CA1126450A - Method for preparing vinyl chloride polymers - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Vinyl chloride polymers or copolymers having a rela-tively low molecular weight or degree of polymerization are prepared by a method in which an organic compound containing at least one mercapto group and at least one hydroxy or car-boxyl group per molecule is added to a monomer or monomer mixture as a chain transfer agent, and the monomer or monomer mixture is dispersed in an aqueous medium containing at least one water-soluble polymeric substance. The chain transfer agent is added in an amount much less than is conventional, due to its very high activity. The quality of the polymer products is satisfactory with respect to heat stability, occurrence of fish-eyes, particle size distribution and plas-ticizer absorption as well as ease of removing residual monomer.

Description

45~

This invention relates to a method for the preparation of vinyl chloride polymers or copolymers. In particular, the invention relates to an improvement in the method of preparing by emulsion or suspension polymerization vinyl chloride polymers or copolymers having a relatively low ~olecular weight or degree of polymerization.
Low molecular weight polyvinyl chloride resins can be produced by polymerization at relatively high temperatures.
; However, the conventional high-temperature polymerization method is undesirable because of an extended gellation time of the resin, inferior flow of the resin in molding, and increased amounts of residual vinyl chloride monomer remaining absorbed in the resin. In order to avoid the above drawbacks, it has been proposed to add certain chain transfer agents to the polymerization mixture. Chain transfer agents suitable for the purpose include, for example, saturated hydrocarbons, such as n-pentane and n-hexane; saturated or unsaturated chlorinated hydrocarbons,such as carbon tetrachloride, trichloroethylene and and perchloroethylene; aldehydes, such as propionaldehyde and n-butyl aldehyde; and certain mercapto-containing organic compounds, such as dodecyl mercaptan.
The above-mentioned chain transfer agents have their respective defects. That is to say ! with respect to the saturated hydrocarbons, they are usually used in a larse amount, say, about 8 to 10~ by weight of the hydrocarbon based on the ; vinyl chloride monomer in order ~o produce vinyl chloride polymers having an average polymerization degree of as low as about 700 by suspension polymerization at about 60~C. As a result, the hydrocarbons tend to be emitted into the atmosphere from the polymer products which are subjected to pos~-polymeri-~' ~
,, ,1 .

45~1 zation processes or released from the finished resin products during the course of storage, resulting in environmental pollu-tion.
With respect to the saturated or unsaturated chlorinated hydrocarbons, they may be used in a reduced amount, say from 0.7 to 1.0~ by weight based on the vinyl chloride monomer.
However, the prohlems of environmental pollution or detrimental effect on the human health can not be solved due to toxicity inherent in the compounds.
With respect to the aldehyde compounds, they are event-ually decomposed by heat in the course of a polymerization run, and the decomposition products exert a retarding effect on the speed of polymerization and also exhibit a toxic effec~ on the human body, though their amount may be Eurther reduced to 0.2 to 0.5% by weight based on the vinyl chloride monomer.
There are also known several methods in which certain mercapto-containing organic compounds are employed with the purpose of controlling the molecular weight of polyvinyl chloride resins. For example, dodecyl mercaptan is used in the emulsion polymerization of various kinds of vinyl monomers, as disclosed in Japanese Patent Publication No. 50-322~1;
several kinds of alkyl thioglycolates are used in the poly~
merization of vinyl chloride, as disclosed in Japanese Patent Publication No. 49-31746; and several kinds of mercapto-containing organic ~ompounds having 4 or less carbon atoms in a molecule are used in the low-temperature bulk pol~merization of vinyl chloride in a continuous process, as disclosed in German OLS 2046143. These prior art me~hods using the mercapto-containing organic compounds have been found defective due to the relatively large amounts of the compounds required which would eventually lead to retardation of the polymerization speed

