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.
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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