Chemistry Reference
In-Depth Information
Polymerizations of methacrylate performed in the presence of sulfuric acid have been reported, with
alkoxyamines prepared with methyl methacrylate, TEMPO, and either benzoyl peroxide or AIBN. Reaction
times are relatively short (45 - 120 min) at reaction temperaturtes of 70
◦
C and 130
◦
C yielding polymers
with M
w
/M
n
=
09 - 1.28.
123
1
.
11.3.8 Rate enhancement of stable free radical polymerization through the use of additives
Initial polymerizations of styrene performed by SFRP using TEMPO as the moderating nitroxide were
quite slow, taking over 40 hours to reach conversions of 76 %.
38
To increase the rate of polymerization,
additives were added to the reaction solutions to reduce the amount of excess free nitroxide, which
increases over time due to unavoidable termination by radical chain - chain coupling. Additives that work
well for styrene polymerization include camphorsulfonic acid (CSA),
124,125
acetic anhydride,
126
2-fluoro-1-
methylpyridinium
p
-toluenesulfonate (FMPTS),
127
and dextrose.
128
Ascorbic acid
109
and ene-diols
110
are
effective for acrylate polymerizations. More recently, the use of DMF as an additive for the polymerization
of
tert
-butyl acrylate initiated by 4-oxo-TEMPO-capped polystyrene macroinitiator has been shown to
be effective.
129
To circumvent the use of additives, a desirable goal in any polymerization, Studer
et al
. employed a
combination of alkoxyamines to polymerize styrene, with one alkoxyamine containing TEMPO (low k
d
)
and the other containing a sterically bulky nitroxide (higher k
d
, lower k
c
), to very good effect.
82
It was
subsequently shown that polymerizations of styrene can proceed quickly and efficiently using a primary
initiator, such as BPO, in the absence of an additive, if the ratio of TEMPO to BPO is varied according
to the targeted molecular weight.
60
11.4 Non-nitroxide-based radicals as mediating agents
The uses of other stable radicals, some isolatable, others generated
in situ
, have been investigated, with
varying degrees of success for living radical polymerizations.
11.4.1 Triazolinyl radicals
Triazolinyl radicals with the general structure
15
, are five-membered ring structures that contain a con-
strained hydrazyl moiety that increases the molecules' stability by allowing
-orbital overlap between the
two adjacent nitrogen atoms.
130,131
Triazolinyl radicals offer some degree of control in the free radical
polymerization of styrene and a variety of methacrylates.
132 - 136
For example, triazolinyl-mediated poly-
merizations using the stable radicals
16
or
17
, or a mixture of both, as mediators have been shown to be
quite effective in the synthesis of both homopolymers and block copolymers.
π
R
R"
R"
N
N
N
N
N
N
N
N
N
R'
15
16
17
Search WWH ::
Custom Search