- 2 -~1 and coloring of -the resultant polyvinyl chloride resin as well as unpleasant odors inherent in the mercapto-compound.
The present invention seeks to provide an improved method by which vinyl chloride polymers or copolymers having a relatively low polymerization degree can be readily obtained in emulsion or suspension polymerization by the use of chain trans-fer agents w.ithout the disadvantages of the prior art described above.
According to an aspect of the present invention there is provided a method of producing polymers comprising polymer-izing monomers dispersed in an aqueous medium to produce low molecular weigh-t polymers, wherein the monomers comprise more than 50% by weight of vinyl chloride monomers, and the aqueous medium contains, as a dispersing agent, at least one water-soluble polymeric subs-tance and, as a chain transfer agent, at least one organic compound having at least one mercapto group and at least one group selected from the functional groups consisting of: hydroxy and carboxyl groups.
Vinyl chloride polymers obtained by methods in accordance wi-th embodiments of the invention have desirable properties, such as porosity and narrow par-ticle size distribution as well as retaining very small amounts of the residual chain transfer ;~ agent or the decomposition products -thereof in the polymers.
The method is further improved with respect to the properties of the polymer products by the use of very specific suspending agents in limited proportions.
~ he method of the present invention gives several advan-tages over the prior art methods. For example, (1) the amount of the specifi.ed chain transfer agent to be added can be greatly reduced because of its very high activity/ (2) the polymerî~ation ràte is not effected by the use of the chain

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transfer agents, (3) the quality of the polyvinyl chloride resins obtained by the me-thod is desirable because the chain transfer acJent itself or the decomposition products thereof do not remain in the polymer produc-ts after completion of the polymerization ancl -the subsequent processing, (4) -the worka-bility oE the po:Lymer produc-ts in fabrication is very good owing to the well-developed porosity of the polymer particles, and (5) any unreacted monomer or monomers absorbed in the polymer par-ticles can very easily be removed from the polymer products after comple-tion of the polymerization.
Embodiments of the method of the present invention will now be illustrated by way of example in further detail. The organic compounds employed as the chain transfer agent in accordance with the present invention have, as has been described above, at least one mercapto group (-SH) in combination with at least one hydroxy group (-OH) or carboxyl (-COOH) group per molecule, and contain preferably from 2 to 7 carbon atoms or, more preferably from 2 to ~ carbon atoms per molecule.
Such compounds are exemplified by SH-con-taining alcohols, such as 2-mercaptoethanol, thiopropyleneglycol and thioglycerine, - and SH-containlng carboxylic acids, such as thioglycolic acid, thiohydracrylic acid, thiolactic acid and thiomalic acid. Com-pounds containing carbon atoms exceeding 7 per molecule have a lower chain transfer activity and are less valuable.
The amount of the chain transfer agent added to the ` polymerization mixture is preferably within -the range from 0.001 to 0.5% by weight, and more preferably in the range from 0.005 to 0.1~ by weight, based on the weight of monomer or monomers used. The amount of chain transfer agent used largely depends on the polymerization conditions, such as the poly-~"~

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merization temperature, the polymerization degree of the polymer to be obtained, and the like. Mixtures of the SH-containing compounds within the above deEinition may be used, instead of using a sin~le compound.
Excepting the use of the chain transfer agent, the polymerization i-tself is much the same as conventional polymeri-zation of vinyl chloride in an aqueous medium. For example, suitable dispersing agents or the suspending agents are water-soluble polymeric substances known in the prior art, including partially saponified polyvinyl alcohol, cellulose ethers (e.g., methyl cellulose and hydroxypropylmethylcellulose), polyvinyl pyrrolidone, vinyl acetate-maleic anhydride co-polymers, starch and gelatine, all of which may be employed in conjunction with an anionic or nonionic surface active agent. The polymerization initiators suitably employed are also known and can be selec-ted from the oil-soluble free-radical initiators belonging to the organic peroxides, such as diisopropylperoxy dicarbonate, di-2-ethylhexylperoxy dicarbonate, acetylcyclohexylsulfonyl peroxide, t-butylperoxypivalate, benzoyl peroxide and lauroyl peroxide, and :- .
azo compounds, such as azobisisobutyronitrile, azobis-2, 4-di-methylvaleronitrile and azobis-2, 4-dimethoxy-2, 4-dime-thylvaler-onitrile.
Although the method of the present invention is most advantageously applicable to the suspension polymerization of vinyl chloride or a monomer mixture mainly composed of vinyl chloride, it is also applicable to emulsion polymerization.
; The monomers copolymerizable with vinyl chloride in the monomer mixture include vinyl esters, such as vinyl aceta-te, vinyl ethers, acrylic and methacrylic acids, and esters thereof;

