CA1274803A

CA1274803A - Vinyl monomer polymerization in presence of photoinitiator and chemical

Info

Publication number: CA1274803A
Application number: CA000527221A
Authority: CA
Inventors: Ratana Kanluen; Brigitte H. Licht
Original assignee: Hart Chemical Ltd
Current assignee: Huntsman Corp
Priority date: 1987-01-13
Filing date: 1987-01-13
Publication date: 1990-10-02

Abstract

Abstract of the Disclosure Vinyl monomers are polymerized in high yield to water-soluble polymers or copolymers by free radical polymerization of an aqueous reaction mixture containing the vinyl monomer(s), a UV-activated photoinitiator and a heat-activated chemical initiator. The polymerization is initially carried out by exposing the monomer mixture to UV radiation to activate the photoinitiator and then is carried out under the influence of free radicals produced by the chemical initiator.

Description

VINYL MONOMER POLYMERIZATION IN PRESENCE OF
PHOTOINITIATOR AND CHEMICAL INITIATOR

The present invention relates to the formation of solid form water-soluble polymers and copolymers by a combination o~ photochemicaily- (photolysis and photoinitiator) and chemically- (thermal initiator) induced process.
A plurality of procedures are known for the photopolymerization of vinyl monomers by free radical mechanisms. Such procedures involve radiation-induced processes which generate free radicals by khe use of high energy ionizing radiation from radio-active sources, electron beams, X-rays or quartz UV-radiation, as described typically in U.S. Patent No. 3,926,756 and U.K. Patents Nos. 1,279,108 and 1,437,281 and photochemically-induced processes, as described typically in U.K. Patents Nos. 567,778, 1,053,097 and 1,332,247.
More recently, U.K. Patents Nos. 1,310,729 and 1,379,088 have described a combination photochemically-and chemically-induced process. In this prior art, polymerization is carried out at pH values in the range of about 8 to 13, which is unsuitable for many vinyl monomers, such as acrylamide, acrylic-type and cationic monomers. In some instances, for example, in the case of acrylic or maleic alkyl (Cl to C4) esters, the high pH increases the hydrolytic decomposition of the monomers to lower alcohols, which may act as chain transfer agents and result in molecular weight limitations.
The specific examples contained in these prior art patents indicate a rather high maximum temperature of the polymerization mass, namely from about 115 to about 150C. These high temperatures can bring about the risk of volatilization of materials in the solution being polymerized and form toxic vapors as well as promote ~L~27 f~

localized cross-linking through chemical or free-radical routes.
In accordance with the pre~ent invention, a novel method of polymerizing vinyl monomers is provided. The method involves the utilization of two di~ferent types of initiator, namely a photoinitiator capable o~
producing free radicals upon exposure to ultraviolet light and a chemical initiator (or so-called 'lthermal initiator") capable of producing ~ree radical~ when heated above a predetermined temperature. The polymerization process is commenced under the influence of free radical produced by e~posing the monomer mixture containing the photoinitiator to ultraviolet radiation o~ long wavelength, such as that usually obtained from a conventional low-pressurs mercury lamp.
Such polymerization r~action brings the temperature of the monomer mixture to the temperature required or the chemical initiator to dissociate into free radicals.
The polymerization then continues to completion under the influence of the chemically-generated ~ree radicals.
Such use of a combination of photochemical and chemical initiation results in high conversion of monomer to polymer without th~ attendant drawbacks of the prior art.
In accordance with the pre~ent invention, there~ore, there is provided a method of polymerizing at least one vinyl monomer to a particulate water-soluble substantially linear polymer or copolymer which comprise~ a plurality o~ 8tep8. An aqueous monomer solution mixture ~irst is formed comprising vinyl monomer(s) polymerizable to the water-soluble polymer or copolymer, an ultraviolet-decomposable free radical initiator capable of producing ~ree radicals upon exposure to ultraviolet radiation and a chemical free radical initiator capahle of producing free radicals upon heating above a predetermined temperature.

