Method for living free-radical polymerization

Details Method for living free-radical polymerization Method for living free-radical polymerization

2001-06-14

2002-11-12

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...

C526S193000, C526S227000, C526S229000, C526S232100, C526S240000, C526S265000, C526S310000, C526S318200, C526S328000, C526S329700, C526S335000, C526S346000

Reexamination Certificate

active

06479603

ABSTRACT:

The invention relates to a process for the living free radical addition polymerization of ethylenically unsaturated monomers in the presence of a stable N-oxyl radical.
Free-radically initiated polymerizations of monomers having at least one ethylenically unsaturated group have the disadvantages that the molecular weight of the polymer chains does not normally increase with the degree of polymerization conversion and that the polymer chains of the resulting polymer are not generally of uniform molecular weight. In other words, and in terms of the molecular weight parameter, the polymer obtainable is generally not monodisperse but usually has a polydispersity index PDI in this regard of ≧2 (PDI={overscore (M)}
w
/{overscore (M)}
n
, where {overscore (M)}
w
=weight-average molecular weight and {overscore (M)}
n
=number-average molecular weight). This can probably be attributed to termination reactions as a result of the irreversible combination of growing free radical polymerization chain ends in particular and to chain transfer reactions, disproportionation, and elimination.
Another disadvantage of conventional free-radically initiated polymerization is that a change made during polymerization to the monomers that are to be polymerized does not generally result in segmented copolymers (block polymers). For example, a change in monomers in the course of emulsion polymerization results in core-shell polymer particles with a core composed of one type of monomer and a shell composed of the other type of monomer, the bond between core and shell being primarily not chemical but merely physical. Accordingly, in the case of conventional free-radical polymerization, the phase attachment of the shell to its core is in some cases inadequate.
It is known from the prior art that by carrying out free-radically initiated polymerization reactions at temperatures above 100° C. in the presence of a stable (essentially noninitiating) N-oxyl radical it is possible to control the free-radically initiated polymerization. For example, the application DE-A 19 803 098, whose priority date is earlier than that of the present application, discloses free-radically initiated aqueous emulsion polymerizations using stable N-oxyl radicals.
The mechanism on which the action is based is presumed to be that the stable N-oxyl radicals do not irreversibly terminate but merely temporarily block reactive radical ends of a growing polymer chain at elevated temperatures. The result of this is a reduction in the steady-state concentration of growing free-radical polymer chain ends, thereby reducing the possibility for irreversible termination of chain growth through the combination of two growing polymer chain ends. This leads on average to polymer chains which grow in (ideally linear) proportion with the polymerization conversion. The result of the latter is an average molecular weight which grows in (ideally linear) proportion with the polymerization conversion, with the resulting polymer having a polydispersity index (PDI) which is ideally 1. At the same time, however, the reduction in the steady-state concentration of growing free radical polymerization chain ends results in a very low polymerization rate.
It is an object of the present invention to increase the polymerization rate in a polymerization of this kind, preferably without substantially increasing the polydispersity of the resulting polymer.
We have found that this object is achieved by a process for the living free radical addition polymerization of one or more ethylenically unsaturated monomers using at least one free-radical polymerization initiator and in the presence of one or more stable N-oxyl radicals. The process of the invention then comprises the feature that at least one stable N-oxyl radical has polymerizable double bonds.
The availability of an easy-to-implement, rapid, controlled, free-radically initiated polymerization for preparing polymers would be advantageous insofar as it would enable the molecular weight of the polymer to be adjusted in a controlled manner within appropriate reaction times. Furthermore, it opens up direct access to block copolymers, since the free radical polymer chain ends are not destroyed by a combination but instead are merely blocked reversibly. In other words, following the consumption of a first type of monomer, the polymerization can be continued with the addition of further types of monomer.
Furthermore, by increasing the polymerization rate and at the same time utilizing the advantages of stable N-oxyl radicals, the long polymerization times are reduced. Accordingly, a practicable and economic process is provided.
In accordance with the invention, the polymerization is carried out using at least one stable N-oxyl radical which has polymerizable double bonds.
The presumed mechanism is based on the ability of the stable N-oxyl radical, owing to its polymerizable double bonds, to participate in the polymerization together with the ethylenically unsaturated monomers. At the same time, the stable N-oxyl radical is able temporarily to block reactive radical ends of a growing polymer chain and so lead to a reduction in the steady-state concentration of growing free radical polymer chain ends, thereby reducing the possibility of irreversible termination of chain growth through the combination of two growing polymer chain ends. As a result of the polymerization of the stable N-oxyl radicals, the concentration of free stable N-oxyl radicals reduces as the polymerization progresses, and fewer polymer chain ends are reversibly blocked. The consequence of this is an increase in the reaction rate.
The stable N-oxyl radical having polymerizable double bonds is preferably a compound with the formula I or II or a mixture thereof.
where
R
1
, R
2
, R
5
, R
6
=independently of one another identical or different straight-chain or branched, substituted or unsubstituted alkyl groups having 1 to 32 carbon atoms, it being possible for R
1
and R
2
and, respectively, R
5
and R
6
to form a ring system;
R
7
, R
8
=independently of one another
M
+
=hydrogen ion or an alkali metal ion,
q=an integer from 1 to 10; and
R
9
in formula I=hydrogen or C
1
-C
8
alkyl; and
R′ and R″ in formula II=independently of one another hydrogen or C
1
-C
8
alkyl and n=0, 1, 2 or 3.
Preferably
R
1
, R
2
, R
5
, R
6
and R
9
in formula I=independently of one another identical or different straight-chain or branched, substituted or unsubstituted alkyl groups having 1 to 3 carbon atoms.
With particular preference, R
1
, R
2
, R
5
, R
6
and R
9
in formula I are methyl groups.
With very particular preference, a compound of the formula I is used where R
1
, R
2
, R
5
, R
6
and R
9
=methyl and R
7
and R
8
=—H.
Accordingly, very particular preference is given to the use of 4-methacryloyloxy-2,2,6,6-tetramethylpiperidine-1-oxy (MTEMPO). MTEMPO can be prepared by a method which is described in T. Kurosaki, J. Polym. Sci.: Polym. Chem. Ed. 1972, 10, 3295.
The stable N-oxyl radical having polymerizable double bonds, with particular preference MTEMPO, is generally used in a molar fraction of from 0.05 to 1%, preferably from 0.1 to 1%, with particular preference from 0.2 to 0.4%, based on monomer employed.
Further suitable stable N-oxyl radicals which can be used together with the stable N-oxyl radical having polymerizable double bonds are all those specified in DE-A-19 803 098, which has an earlier priority date than, but was unpublished at the priority date of; the present application.
Further stable N-oxyl radicals which can be used are preferably compounds of the formula III, which are derived from a secondary amine:
where R
10
, R
11
, R
14
and R
15
=identical or different straight-chain or branched, substituted or unsubstituted alkyl groups, and
R
12
and R
13
=identical or different straight-chain or branched, substituted or unsubstituted alkyl groups or R
12
CNCR
13
=part of a cyclic structure with or without anothe

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