maleic and fumaric acids and esters thereof; maleic anhydride, aromatic vinyl compounds; unsaturated ni-trile compounds, such as ,, 6~59 acryloni-trile; vinylidene halides, such as vinylidene fluoride and vinylidene chloride; and olefins, such as ethylene and propylene.
Other conditio.ns of polymerization with respect, for example, to the amount of the polymerization initiators, poly-merization temperature, time of polymerization, and the like are not critical, and can be determined as in the conventional polymerization of vinyl chloride in an aqueous medium.
It has been noted that the dispersion of the monomer or monomers in the aqueous medium some-times became degraded or unstable during the polymerization run, resulting in the pro-duction of inferior polymers with a coarser particle size dis-tribution or increased occurrence oE fish-eyes.
It has been established that the dispersion of the . monomer can be stabilized if the monomer or monomers are dis-; persed in the aqueous medium in the presence of combined dis-` persing agents (i.e., composed of a partially saponified poly-vinyl alcohol and a cellulose ether in a specific proportion), . whereby the polymer particles can be effectively prevented from coarsening,. regardless of the intensity of agitation, the p~
of the aqueous medium and other conditions of polymerization.
Further, the porosity of the polymer particles is improved by use of the combined dispersing agents, bringing about the advan-tages that the removal of the residual monomer from the polymer product is very rapid and the occurrence of fish-eyes in the ~: articles fabricated from the polymer product is greatly reduced.
The proportion of a partially saponified polyvinyl alcohol and a cellulose ether is preferably within the range from 80 to 20~ by weight, and more preferably from 30 to 70~ by weight, of the former and from 20 to 80% by weight, and more E

L5~

preferably from 70 to 30~ by weight, of the latter. When the amoun-t of the cellulose ether i.s increased relative to that of the par-tially saponified polyvinyl alcohol to outside -the above ranges, the particle size distribution oE the polymer product becomes broadened and the occurrence of fish-eyes in the fabri-cated articles is increased. On the contrary, an increased relative amount of the partially saponified polyvinyl alcohol to outside the above ranges leads disadvantageously to the coarser particle size distribution of the polymer product. The amount of the dispersing agent is such that the total of the partially saponified polyvinyl alcohol and the cellulose ether is pre-ferably in the range from 0.01 to 0.5% by weight, and more preferably from 0.02 to 0.2% by weight, based on the monomer or monomers in the polymerization mixture.
: The partially saponified polyvinyl alcohol preferably has a degree of saponification in the range of from 65 to 93%, more preferably from 75 to ~0% and a molecular weight such that its 4~ by weight aqueous solution has a viscosity ranging pre-ferably from 20 to 70 centipoise at 20C. On the other hand, `~ 20 preferably the cellulose ether is selected from methylcellulose, ethylcellulose, hydroxyethylcellulose, hydroxypropylmethyl-cellulose and the like, and its 2~ by weight aqueous solution has a viscosity preferably in the range of from 10 to 100 centi-poise at 20C.
In addition to the above defined partially saponified polyvinyl alcohol and cellulose ether, certain water-soluble polymeric substances, for example, vinyl acetate-maleic anhy-dride copolymers, starch, gelatine, and nonionic or anionic sur-face active agents may be present in the polymerization mix-ture, if desired, in order to promote the dispersion of the monomer or monomers, though the amounts of such auxiliary dispersing agen-ts s~-~ 7 ~
I~A j should be as small as possible.
The following examples further illustrate embodiments of the method of the present invention but do not limit the scope of the inventi.on.
Example 1.
Into a 100-liter capacity stainless steel polymeriza--tion reactor were introduced 60 kg of deionized water, 33 g of a partially saponified polyvinyl alcohol having a degree of saponification of about 80~ and having a viscosity of about 35 centipoise at 20C as measured in a 4% by weight aqueous solution, 12 g of a methylcellulose with a methoxy content of 30% by weight, having a viscosity of about 15 centipoise at 20C as measured in a 2% by weight aqueous solution, 30 kg of vinyl chloride monomer, azobis-2, 4-dimethylvaleronitrile as the polymerization initiator in an amount of 12 or 9.6 g and the various chain transfer agents as indicated in Table 1.
Polymerization was conducted at a temperature of 62 or 57C
under agitation at 300 r.p.m. for the various periods of time as indicated in -the table, during which unreacted monomer was removed, and then the polymer product was recovered by dehydra-tion and drying. The monomer-to-polymer conversion in ~ and the averaye polymerization degree of the polymer pro~ucts are set out in the same table.
The heat stability of the polymer products ob-tained above was tested as follows. A blend of 100 parts of the poly-vinyl chloride resin to be tested, 15 parts of lead stearate, 0.5 part of tribasic lead sulfate and 0.5 part of dibasic lead stearate, all parts being by weight, was milled homogeneously and fabricated into a sheet 2 mm thick by passing through a hot roller at 170C. These sheets were heated in a Geer's oven at ~z~s~

