversion of amide to amine groups was limited to about 25% and was accompanied by hydrolysis and chain scission. Hofmann rearrangement of crosslinked polyacrylamides as well as amide function attached to styrene-based copolymers are discussed in this chapter. Hofmann degradation reaction was carried out so as to facilitate the preparation of polymeric amines and to study the effect of various reaction parameters on the extent of Hofmann rearrangement in polymeric networks.

2 This section deals with the preparation of polymeric amides and its conversion to polymeric amine through an intrapolymeric rearrangement. For the preliminary investigations, 2% DVB-crosslinked gel-type polymer was used. An amide function was introduced into the polymer through the following steps: (i) Chloromethylation of the resin (ii) Oxidation of chloromethyl polystyrene into polymeric aldehyde (iii) Oxidation of aldehyde into acid (iv) Conversion of acid into acid chloride and (v) Reaction of acid chloride with dry ammonia giving amide. Rearrangement condition was applied and the products were analysed. A temperature-dependent competition between rearrangement and hydrolysis was observed. Polyacrylamide resins with three different crosslinking agents (in 5-20 mole per cent crosslink densities) were prepared by copolymerization. The resins were treated with hypobromite and the products were characterised by chemical and spectroscopic methods. The relation between the molecular character and extent of crosslinking of the polymer and the extent of rearrangement was derived in terms of the amino function in the rearranged products.

3 RESULTS AND DISCUSSION Preparation of Polymeric Amide from DVB- Crosslinked Polystyrene 2% DVB-crosslinked polystyrene support was selected for the preliminary investigations of the Hofmann rearrangement in crosslinked polymeric matrices. The support was prepared by the copolymerization of styrene and divinylbenzene by the free radical suspension polymerization technique using benzoyl peroxide as the initiator. The macroreticular resin thus produced was chloromethylated by treating with chloromethyl methyl ether and SnC14. The chloromethyl polystyrene was oxidised into polymeric aldehydes by treating with dimethyl sulphoxide and sodium bicarbonate (Scheme IV.1). For introducing a rearrangeable amide function into the polymeric backbone, resin 4 was first converted into the polymer analogue of aldehyde. The polymeric aldehyde was treated with sodium dichromate in glacial acetic acid containing a few drops of concentrated H2S04. heating and stirring is required for the conversion of the aldehyde into the carboxylic acid. Prolonged effective The polymeric acid (17) was converted into the corresponding acid chloride analogue (18) by treating with thionyl chloride 191. For this purpose, resin 17 was thoroughly

4 dried in an air oven and swelled in benzene. The preswollen resin was treated with SOC12. The apparatus used were completely free from moisture and a calcium chloride trap was used. The acid chloride thus produced was converted into polymeric amide (19) by passing dry ammonia after swelling in dried dioxane (Scheme IV.1). C1CH2nlg DMSO CH2C1 -$ m c H O SnC14/CH2C$ NaHC03 Na2Cr207, HAc CO OI-I Scheme IV.l. preparation of polymeric amide 2. Synthesis of polymeric mine from Polymeric Amide by Hofmann Rearrangement Polystyrene supported amide was subjected to Hofmann rearrangement. The resin was treated with sodium hypobromite in strong alkaline medium. The reaction

5 temperature was varied from OOC to 70 c. washed with water and organic solvents. under vacuum. The product was It was dried The resulting resin was subjected to chemical and spectroscopic analyses. The rearrangement was observed to be facile in these crosslinked polymeric matrices. The amide undergoes a Hofmann type rearrangement yielding polymer-bound amine as the product (Scheme IV.2). Scheme IV.2. Hofmann rearrangement of polymeric amides into polymeric amines

