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CHAPTER IV
HOFMANN REARRANGEMENT IN CROSSLINKED POLYMERIC MATRICES
The Hofmann degradation reaction has been used as a
synthetic route for the preparation of amines 180-187
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 89,186-190. 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.
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.
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. Prolonged
heating and stirring is required for the effective
conversion of the aldehyde into the carboxylic acid. The
polymeric acid (17) was converted into the corresponding
acid chloride analogue (18) by treating with thionyl
191 chloride . For this purpose, resin 17 was thoroughly
dried in an air oven and swelled in benzene. The pre-
swollen 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).
DMSO CH2C1 -$ m c H O C1CH2nlg'm NaHC03 SnC14/CH2C$
Na2Cr207, CO OI-I
HAc
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
temperature was varied from OOC to 70°c. The product was
washed with water and organic solvents. It was dried
under vacuum.
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
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 pre-
swollen 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 DVB-
crosslinked 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.
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. 4 5 . 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).
Figure IV.2. Rearrangement Vs hydrolysis
70 -
60 - 0 = Rearrangement = Hydrolysis
d# - E 0
.rl
u E 0 u X W
20 -
10 -
.
0 20 4 0 6 0 8 0 100
Temperature ( O C )
CONH
NH2
COOH
Scheme IV.3. Rearrangement Vs hydrolysis -
4. Synthesis of Crosslinked Polyacrylamide Resins
Differently crosslinked polyacrylamides (PA) were
192 designed for studying Hofmann rearrangement . 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 amide
functions. These polymeric amides can be subjected to
Hofmann rearrangement.
(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 the-
solvent. The precipitated polymer was purified by soxhlet
extraction.
CONH
- CH2 - CH- -CH2 I
CONH2
-CH - CH2 - CH - CH2 - CH - C H y I I CONH2 CONH
21
Scheme IV.4. Preparation of DVB-crosslinked polY- acrylamides
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.
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.
Scheme IV.5. Preparation of TTEGDA-crosslinked poly- acrylamide
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
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 poly- acry lamide
Table IV.4. Preparation of NNMBA-crosslinked polyacrylamide
Wt. of monomers (g) Crosslink ...................... Yield
Resin density ( % ) Acrylamide NNMBA ( g )
23a 5 13.5 1.54 13.7
5. 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.
(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).
NaOBr CONH2 I
NaOH
2 4
scheme IV.7. Conversion of polymeric amide into polymeric amine
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
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
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)
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 )
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.
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. As
the degree of crosslinking increases, the ability of the
reagent to penetrate into the interior decreases. 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.
EXPERIMENTAL