Polymerization of the monomer mixture then is initiated by irradiating the monomer mixture with long wave ultraviolet radiation in the wavelength range from about 300 to about 400 nm. The pol~merization is permitted to be effected while the monomer mixture increases in temperature to the temperature at which the chemical initiator is activated. The irradiation is discontinued and the pol~merization then i~ effected above the predetermined temperature until complete.
The procsss of the invention allows full control over the rate of poly~erization which~ in turn, provides the advantage of carrying out the polymerization of highly concentrated monomex solution, while maintaining the maximum temperature of polymerization in th~ range of about 60 to about 105C, which substantially prevents any localized cross-linking and further minimizes the risk of generating volatile toxic material promoted by high temperature.
A further advantage of the proce~s of the present invention i that it i8 efficient, industrially simple and can be carried out batch-wise in shallow trays on a continuous basis. In such mode o~ operation, the monomer solution to be polymerized contained in a shallow tray i~ brought under the W light source and held there until the temperature of the system reaches the level at which the chemical initiator dissociates, that tray then is removed from below the W light source and a new tray is moved into place for irradiation.
One important feature of the process of the invention is that the pH of the monomer ~olution is in the range of about 3 to about 9, so that the process is suitable for a whole range o~ vinyl monomer~, including nonionic, anionic and cationic vinyl monomer3.
The present invention provides a process for the manufacture of water-soluble polymers and aopolymers of vinyl monomers or mixtures of such monomers. The vinyl 3a monomers with which the presen~ invention i8 concerned pre~erably have the formula:

R Q
H2C = 1 - C - Y
wherein:
R represents hydrogen, a methyl group or an ethyl group; and Y represents -NH2, -OH, ~
R ~
-OC2H4 - N, C2H4 - R2 ~' R2 \ R3 / 1 ~Rl - NH(CH2)3 - N \ R2 -OC2H4 - NH - C~ R2 Rl - NH - ~ - CH2SO3H or its alkali metal salts in which Rl, R2 and R3 individually represent alkyl groups containing from 1 to 4 carbon atoms, represents NH463 or an alkali metal cation, and represents an anion.
Using the process of the present invention, such vinyl monomers may be polymerized to a wide range of molecular weight, usually in the range of about 50,000 to about 15,000,000.
The present invention involves the use of both photoinitiators and chemical initiators. Photo-initiation is effected by exposing the monomer mixture to long wave ultravio]et radiation, generally about 300 to about 400 nm, usually at a radiation intensity in the range of about 1000 to about 4000 microwatts/cm2.
The amount of photoinitiator employed in the monomer mixture may vary widely, depending on the activity of the initiator and the intensity of the radiation which is applied. The amount of photoinitiator may range from about 40 to about 200 ppm, based on the weight of monomer. Any known photoinitiator may be employed, for example, benzophenone, benzoin and its alkyl ethers or other carbonyl-containing compounds.
The polymerization under the influence of the free radicals produced by the photoinitiator is exothermic, so that the temperature of the reaction mixture rises towards the temperature at which the chemical initiator dissociates to free radicals. The dissociation temperature depends on the nature of the chemical , ~ .

3tP~.~

initiator and preferably is in the range o~ about 35 to about 70C.
The chemical free-radical initia~ors may be selected from compounds known to genera~e ~r~e radicals upon heating, including azo-type compounds, persulfates and organic peroxides. Specific examples o~ azo-type compounds which are effective at temperatures of about 35 to 70C are 2,2~-azobis(2-amidinopropane) dihydrochloride, azobisisobutyronitrile, 2,2'-azobis-~2-methylpropionitrile) and 4,4'-azobi~;(4-cyano-pentanoic acid). Such chemical ~ree~radical initiators may be employed in amounts from about 50 to about 400 ppm, based on the weight of monomer.
Chain transfer agents also may be pre~ent and usually are employed to control molecular weight.
Examples of suitable materials are formic acid, alkali metal salts thereof, secondary alcohols, such as isopropanol, and water-soluble mercaptansO When pre~ent, such chain transfer agents are employed in amounts up to about 25,000 ppm, based on the weight of the monomer, depending on the effectiveness of the chain transfer agent and the required polymer molecular weight.
The polymer~ which are obtained from the polymerization process are homogeneous, ~ubstantially linear, of a wide range of molecular weight, usually ~rom about 50,000 to about 15,000,000, and water-solubl~. The polymerization proce ~; is very efficient and produces pol~mer in very high yield, over 99%.
The ~ree-radical polymerization reaction~ are exothermic which, at high monomer concentrations, can lead to a sub3tantial increa~e in temperature, which, in turn, leads ~o an increase in reaction rates. To produce the desired polymer~ and to avoid undesirable 7~:~J'~'~