6 The product polymer gives the characteristic tests for primary amines. The amino capacity was determined by the acetylation method. The extent of rearrangement was calculated from the results. The percentage migrations observed during these studies are less than expected. IR spectral analysis shows that the carbonyl absorption of the polymeric amide does not disappear completely during the rearrangement. However, a slight shift was observed to the longer wavenumber region (Figure IV.1). The product was tested for the presence of acid function in the resin. The carboxylic capacity was determined by equilibrating a weighed quantity of preswollen sample with standard alkali. The unreacted alkali was estimated by titration with acid. The carboxyl capacity was found to be higher than the amino capacity (Table IV.l). These results indicate simultaneous hydrolysis with the rearrangement. 3. Rearrangement/Hydrolysis - Effect of Temperature Hofmann rearrangement was carried out using DVBcrosslinked polystyrene supported benzamide at different temperatures varying from OOC to 70 c. The product was isolated, purified and the amino and carboxyl groups were estimated by chemical methods. Typical results are given in Table IV.l.

8 Table IV.1: Temperature dependence of Hofmann rearrangement in polystyrene matrices: Competition between hydrolysis and rearrangement Tempe- Capacity Amino Carboxyl Percent- Percent- Hydrolysis/ rature of amide group capacity age mig- age hyd- migration ration rolysis ratio (Oc (meq/g ) (meq/g ) (meq/g ) (%) (%) 39.5% rearrangement was observed at OOC whereas only 13.3% rearrangement occured at 70 c. 45.8% hydrolysis was observed at OOC and 68.75% hydrolysis was observed at 70 c. These results suggest a competition between the rearrangement and hydrolytic reactions and the ratio of these two reactions is temperature-dependent. The percentage migration is inversely proportional to the temperature whereas the percentage hydrolysis is directly proportional to the temperature (E'igure'1~.2).. At higher temperatures, the hydrolytic reaction is dominant resulting in the formation of polymeric acids (Scheme IV.3).

9 = Rearrangement = Hydrolysis d# - E 0.rl u E 0 u X W Temperature (OC) Figure IV.2. Rearrangement Vs hydrolysis

10 NH2 CONH COOH Scheme IV.3. Rearrangement - Vs hydrolysis 4. Synthesis of Crosslinked Polyacrylamide Resins Differently crosslinked polyacrylamides (PA) were designed for studying Hofmann rearrangement 192. In the previous cases, the rearrangeable amide function was anchored to the polystyrene support by a series of polymer analogous reactions. The amide group w,as attached to the support as a pendant group. Acrylamide on copolymerization with crosslinking agents like DVB, TTEGDA or N,N1-methylene bisacrylamide (NNMBA) gave the corresponding crosslinked polymer network with functions. Hofmann rearrangement. amide These polymeric amides can be subjected to

11 (a). Synthesis of DVB-Crosslinked Polyacrylamide (21) DVB-crosslinked polyacrylamide was prepared by solution polymerization (Scheme IV.4) by using benzoyl peroxide as the free radical initiator and ethanol as thesolvent. The precipitated polymer was purified by soxhlet extraction. CONH -CH2 - CH2 - CH- I CONH2 -CH - CH2 - CH - CH2 - CH - C H y I CONH2 21 I CONH Scheme IV.4. Preparation of DVB-crosslinked polyacrylamides DVB-crosslinked polyacrylamides with varying crosslink densities were prepared by adjusting the molar ratio of the acrylamide and DVB. PA-DVB resin with 5, 10, 15 and 20 mole per cent DVB contents were prepared. The details of the copolymerization are given in Table IV.2.

12 Table IV.2. Preparation of PA-DVB resin Wt. of monomers (g) Crosslink... Yield Resin density (%) Acrylamide DVB (9) (b). TTEGDA-Crosslinked Polyacrylamide (22) The polymerization was carried out at 60 c using methanol as the solvent. The purified monomers were dissolved in methanol and mixed with ammonium persulphate as the initiator (Scheme IV.5). PA-TTEGDA resins with 5, 10, 15 and 20 mole per cent crosslink densities were prepared by adjusting the rat0 of the monomers. The resins were purified by soxhlet extraction technique. The details of the preparation of the PA-TTEGDA resins are given in Table IV.3.

