CA1059698A - Controlling the mass polymerization of vinyl chloride monomer - Google Patents

Abstract

CONTROLLING THE MASS POLYMERIZATION
OF VINYL CHLORIDE MONOMER
ABSTRACT OF THE DISCLOSURE
Controlling mass polymerization of vinyl chlor-ide monomers, whether alone or with other olefinically unsat-urated monomers, is readily accomplished by adding small amount of nitric oxide to the polymerization.

Description

i9~3 ~ACKG~OUND OF THE INVENTION
Ma~s or bulk polymerlzation o~ vinyl ~hloride mono-mers to provide polyvlnyl chloride or vinyl chloride copolymers is now commercial. Such poly~erizations are conducted in the ~bsence of any di~solving ~olvents in whlch either the vinyl chloride and/or vinyl chloride pol,~mer ls soluble, or ~uspend- ~ing agents such as water, alcohol and the llke. Such polymeri- :
z~tions are in~tlated and carried ~orward by the u~e of free radical formers 8uch as organic peroxy ~nitiator~. It i5 o~ten -~de~ired or nece~sary to control or stop such polymerization reactions within a relatively short time.
Under one set o~ c~ndition~, one may wi~h to stop the polymerization rapidly because of desired phy~ical proper-ties o~ polymers obtained at a given percent of conver~ion, less t~an complete conversion. Uhder other conditions, it is de~irable, even though sub~tantial conversion of monomer to ' polymer is obtained, to co~pletely destroy the cat~ly~t. Of more importance, however, is a safety ~actor. In mass or bulk pol~meri~ation Or vinyl chloride monomers the reactlons ma~
run out o~ control for a variety of reasons and emeræency shortstop condltlons are n~ces~ary due to the large volume o~
material belng polymerized. Such conditions may aris~ as a consequence o~ fallure o~ pump~ or other equipme~t providln~ -coollng means to the reactors, charging exces~ive ~mounts o~
catalyst to the reactor either becau~e of equlpment ~ailure monltoring the equipm~nt, breakdown or operator error æ~d the llke~
Whlle nitr~c oxlde has been propos~d as a ~hortstop ~or certain aqueous su9penslon polymerizations of but~diene 3o polymers and the like, it has not been u~ed com~ercially ~or many reasons. In studie~ related to mas$ or bulk polymeriza-tion of ~inyl chloride monomer~ the critic~l perlod ~here it

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~S~9~ , is most difficult to stop the polymerization reaction 1~ after about 25% conversion o~ monomer to polymer has occurred, at which time there is in the reactor a substantial solid wet mass of material as opposed to a solution or su~pension o~
vinyl chloride polymer in vinyl ch:Loride monomer. To date, none of the conventional shortstops for vinyl chloride poly~
merization have been found to be e~ective as a~ emergency shortstop in mass polymerization systems under out-of-control conditions.
SUMMARX OF THE INVENTION
r ~. . . _ __ ~ ' It has now been discovered that in the mass polymeri-zation o~ vinyl chloride monomer~ alone or mixed with other ~
olefinically unsaturated v~nylidene monomers, for example, ;`
that when the con~ersion o~ monomer to polymer has reached about 30% and the polymerization mass has been converted to a ~et mass with an organo peroxy ~ree radical forming catalyst, that the polymerization reaction mæy be stopped quickly even under emergency and out-o~-control situations wlth small amounts of nitric oxide.
An unexpected advantage o~ the inYention is that an amount o~ nitric oxide les~ than th~t requtred to react with all of the initia~or or catalyst may be added so that when this amount o~ nltric oxide ha~ reacted with ca~alyst ~ree radicals and the nitrlc oxide is all reacted or used up, the polymerization will begin ~g~ln and may be continued to com pletion. The use of nltri~ oxide ha~ another advanta~e in that one i8 not ~dding solid material~ to the ~inyl chlor~de polymer that have to be later remo~ed. Also, the rate of polymerization may be controlled by use o~ small amounts o~
nitric oxide.
D~TAILED DESCRIPTION
The polymers of the pre~ent in~ention are obtained

