Process for preparing polymers in the presence of triazolyl...

Details Process for preparing polymers in the presence of triazolyl... Process for preparing polymers in the presence of triazolyl...

1999-03-10

2002-04-30

Henderson, Christopher (Department: 1713)

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

Synthetic resins

Mixing of two or more solid polymers; mixing of solid...

C526S204000, C526S205000, C525S299000, C525S318000, C525S328500

Reexamination Certificate

active

06380315

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a process for preparing polymers. The present invention also relates to the use of the polymers obtainable in that process for preparing moldings, films or fibers. Furthermore, the present invention relates to the polymers obtainable by the novel process and to their use.
BACKGROUND ART
Free-radical polymerization is the most widely applicable technique for polymerizing unsaturated monomers such as those containing vinylic double bonds. It permits the polymerization of a large number of monomers varying in structure, functional groups and polarity. The copolymerization of different monomers with one another is also possible. Owing to unavoidable side reactions such as chain transfer, disproportionation, recombination or elimination, however, it is very difficult to control the molecular weight distribution. Normally, polymers having a polydispersity index PDI of 2.0 or more are obtained. The relevant definition is PDI=M
w
/M
n
, M
w
being the weight-average and M
n
the number-average molecular weight. In addition, little influence can be exerted over the architecture and structure of the polymers.
To prepare polymers with a narrow molecular weight distribution, therefore, the technique of controlled free-radical polymerization was developed, also sometimes called “living” free-radical polymerization, which is described, for example, in M. K. Georges et al., Trends in Polymer Science, Vol. 2, No. 2 (1994), pages 66 to 72. The fundamental strategy of this technique consists in temporarily blocking and then reactivating, in a controlled manner, the reactive free-radical chain ends of a growing polymer chain. The dynamic equilibrium between active and dormant form leads to a low steady-state concentration of free polymer radicals.
A variety of techniques are available for blocking and stabilizing the free-radical chain end. They employ stable free radicals and/or metal salts.
For instance, it is known to use “iniferters”, i.e. agents which both free-radically initiate a polymerization and terminate the chain end by combination. Examples of photochemically activated iniferters, such as dithiocarbamates, are described in T. Otsu et al., Eur. Polym. J., Vol. 25, No. 7/8 (1989), pages 643 to 650. However, these photochemical iniferters are very expensive compounds, and photochemically initiated polymerization is highly uneconomic in industrial practice. Furthermore, the polydispersity index is very high in some cases. There are also thermal iniferters, such as tetramethylene disulfides, which are described, for example, in K. Endo et al., Macromolecules, Vol. 25 (1992), pages 5554 to 5556. In this case the PDI, at levels of between 3 and 4, is too high to be satisfactory.
EP-A 135 280 describes the use of stable N-oxyl radicals which combine reversibly with the reactive chain ends. However, this process produces not high molecular mass polymers but oligomers instead.
U.S. Pat. No. 5,322,912 discloses cyclic, sterically shielded N-oxyl radicals which are used in combination with conventional initiators. However, these systems do not permit the polymerization of alkyl acrylates.
A particular group of initiators for controlled free-radical polymerization is formed by compounds which can be cleaved into free-radical initiators and N-oxyl radicals (Trends in Polymer Science, 4(6), 1996, 183-188). These compounds make it possible, for example, to construct branched polymers. However, only selected monomers can be polymerized, and the reaction temperatures are too high to be satisfactory.
In general, the reaction rates for the polymerization of monomers in the presence of N-oxyl radicals are too low for many industrial purposes. For this reason use has been made, for instance, of strong organic acids (U.S. Pat. No. 5,322,912). These acids can give rise to difficulties during the workup of the products, however.
DE-P 195 16 967.0 describes techniques in which vinylic monomers are polymerized in the presence of common free-radical initiators and of electron donors which stabilize the free radical chain end.
DISCLOSURE OF INVENTION
It is an object of the present invention to provide a novel process for preparing polymers which can be used to polymerize a broad range of unsaturated monomers. It is also intended that the novel process should permit very good control over both the molecular weight distribution and the architecture and structure of the polymers. The intention, furthermore, was to find a process having a sufficiently high reaction rate even at relatively low temperatures.
We have found that these objects are achieved by a process for preparing polymers, in which polymerization is conducted in the presence of free radicals of the formula I
in which Q is NR
2
or S and T is CR
3
R
4
or S and R
1
, R
2
, R
3
and R
4
can be identical or different and independently are hydrogen, C
1
to C
20
-alkyl or C
6
- to C
8
-aryl. Reference to the free radicals shown in the formula I should also be understood according to the invention as referring to their tautomers and positional isomers. The alkyls can be linear, branched or cyclic. They can be either unsubstituted or substituted, for example by one or more halogens such as chlorine, nitrile groups, NO
2
, sulfonic acid radicals, hydroxyls, alkyl or aryl ester radicals. Furthermore, the alkyls may contain sulfoxide or carbonyl radicals. The alkyls include C
1
to C
12
-alkyl, preferably C
1
- to C
10
-alkyl, for example methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, t-butyl, n-pentyl, cyclopentyl, n-hexyl or cyclohexyl. Among these, methyl is particularly preferred. The preferred aryls include phenyl, naphthyl and biphenylyl. The aryls can be either substituted by one or more substituents or else unsubstituted. Suitable substituents are alkyls, for example C
1
- to C
10
-alkyl, preferably C
1
- to C
6
-alkyl, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl or t-butyl, or else hydroxyls or halogens such as chlorine. In addition, the aryls can also be substituted by one or more halogens such as chlorine, nitrile groups, NO
2
, sulfonic acid radicals, alkyl or aryl ester radicals. Among the aryls, phenyl is particularly preferred.
Examples of suitable free radicals I are thiatriazolyls of the formulas
or dithiadiazolyls of the formula
Preference is given to 2,5-dihydro-1H-1,2,4-triazol-2-yl (triazolyl radicals) of the formula
Particular preference is given to free triazolyl radicals in which R
3
and R
4
are identical. In the very particularly preferred free triazolyl radicals, R
1
is phenyl, R
2
is phenyl or methyl and R
3
and R
4
are each phenyl, biphenyl-2,2′-diyl, 6,6′-dimethylbiphenyl-2,2′-diyl or 5,5′-dimethylbiphenyl-2,2′-diyl.
2,5-Dihydro-1H-1,2,4-triazol-2-yl radicals are known per se or are obtainable by methods known per se. Thus the free triazolyl radicals can be obtained, for example, by irradiating 1H-1,2,4-triazoles with &ggr; radiation or can be prepared by dehydrogenating 4,5-dihydro-1H-1,2,4-triazoles with basic potassium hexacyanoferrate solution. Another method of obtaining free triazolyl radicals is the ring contraction of tetrazines in the presence of acids (Tetrahedron, 51 (47), 1995, 12883-12898).
Thiatriazolyls can be prepared, for example, by reducing the corresponding thiatriazol-1-ium salts (J. Am. Chem. Soc. Perkin Trans 2 (1990) 1619). Dithiadiazolyls are obtainable, for example, by reducing the corresponding dithiadiazolium salts (Chem. Ber. 118 (1985) 3781).
The free radicals I can be generated in situ by, for example, one of the abovementioned methods. The free radicals I are preferably prepared separately, isolated, and employed as they are. In addition, the free radicals I can be employed in the novel process alternatively in the form of compounds II which can be cleaved into free-radical initiators and free radicals I. Compounds of this kind can be summarized, for example, by the formula II
where R
5
is a radical which, when eliminated, is able to initiate a free-radical reaction. Preferred

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