13 Scheme IV.5. Preparation of TTEGDA-crosslinked polyacrylamide

14 Table IV.3. Preparation of TTEGDA-crosslinked polyacrylamide resins Wt. of monomers (g) Crosslink... Yield Resin density (%) Acrylamide TTEGDA (9) The resulting polymers were characterized by IR spectroscopy. The IR spectra of PA-TTEGDA resins showed absorption peaks at 1690 (C=O, arnide) and 1740 cm-i ( C=O, ester). The appearance of the peak near 1740 cm-l (ester) indicates the incorporation of the TTEGDA crosslinking units in the polymer. (c). Preparation of NNMBA-Crosslinked Polyacrylamide (23) NNMBA-crosslinked polyacrylamide resins were prepared by solution polymerization using water as the solvent (Schme IV.6). Ammonium persulphate was used as the free radical initiator and the reaction was carried out at 70 c. Crosslink densities were adjusted by varying the acrylamide/nnmba ratio. Resins with different crosslink

15 densities such as 5, 10, 15 and 20 mole per cent of the bifuctional crosslinking agent were prepared. The details are given in Table IV.4. The precipitated polymers were purified by soxhlet extraction and characterized by IR. I CONH2 CO CONH I I I Scheme IV.6. Preparation of NNMBA-crosslinked polyacry lamide

16 Table IV.4. Preparation of NNMBA-crosslinked polyacrylamide Wt. of monomers (g) Crosslink... Yield Resin density (%) Acrylamide NNMBA (g) 23a Hofmann Rearrangement in Crosslinked Po1yacrylami.de Matrices As part of the studies of molecular rearrangement in crosslinked macromolecular matrices, Hofmann rearrangement reaction in 2% DVB-crosslinked polystyrene supported amide functions was investigated. About 40% migration was reported in these studies. Formation of carboxylate functions was also observed which appears to be due to the hydrolysis of the amide groups. The investigations on Hofmann rearrangement were extended into crosslinked polyacrylamide resins. DVB, TTEGDA and NNMBA-crosslinked polymers were used for these studies.

17 (a). Hofmann Rearrangement in DVB-Crosslinked Polyacrylamide Matrices DVB-crosslinked polyacrylamide resins with different crosslink densities were subjected to Hofmann rearrangement. The rearrangement was observed to be facile in these polymers which was established by the analysis of amino group in the resulting product. The polymeric amide undergoes a Hofmann type rearrangement resulting in the formation of the polymeric amine (Scheme IV.7). CONH2 NaOBr I NaOH 2 4 scheme IV.7. Conversion of polymeric amide polymeric amine into Polyacrylamide resins with 5, 10, 15 and 20 mole per cent crosslinking agent were subjected to Hofmann rearrangement using sodium hypobormite. The amino group was detected by usual chemical tests and the amino

18 capacity was determined by the estimation of the amino group by acetylation method. The results are given in Table IV.5. Table IV.5. Hofmann rearrangement in DVB-crosslinked polyacrylamide matrices Crosslink Amino Resin density capacity (mole %) (meq/g ) The results suggest that the conversion of amide into amine is not quantitative. 5% crosslinked resin shows 2.79 meq/g amino capacity. For 20% crosslinked resin, the amino capacity was only 1.51 meq/g. As the frequency of crosslinking units increases, the extent,of rearrangement was found to be decreased. The decrease in amino capacity with increasing crosslink density is explainable based on the polymeric effect of the backbone. As the DVB content increases, the polymer becomes more rigid and hydrophobic and the accessibility of the rearranging functional group is reduced. In all the cases, carboxyl function was

