Initiators for radical polymerization

Details Initiators for radical polymerization Initiators for radical polymerization

1999-12-27

2001-01-09

Pezzuto, Helen L. (Department: 1713)

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

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S204000, C526S209000, C526S217000, C526S219100, C526S220000, C526S222000, C526S225000, C502S150000, C502S162000, C502S167000, C502S200000, C544S224000, C544S234000, C544S235000, C544S236000

Reexamination Certificate

active

06172170

ABSTRACT:

DESCRIPTION
The invention relates to initiators for free-radical addition polymerization, comprising as structural feature the Diels-Alder adduct of an azo group (—N═N—) with a conjugated double bond (diene).
Initiators for free-radical addition polymerization are compounds which decompose into free radicals and so initiate the free-radical addition polymerization. The rate of decomposition is generally dependent on temperature and can be described by the Arrhenius equation. Accordingly, decomposition to free radicals begins even at relatively low temperatures and is accelerated by temperature increase.
For some fields of use, for example for the curing of powder coatings, initiators are desired which become effective only at high temperatures. Following application to the substrate surface, powder coatings must generally be heated to flowability in order to obtain a smooth and level coating film. Curing (free-radically crosslinking), which begins simultaneously by decomposition of the initiators to free radicals, leads in this case to unevennesses or defects in the coating surface. Instead, curing should not begin until after a smooth coating film has been formed by the melting of the polymer powder.
This requires initiators which decompose to free radicals only at high temperatures. The temperature dependency of the rate of decomposition in this case should be as low as possible; in other words, no decomposition below a certain temperature as complete as possible a decomposition above this temperature (non-Arrhenius behavior).
The object of the present invention was to provide appropriate initiators.
Accordingly, the initiators defined at the outset have been found.
The initiators according to the invention comprise as structural feature the Diels-Alder adduct of an azo group, that is an —N═N—group, with a diene.
A Diels-Alder reaction is a 1,4-cycloaddition of a C,C double bond (here an azo group instead) onto a conjugated double bond (diene).
Accordingly, the initiators according to the invention can be obtained by 1,4-cycloaddition of the azo group of an azo compound (dienophile) onto a diene.
The azo compound is a compound which is already an initiator for free-radical polymerization and which decomposes to free radicals above certain temperatures (azo initiator). Furthermore, the azo compound should be able to undergo a Diels-Alder reaction.
Suitable azo compounds are, for example, those of the formula
in which R
1
and R
2
independently of one another represent organic radicals.
The radicals R
1
and R
2
are preferably organic radicals which contain up to 30, in particular up to 20 C atoms and, if desired, also heteroatoms such as N, O, S.
With particular preference, R
1
and R
2
independently of one another are a C
1
-C
20
-alkyl group, a C
5
-C
20
-aryl group or cycloalkyl group, a C
6
- to C
20
-alkaryl or aralkyl group. In particular they are a C
1
-C
8
alkyl group. The alkyl groups can be linear or branched.
Another azo compound capable of the Diels-Alder reaction is an imide of the formula
R
1
has the meaning indicated above.
The initiating action of the azodicarboxylic esters has been described in the literature (T. Schmelzer and J. Springer; Eur. Polym. J. 23(3), pp. 243-248 (1987)).
Examples of suitable dienes which form the Diels-Alder adduct with the azo compounds are butadiene, cyclopentadiene, cyclohexadiene, or derivatives thereof in which one or more of the hydrogen atoms are substituted by organic radicals.
All that is essential is that a conjugated double bond is present which is capable of a Diels-Alder reaction.
The implementation of the Diels-Alder reaction is known.
For preparing the initiators according to the invention it is possible to react the azo compounds with the diene, for example, at temperatures from 0° C. to 100° C. in a solvent.
In the reaction of the azo compounds of the formula I, for example with cyclopentadiene, the following Diels-Alder adduct is obtained:
With the initiator according to the invention as well, the original azo compound is the compound which decomposes to free radicals and so acts initiatingly. All that is necessary is that this azo compound must first of all be released from the Diels-Alder adduct by a reversal of the Diels-Alder reaction (retro-Diels-Alder reaction).
The temperature which is required for the retro-Diels-Alder reaction depends on the choice of the azo compound with the diene components. In general, the temperatures lie above 100° C., in particular between 150 and 250° C. These temperatures generally lie above the decomposition temperature of the azo compounds. In the case of temperature increase, the azo compound is released only after the retro-Diels-Alder reaction has taken place, so that the temperature of decomposition of the initiators according to the invention to free radicals is higher than that of the original azo compounds.
The retro-Diels-Alder reaction also follows the Arrhenius Law, i.e. it shows a certain temperature dependency. A further improvement of this invention can be achieved if this temperature dependency is reduced still further.
The literature reference Fr{acute over (e)}chet, J. M. J.; Eichler, E, HO, H; Willson C. G. Polymer 1996, 24, 995-1000 discloses that phenyl tert-butyl carbonate decomposes abruptly when heated into isobutene:
The decomposition is autocatalytic and does not follow the Arrhenius Law.
Corresponding addition of substituents onto the Diels-Alder adduct of the formula IIIa, for example, gives the compound
in which R
3
represents a phenyl group which may additionally carry further substituents.
The compound of the formula IIIb decomposes at elevated temperature autocatalytically, giving off CO
2
and eliminating phenol as it does so, to the Diels-Alder adduct of the formula IIIa, which then decomposes further in a retro-Diels-Alder reactor [sic] to the azo compound which is active as initiator. By this means the temperature dependency of the decomposition of the Diels-Alder adduct is suppressed and the initiator present in the Diels-Alder adduct becomes effective to its full extent only above the temperature of the autocatalytic decomposition.
Diels Alder adducts which are subject to such an autocatalytic decomposition are those which are substituted by at least one group of the formulae
A and A′ in the above formulae independently of one another represent organic radicals having 1 to 30 C atoms, preferably 2 to 20 C atoms.
The radical involved is in particular one which has an aromatic ring system. With particular preference it is a phenyl group, which may if desired carry further substituents, e.g. C
1
-C
8
alkyl groups. very particular preference is given to a phenyl group. Preferred- substituents are groups of the formula IIa or IId, especially phenylcarbonate or phenylsulfonate.
The Diels Alder adduct preferably contains one or two, with particular preference one substituent of the formula IIa to IIe.
The phenyl group of the phenylcarbonate or phenylsulfonate respectively may contain further substituents, in particular for example C
1
-C8 alkyl groups. By this means it is possible to vary the autocatalytic decomposition temperature.
The above groups of the formulae IIa) to IIe), especially the phenylcarbonate or phenylsulfonate group respectively can be attached to the Diels-Alder adduct, for example, by means of customary addition reaction onto the double bonds obtained in the course of the Diels-Alder reaction. Also particularly suitable are esterifications of the acids, acid anhydrides or acid chlorides derived from IIa to IIe with Diels-Alder adducts containing hydroxy groups. The attachment of the phenylcarbonate or phenylsulfonate group, or of derivatives thereof, is achieved with particular simplicity by reacting
with Diels-Alder adducts containing hydroxy groups.
The initiators according to the invention are suitable as initiators for free-radical addition polymerization. This may involve the addition polymerization of low-molecular monomers, examples being acrylates, vinylaromatic compounds, vinyl es

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