molecular weight distribution, control of these ef~ects is reguired.
The rate and degree of polymer~zation also depend on a number of other factors, including monomer concentration, p~ of the reaction medium, concentration of initiators, the presence of any chain trans~er agent, thickness oP polymerization mass and W radiation intensity.
The polymerization process typically iB effected in a batch operation, although continuous operation also is possible, as de~cribed above. In the batch proces , an aqueous solution of the monomer is position~d in the reaction vessel to a depth, which may vary from a few millimeters to about 10 cms. The concentration of monomer in the solution also may vary widely, g~nerally from about 30 to about 70 wt.%, depending on the chemical nature of the vinyl monomer and the required molecular w~ight o~ polymer.
The invention is illustrated further by the following Examples:
Exam~
Thi~ Example illustrates the polymerization of acrylamide using photochemical and chemical initiators.
A 40~ ~w/w) aqueous acrylamide solution at pH 5.0 was transferred into a polyethylene tray. The depth of the monomer solution wa 70 mm. The monomer solution was deaerated with nitrogen To this solution were added 1.15% of isopropyl alcohol (chain transP~r agent), 135 ppm of benzoin methyl ether (photoinitiator) and 160 ppm of 2,2'-azobi~(2-amidinopropane) dihydrochloride (chemical initiator), all based on the wei~ht of monomer.
The monomer mixture then was irradiated with ultraviolet light from a low pres~ure mercury lamp ~wavel~ngth 365 nm) at a radiation 1QVe1 0~

~27~?3~ ~3 approximately 2500 microwatts/cm2. The radiation was continued until the temperature of the polymerization mass had reached 42c. The radiation thereupon was terminated and the polymerization wa~ allowed to conti~ue under the influence of ~ree radicals produced by the chemical (thermal~ initiator. The product was a firm rubbery gel and was obtained in a yield exceeding 9g% .

D~

The product was permitted to cool to room temperature, before being particulated and dried in a f]uid bed dryer at 40C for two hours.
The polymer so produced was completely water-soluble, i.e. no insoluble gel was obtained. The Brookfield viscosity of an 0.5% aqueous polymer solution was 60 cps (spindle # 1, at 12 rpm).
Example 2 This Example illustrates the synthesis of anionic copolymers of vinyl monomers using the procedure of Example 1.
(a) The procedure of Example l was repeated for an aqueous monomer solution containing 42.5 wt.% of a mixture of 70 mole % acrylamide and 30 mole % acrylic acid, adjusted to pH 5.5 by the addition of sodium hydroxide and cooled to 10C. To this solution, 60 ppm of sodium formate, 135 ppm of benzoin methyl ether and 185 ppm of 2,2'-azobis(2-amidinopropane) dihydrochloride were added. In this instance, UV irradiation was

2~ continued until the temperature reached 45C.
The copolymer so produced was completely water-soluble. The Brookfield viscosity of an 0.1% aqueous solution was 850 cps (spindle # 2, at 12 rpm).
(b) The procedure of Example 1 was repeated for an aqueous monomer solution containing 55 wt.% of a mixture of 80 mole % acrylamide and 20 mole %
2-acrylamido-2-methylpropane sulfonic acid which was ` adjusted to pH 9.0 using sodium hydroxide and to which ? was subsequently added 50 ppm of sodium formate, 45 ppm of 2,2-dimethoxy-2-phenylacetophenone and 120 ppm of 2,2'-azobis(2-amidinopropane) dihydrochloride. In this instance, UV radiation was continued until the temperature reached 45C.
The copolymer so produced was readily soluble in water and an 0.1~ solution of the copolymer had a Brook~ield viscosity of 450 cps (spindle # l, at 6 rpm).