19 observed in the product. This indicates the hydrolysis of the amide groups into the carboxylic acid function as a parallel reaction alongwith the rearrangement. (b). Hofmann Rearrangement in TTEGDA-Crosslinked Polyacrylamide Matrix The resins prepared by the copolymerization of acrylamide and TTEGDA containing rearrangeable amide groups were subjected to Hofmann rearrangement. The amino group in the products obtained by the degradation reaction was monitored quantitatively. The results are presented in Table IV.6. Table IV.6. Hofmann rearrangement in WEGDA-crosslinked polyacrylamide matrices Crosslink Amino Resin density capacity (mole %) (meq/g)

20 In the case of PA-TTEGDA resin also, the extent of rearrangement was found to be inversely proportional to the crosslink density. 5% crosslinked resin gives 1.95 meq/g amino capacity whereas the 20% resin gives only 0.70 meq/g. This decrease is related to the increased rigidity of the network and hence the decreased accessibility of the reactive sites. (c). Hoffman Rearrangement in NNMBA-Crosslinked Polyacrylamide Matrix Hofmann rearrangement condition was applied to PA- NNMBA resin with 5, 10, 15 and 20% crosslink densities. The products were isolated and the amino capacity was estimated by acetylation method. The results are given in Table IV.7. Table IV.7. Hofmann rearrangement in NNMBA-crosslinked polyacrylamide matrix Crosslink Amino Resin density capacity (mole %) (meq/g)

21 Comparatively high amino capacity was observed in the case of PA-NNMBA resin. The 5% resin gives 3.10 meq/g amino group and the 20% resin gives 1.75 meq/g amino capacity. This may be due to the partial bond scission of the NNMBA crosslinking units in the polymeric networks. Chemical and spectral analyses of the rearrangement products of all the three different types of polyacrylamide resins showed that there is no quantitative conversion of the amide into amine through the rearrangement step and the presence of some other functional groups are also observed. Carboxyl group was detected in all the cases, which might be produced by the hydrolysis of the pendant amide groups or the amide linkages of the crosslinking units. Good yield of the amino functional group can be achieved by adjusting the reaction conditions. The use of excess alkali facilitates the rearrangement reaction. However the use of large excess bromine will cause some side reactions. PA-TTEGDA and PA-NNMBA resins are superior to PA-DVB resin due to the polar character and accessibility of the crosslinking units and hence the reactive sites to the attacking species. But the ester and amide linkages in the crosslinking units are labile for hydrolytic reactions. Therefore, these resins are least preferred.

22 One of the important observations of the studies of Hofmann rearrangement in polymeric matrices is the dominance of the 'polymer effect' on the course of the rearrangement. The purity of the Hofmann product was doubtful due to the side reactions of the amide analogue and bond scission of the crosslinking units. However the polymer influences the extent of the reaction by its topographical peculiarities. From the results of the previous investigations with benzil-benzilic acid rearrangement in crosslinked macromolecular systems, it might be expected that the migratory aptitude in polymeric. analogous Hofmann rearrangement is dependent on the crosslink density of the backbone and on the molecular character of the crosslinking agents. Due to the heterogeneity of the polymeric systems, the reagent present in the continuous phase must penetrate into the interior of the network to attack the reactive sites. the degree of crosslinking increases, the ability of the reagent to penetrate into the interior decreases. As This will result in a reduced extent of migration in highly crosslinked polymers. The amino capacity of the rearranged product and the extent of side reactions are different for the various acrylamide resins. But in all the cases, an inverse relation was observed between the extent of rearrangement and the extent of crosslinking.

23 EXPERIMENTAL

CHAPTER I1 BENZIL-BENZILIC ACID REARRANGEMENT IN CROSSLINKED MACROMOLECULAR SYSTEMS

CHAPTER I1 BENZIL-BENZILIC ACID REARRANGEMENT IN CROSSLINKED MACROMOLECULAR SYSTEMS Control over reactivity, rate and specificity can be attained in functional transformations of organic compounds by attaching