180C till they were blackened. The periods of time for such blackening were recorded in minutes and set out in the table as the heat stability.

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Example 2.
Into the same polymeriza-tion reactor as employed in Example 1 were introduced 60 kg of deionized water, 33 g oE
the same par-tially saponified polyvinyl alcohol as in Example 1, 12 g of the same me-thylcellulose as in Example 1, 24 kg of vinyl chlor.ide monomer, 6 kg of v:inyl acetate monomer, 12.0 g of azobis-2, 4-dimethylvaleronitrile and 2-mercaptoethanol or trichloroethylene as indicated in Table II. Polymerization was conduc-ted at 62C for 8 hours, during which unreacted monomer was removed, and then the vinyl chloride-vinyl acetate copolymer product was recovered by dehydration and drying. The monomer-to-polymer conversion and the average degree of poly-merization of the polymer products are set out in the table.
; These vinyl chloride-vinyl acetate copolymers were subjected to the test for heat stability in the same manner as in Example 1 except that the temperature of the oven was set at 120C instead of 180C. The results of the test are also shown in the table.

Table II
Experiment No.
8 9*
Chain transfer agent (g) 2-Mercaptoethanol Trichloroethylene (17) (600) Conversion, ~ 92 90 Average degree of 690 700 polymeriza-tion Heat stability, min. 75 70 * Control 5~

Example 3.
Into a 100~ ter capacity s-tainless steel polymeri~.a-tion reactor were lntroduced SO kg of deionized water, the mercapto-containin~ organic compound or trichloroethylene as the chain transfer agent and the combined dispersing agents composed of a partially saponified polyvinyl alcohol and a cellulose e-ther, as indicated in Table III. Vinyl chloride monomer or vinyl chloride ~ vinyl acetate monomer mixture and polymerization initiators used are also indicated in the table.
Polymerization wa.s conducted at the polymerization -temperature with the velocity of agitation as set forth in the table.
In the table, the shortened names of compounds are as follows:

PVA-A: Partlally saponified polyvinyl alcohol with the degree o~ saponificati.on 80 ~nd vi~coslty 35 centlpol&e at Z0C a~
measured in a 4~ by wei~ht a~ueou~
80111tiOltl.
PVA-B: Partially sapo~ified polyvlnyl alcohol with the degree o~ ~aponi~icatio~
and visco~ity 30 csntlpoise at 20C a~
mea~ur~d in a 4% by weight ~que~u3 solution.
PVA-C: Partially saponifled p~lyvlnyl alcohol with the dsgre3 oi saponi~ication 80%
and vlsco~ity 50 centipoi~ at 20C a~
measured in a 4% by weight aqueous solution.

~ 12 _ .~.7 s~

o1 Cel-A: Hydroxypropylmethylcellulo~e with ths content of methoxy groupa 30~ by welght, content of hydroxypropoxy groups 10~ by welght and vi~cosity 60 centipoi~e at 20C
a~ mea~ured in a 2% by weight aquaous solution.
Cel-B; ~Iydroxypropylcellulose with the content o~ hydroxypropo~y ~roup~ 65% by weight and vi~co~ty 50 centipoi~e at 2aQC as lo measured in a 2% by wei~ht aqueou~
301uti on.
C~l-C; Hydroxypropylmethylcellulc3~ with th~
content of methox~ group~3 2096 by w~ght, oontent o~ hydroxypropoxy group~ a% by weight and vi 8co~ity 100 o~ntllpol~e at ~0C a~3 measured in a ~% by w01~ht aqueou~
801ution.
IPP: Dii~opropyl p~roxydicarbona~0 P~: t-13utyl pero~yplvala~
: 20 DMVN: Azobis-2 ,, 4 diolethyl Yall3ronitrlle .
~C: Vinyl chlorid~.
YAc: V~nyl ~cetata~