Example 3 This Example illustrates the synthesis o~ cationic copolymers of vinyl monomers using the procedure of Example 1.
The pxocedure of Example 1 was repeated for an aqueous monomer solution containing 40.O wt.~ of a mixture of 90 mole % acrylamide and 10 mole % of quaternized dimethylaminoethylmethylmethacrylate: the pH
was adjusted to 4.0 by the addition of acetic acid. To this solution~ were added 150 ppm of 2,2-azobis(2-amldinopropane) dihydrochloride and 100 ppm of 2,2-dimethoxy-2-diphenylacetophenone. This monomer mixture was irradiated with ultraviolet light until the temperature of the polymerization mass reached 50C.
The copolymer so obtained was completely water-soluble and had Brookfield viscosity of an 0.5 polymer solution of 1750 cps (spindle # 2, at 6 rpm).
In summary of this disclosure, the present invention provides a novel method of producing water-soluble polymers and copol~mers of vinyl monomers by free radical polymerization, using two different types o~ free radical initiator which are sequentially activated. Modifications are possible within the scope of the invention.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of polymerizing at least one vinyl monomer to a particulate water-soluble substantially linear polymer or copolymer, which comprises:
forming an aqueous monomer solution mixture having a pH of about 3 to about 9 comprising at least one vinyl monomer polymerizable to said water-soluble polymer or copolymer, an ultraviolet-decomposable free radical initiator capable of producing free radicals upon exposure to ultraviolet radiation and a chemical free radical initiator capable of producing free radicals upon heating above a predetermined temperature, initiating polymerization of said at least one monomer by irradiating said monomer mixture with long wave ultraviolet radiation in the wavelength range from about 300 to about 400 nm, permitting said polymerization to be effected while said monomer mixture increases in temperature to said predetermined temperature, so as to activate said chemical initiator, discontinuing said irradiation and permitting further polymerization of said at least one monomer to effected above said predetermined temperature until complete while maintaining the maximum temperature of said monomer mixture in the range of about 60° to about 105°C, and particulating the polymer so produced.

2. The method of claim 1 wherein said vinyl monomer has the formula:
H2C = ? - ? - Y

wherein:
R represents hydrogen, a methyl group or an ethyl group; and Y represents -NH2, -OH, -O?M?, -OC2H4 - , -OC2H4 - . X?, -NH(CH2)3 - . X?, -OC2H4 - NH - , - NH - - CH2SO3H or its alkali metal salts in which R1, R2 and R3 individually represent alkyl groups containing from 1 to 4 carbon atoms, M?
represents NH4? or an alkali metal cation, and X?
represent an anion.

3. The method of claim 2 wherein said ultraviolet radiation is applied to said monomer mixture at an applied radiation intensity of about 1000 to about 4000 microwatts/cm2.

4. The method of claim 3 wherein said photoinitiator is used in an amount of about 40 to about 200 ppm, based on the weight of monomer.

5. The method of claim 4 wherein said predetermined temperature of dissociation of said chemical initiator is about 35° to about 70°C.

6. The method of claim 5 wherein said chemical initiator is used in an amount of about 50 to about 400 ppm, based on the weight of monomer.

7. The method of claim 6 wherein said monomer mixture also contains at least one chain transfer agent.

8. The method of claim 7 wherein said chain transfer agent is present in an amount up to about 25,000 ppm, based on the weight of monomer.

9. The method of claim 8 wherein said monomer(s) is present in the monomer mixture in a concentration of about 30 to about 70 wt.%.

10. The method of claim 9 wherein the polymerization process is effected in a batch operation using a monomer mixture having a depth of from a few millimeters to about 10 cms.

11. The method of claim 9 wherein the polymerization is effected in a continuous operation in which successive batches of monomer mixture contained in shallow trays to a depth of from a few millimeters to about 10 cms are exposed to ultraviolet radiation until said predetermined temperature is reached and the tray is removed from said exposure to be replaced by the next tray.

12. The method of claim 1 effected to produce a vinyl polymer or copolymer of molecular weight in the range of about 50,000 to about 15,000,000.