N-oxyl radicals

Details N-oxyl radicals N-oxyl radicals

2002-02-19

2003-10-28

Teskin, Fred (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S204000, C526S217000, C526S220000, C526S346000, C526S347100, C525S256000, C525S267000

Reexamination Certificate

active

06639033

ABSTRACT:

The present invention relates to N-oxyl radicals, N-oxyl radical formers and their use in the free radical polymerization of ethylenically unsaturated monomers.
N-oxyl radicals and N-oxyl radical formers which are derived from a secondary amine which carries no hydrogen atoms on the &agr;-carbon atom are known. They are or form extremely stable free radicals and as a rule can be prepared as a pure substance.
Such stable N-oxyl radicals are used, inter alia, as inhibitors of undesired free radical polymerization (cf. for example U.S. Pat. No. 5,322,960). A further use of such stable N-oxyl radicals consists in the fact that their presence permits a certain control of free radical polymerizations of compounds (monomers) having at least one ethylenically unsaturated group (cf. for example EP-A 135280, U.S. Pat. No. 5,412,047 and U.S. Pat. No. 5,322,912).
In fact, free radical polymerizations of monomers usually have the disadvantage that the molecular weight of the polymer chains does not increase linearly with the conversion in the polymerization and that the polymer chains of the resulting polymer do not as a rule have a uniform molecular weight, i.e. the polymer obtainable by free radical polymerization is as a rule not monodispersed with respect to the property of molecular weight but usually has a polydispersity index PDI in this context of ≧2 (PDI={overscore (M
w
)}/{overscore (M
n
)}, where {overscore (M
w
)} is the weight-average molecular weight of the polymer and {overscore (M
n
)}=is the number average molecular weight of the polymer). Both of the abovementioned phenomena adhere to determination reactions as a result of the irreversible combination of growing free radical polymer chain ends.
The controlled influence of stable N-oxyl radicals (which are usually not capable of initiating a free radical polymerization of monomers) is presumably due to the fact that the stable N-oxyl radicals do not irreversibly terminate but only temporarily block reactive free radical ends of a growing polymer-chain. This results in a reduction in the steady-state concentration of growing free radical polymer chain ends, which reduces the possibility of an irreversible termination of the chain growth of two growing polymer chain ends. This leads on average to polymer chains growing (ideally linearly) with the conversion in the polymerization. The latter results in an average molecular weight of the resulting polymer which increases (ideally linearly) for the conversion of the polymerization and has a polydispersity index frequently <2. A controlled free radical polymerization carried out as described thus permits the preparation of tailor-made polymers having a specific molecular weight and appropriately tailored physical properties. Furthermore, it opens up the possibility of the preparation of block copolymers.
The stable N-oxyl radicals may be added to the polymerization system as such and/or in the form of N-oxyl radical formers. In the first case, the addition of classical free radical polymerization initiators or initiation by heat or high-energy radiation is usually essential for initiating the controlled free radical polymerization. In the second case, the polymerization can be initiated simply by the free radicals formed in addition to the stable N-oxyl radicals in thermal decomposition of the N-oxyl radical formers.
Stable free N-oxyl radicals used todate are, inter alia, TEMPO [(1): 2,2,6,6-tetramethylpiperidin-1-oxyl] and 4-OH-TEMPO [(2): 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl].
However, the disadvantage of the abovementioned N-oxyl radical is that their range of-use is limited essentially to the polymerization of systems which contain styrene monomers or a mixture of styrene monomers and other monomers. Nonstyrene monomers cannot be expediently subjected to homopolymerization in the presence of TEMPO or 4-OH-TEMPO.
U.S. Pat. No. 4,528,370 relates to polysubstituted 2-morpholones. When the term nitroxyl compounds is used in this publication, only the preparation of the nitroxyls of precursor compounds in the form of substituted hydroxyethylaminoacetates is mentioned.
EP-A-0 869 137 discloses a polymerization process for the preparation of (meth)acrylate-containing homopolymers or block copolymers in the presence of specific nitroxyls of the piperazinone or morpholone structure type.
However, the known free radical polymerization processes in the presence of nitroxyl radicals have the disadvantage that long reaction times are required for achieving practicable conversions.
DE-A-2 351 865 discloses, inter alia, the compound 2,2,6,6-tetramethyl-1-aza-4-thiacyclohexane-1,4,4-trioxide. The use of this compound for controlling free radical polymerizations is neither taught nor suggested.
It is an object of the present invention to provide novel N-oxyl radicals or formers of novel N-oxyl radicals, in the presence of which the controlled free radical polymerization leads to relatively high conversions in relatively short times. They should furthermore be suitable for carrying out controlled free radical polymerizations of nonstyrene monomers, in particular of esters of acrylic and/or methacrylic acids.
We have found that this object is achieved, according to the invention, by N-oxyl radicals of the formula I
where X is S, NR
5
, O or SO and
R
1
, R
2
, R
3
, R
4
and R
5
independently of one another, are organic radicals or
R
1
and R
2
and/or R
3
and R
4
together with the carbon atom carrying them, are a ring consisting at least of 3 atoms.
We have found that the object is furthermore achieved by a process for the free radical polymerization of ethylenically unsaturated monomers, wherein the polymerization is carried out in the presence of N-oxyl radicals of the formula I, where X may additionally be SO
2
.
In a preferred embodiment, the N-oxyl radicals of the formula I comprise a spiro ring system formed from R
1
and R
2
and/or R
3
and R
4
, together with the carbon atom carrying them, particularly preferably from R
1
and R
2
, together with the carbon atom carrying them. Two nonneighboring ring atoms in the ring may be bridged via a divalent organic radical and/or the ring may be fused to a further ring. Expediently, the ring formed from R
1
and R
2
or R
3
and R
4
, together with the carbon atom carrying them, consists of from 3 to 10, preferably of three, five, six or seven, atoms, the free valences of the atoms forming the ring being saturated with hydrogen and/or organic radicals, preferably C
1
-C
20
-alkyl, especially C
1
-C
4
-alkyl (e.g. methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl and tert-butyl). However, aryl groups are also suitable as such saturating radicals. Among these, the phenyl and the naphthyl group are preferred. Usually, the ring is a saturated ring and generally the ring-forming atoms are carbon atoms, but the ring-forming atoms may also include, for example, one or two heteroatoms, such suitable heteroatoms being in particular, N, O and/or S.
The present invention relates in particular to N-oxyl radicals of the formula I, where R
1
, R
2
, R
3
, R
4
and R
5
independently of one another, are C
1
-C
20
-alkyl, C
5
-C
8
-cycloalkyl, C
7
-C
20
-aralkyl, unsubstituted or substituted C
6
-C
10
-aryl, heteroaryl,
where
R
6
is straight-chain or branched C
1
-C
20
-alkylene,
R
7
in each case independently H, C
1
-C
20
-alkyl, C
3
-C
20
-cycloalkyl which may be interrupted by oxygen atoms, or aryl,
R
8
in each case independently C
1
-C
20
-alkyl or aryl,
R
9
is a chemical bond or straight-chain or branched C
1
-C
20
-alkylene, and
R
10
is H, C
1
-C
20
-alkyl, aryl or an alkali metal,
k is 0 or 1 and
n is an integer from 1 to 100,
or R
3
and R
4
together are an oxygen atom and/or R
1
and R
2
and/or R
3
and R
4
, together where the carbon atom carrying them, form a carbocyclic ring which comprises 3 to 10 atoms and in which 1 or 2 carbon atoms may be replaced by O, S or N and/or in which two nonneighboring atoms may be bridged via a divalent organic radical, preferably C
1
-C
3
-alkylene,

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