- ].3 i45~

Table III
Tem-pera- Agi-Exp. Monomer Chain Transfer Dispersing Initi- ture, tation, No. (kcJ) agent ( ~ a~ent tg) a-tor(g) C r.p.m.
10 VC 2-Mercapto- PVA-A(10) IPP 61 350 (25) ethanol t8) Cel-A(7.5) (10) 11 VC 2-Mercapto- PVA-A(10) IPP 61 300 (25) ethanol (8) Cel-A(7.5) (10) 12 VC 2-Mercapto- PVA-A(10) IPP 61 270 (25) ethanol (8) Cel-A(7.5) (10) 13 VC 2-Mercapto- PVA-A(10) PV 63 350 (25) ethanol (2) Cel-A(7.5) (10) 14 VC 2-Hydroxy- PVA-C(10) DMVN 61 350 (25) propyl Cel-B(7.5) (10) mercaptan (8) 15 VC Thioglycer- PVA-B(4) DMVN 61 350 (25) ine (8) Cel-C(12) (10) 16 VC 2-Mercapto- PVA-B(7) IPP 55 350 (25) ethanol (15) Cel-A(ll) (10) 17VC (22) 2-mercapto- PVA-A(10) IPP 56 300 VAc (3) ethanol ~15) Cel-A(15) (10) 18VC Trichloro- PVA-A(10) PV 61 350 (25) ethylene(200) Cel-A(7.5) (10) 19VC Dodecyl- PVA-A(10) DMVN 61 350 (25) mercaptan(20) Cel-A(7.5) (10) 20VC 2-Ethylhexyl PVA-A(10) DMVN 60 350 (25~ thioglycolate Cel-A(7.5) (10) (50) 21 VC 2-Mercapto- PVA-B(25) DMVN 61 350 (25) ethanol (8) Cel-A (1) (10) 22VC 2-Mercapto- PVA-A(18) DMVN 61 350 (25) ethanol (8) Cel-A (2) (10) 23VC 2-Mercapto- PVA-A (1) DMVN 61 350 (25) ethanol (8) Cel-A(20) (10) Notes: Experiments 18 to 20 are controls. Experiments 21 to 23 are for demonstrating the effect of the combin~d dispersing agents with the proportions outside the pre-ferred range.

_ 1~ --.~
' The polymer products resul-ting from the above polymer-ization tests were then -tested for average degree of polymer-ization, particle size distribution, occurrence of fish-eyes and deoctyl phthalate (DOP) absorption, and the results are set ou-t in Table IV. Also in the table, the speed of monomer removal from each polymer is shown. The manner of determining each of the properties i.e. occurrence of fish-eyes, DOP
absorption and speed of monomer removal was as follows.

Occurrence of fish-eyes: A mixture of 50 g of the polymer product to be tested, 25 g of DOP, 0.3 g of tribasic lead sulfa-te, 1.0 g of lead stearate, 0.01 g of titanium dioxide and 0.005 g of carbon black was kept standing for about 30 minutes and then milled in a hot roller mill a-t 1~0C
for 7 minutes. The blended mixture was taken out from the roller mill in the form of a sheet 0.2 mm thick, and the number of the transparent particles (fish-eyes) ~as counted over an area of 100 cm2 of the sheet.
DOP absorption: A mixture of 10 g of the polymer product to be tested and 20 g of DOP was, after 1 hour of standing, subjected to centrifugal separation to remove the un~
absorbed DOP and the percentages of the DOP absorbed in the polymer were de-termined by weighing.
Speed of monomer removal: A liter of the polymer slurry after completion of the polymeri~ation _ 15 -~' .

1~;26~5~

was kept at 80C by heating in a flask with agita-tion while nitrogen gas was blown into the slurry at a rate of 0.1 li-ter/minute. Small portions of the polymer slurry were taken at certain time i.ntervals and analyzed for the monomer content to determine the time required for the decrease of the monomer content to a half of the value.

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Claims (22)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of producing polymers comprising poly-merizing monomers dispersed in an aqueous medium to produce low molecular weight polymers, wherein said monomers comprise more than 50% by weight of vinyl chloride monomers, and said aqueous medium contains, as a dispersing agent, at least one water-soluble polymeric substance and, as a chain transfer agent, at least one organia compound haviny at least one mercapto group and at least one group selected from the functional groups consisting of: hydroxy and carboxyl groups.

2. The method of claim 1, wherein said method is selected from the group consisting of: emulsion-polymerization and suspension-polymerization.

3. The method of claim 1, wherein said chain transfer agent comprises an organic compound having at least one mercapto group and at least one hydroxy group per molecule.

4. The method of claim 1, wherein said chain transfer agent comprises an organic compound having at least one mercapto group and at least one carboxyl group per molecule.

5. The method of claim 1, 3 or 4, wherein said chain transfer agent has from 2 to 7 carbon atoms per molecule.

6. The method of claim 1, 3 or 4, wherein said chain transfer agent has 4 carbon atoms per molecule.

7, The method of claim 1, 2 or 3, wherein said chain transfer agent is se.lected from the group consisting of:
2-mercaptoethanol, thiopropyleneglycol and thioglycerine.

8. The method of claim 1, 2 or 4, wherein said chain transfer agent is selected from the group consisting of:
thioglycolic acid, thiohydrocrylic acid, thiolactic acid and thiomalic acid.

9. The method of claim 1, 3 or 4, wherein said chain transfer agent is added to said aqueous medium in an amount ranging from 0.001 to 0.5% by weight relative to the weight of said monomers,

10. The method of claim 1, 3 or 4, wherein said chain transfer agent is added to said aqueous medium in an amount ranging from 0.005% to 0.1% by weight relative to the weight of said monomers.

11. The method of claim 1, wherein said dispersing agent comprises a combination of partially saponified polyvinyl alcohol and a cellulose ether.

12. The method of claim 11, wherein said combination is composed of from 80% to 20% by weight of said partially saponified polyvinyl alcohol and from 20% to 80% by weight of said cellulose ether.

13. The method of claim 11, wherein said combination is composed of from 70% to 30% by weight of said partially saponified polyvinyl alcohol and from 30% to 70% by weight of said cellulose ether.

14. The method of claim 11, 12 or 13, wherein said dispersing agent is added to said aqueous medium in an amount ranging from 0.01% to 0.5% by weight relative to the weight of said monomers.

15. The method of claim 11, 12 or 13, wherein said dispersing agent is added to said aqueous medium in an amount ranging from 0.02% to 0.2% by weight relative to the weight of said monomers.

16. The method of claim 11l 12 or 13, wherein said partially saponified alcohol has a degree of saponification in the range of from 65% to 93%.

17. The method of claim 11, 12 or 13, wherein said partially saponified alcohol has a degree of saponification in the range of from 75% to 90%.

18. The method of claim 11, 12 or 13, wherein said partially saponified alcohol has a molecular weight such that the viscosity of a 4% by weight aqueous solution at 20°C of said partially saponified alcohol is in the range from 20 to 70 centipoise.

19. The method of claim 11, 12 or 13, wherein said cellulose ether is selected from the group consisting of:
methylcellulose, ethylcellulose, hydroxyethylcellulose and hydroxymethylcellulose.

20. The method of claim 11, 12 or 13, wherein said cellulose ether has a molecular weight such that the viscosity of a 2% by weight aqueous solution at 20°C of said cellulose ether is in the range from 10 to 100 centipoise.

21. The method of claim 1, 3 or 4, wherein said monomers are 100% vinyl chloride monomers.

22. The method of claim 1, 3 or 4, wherein said monomers other than vinyl chloride monomers are selected from the group consisting of: vinyl esters, vinyl ethers, acrylic and methacrylic acids and esters thereof, maleic and fumaric acids and esters thereof, maleic anhydride, aromatic vinyl compounds, unsaturated nitrile compounds, vinylidene halides and olefins.