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1~)59~i~8 by mass polymerization of a vinyl h~lide or vinylidene halide monomer, or mixtures thereo~, with other olefinically unsat urated or vinylidene comonomers. The vinyl or vinylidene hailde monomer~ corre~pond to the ~t;ructural formula ~Y ~.
H2C c\x wherein X is a halogen ~elected from the group consisting o~
chlorine9 bromine or fluorine and Y i~ hydrogen or a halogen the æame as de~lned ~or XO An especi~lly u~eful vinyl halide monomer o~ the above type i8 vinyl ~hloride. The amount of vinyl chloride mon~ner will range irom about 40~ or more~ pre-~era~ly about 55%, to 100% by weight of the total monomer compo~ition.
In addition to the Ylnyl chloride monomer, one or more other polymerizable olefinically un~aturated, vinylidene ,~
comono~er~ pre~erably containing at lea~t one ter~inal methyl-ene group (H2C=C ~) may al80 be emplo~ed ~herewith. U~ually less than about 50~ by welght o~ the~e polymerizable co~ono-mers i~ pre3ent. Copolymerizable co~onomer~ include dIene~ .
o~ 4 to 10 carbon at~s including such con~u~a~ed diene~ a~
butadiene, isoprene and plperylen~;~ ethylldene norbornene and dlcyclopentadlene; ~-olefins such as ethylene, propylene, i~o-butylene, butene-l and 4-methylpentene-1, pre~erably contain-:ing 1 to 8 carbon atoms, vinyl bromide, ~inyhidene chloride, vinyl rluoride; vinyl e~ters such as vlnyl acetate, vinyl laurate and vin~rl chloroacetate, vin~rl aromatics euch as sty- :.rene,~ chlorostyrene? ~methyl 3tyrene, vinrl tolu~ne ~nd ~inyl naphthalene, alkyl v~ nyl ether~ and ketone~ 3uch ~ ethyl vinyl ether, lsobutyl vlnyl ether, N-butyl vinyl ether, chloro- -eth~rl vinyl el;her, methyl Yinyl ketone and ieobutyl vinyl 3o ether; a"B-olefinically un~aturated nitrile~ such as acrylo-nitrile ~ ~thacrylonitrlle; cyanoalkyl acrylates such as a-. .
, -.. ~, ... .. . . ......................... . . .

- . , ~ 69 ~ , cyanomethyl acrylate and the ~ and y-cyanopropyl acryl-ates; olef~nically unsaturated carboxylic acids as acrylic acid, methacrylic acid and the l$kle; ester~ of olefinically unsaturated carboxylic acid~ including ~ ole~inically un-saturated a¢ids such as methyl acrylate, ethyl acrylate where- .
in the alkyl groups contain 1 to 8 rarbon atoms, chloropropyl acrylate, methyl methacrylate, et~yl methacrylate, 2~ethyl-hexyl acrylate, cyclohexyl acrylatls, phenyl æcrylate, glycidyl acrylate, glycidyl methacrylate, ethoxyethyl acrylate, esters of maleic and fumaric acid, amideæ of the a,~ olefinically unsaturated carboxylic acids, and the like; polyfunctional monomers such as methylene bis-acrylamide, ethylene glycol dimethacrylate, diethylene glycol diacrylate, di~nyl benzene and allyl pentaerythritol; bis(~-haloalkyl)alkenyl phosphon-ates such as bis(~-chloroethyl)~lnyl phQsphonate; ~,~-ole~
finically uns~turated ~-alkylol amides or ~ ole~inlcally unsaturated N-alkoxyalkyl amide of the formula CH2=C-C-X~ ( CH2 )n~Rl wherein R is hydrogen or an alkyl group containing ~r~m 1 to

4 carbon atoms, Rl i8 hydrogen or an alkyl group:containing .-.
from 1 to 8 carbon atoms and n is an integer from 1 to 4, ~ncluding N-met4ylol acrylamide, ~-~ethylol methacrylamlde;
amide monomer~ derived ~rom an ~ oleMnically un~aturated carboxylic acid and havln~ th~ 8tructural ~ormula 0 ,R4 C~ =C-C-N R3 wherein R2 i~ hydrogen or an al}url group containing from 1 to 4 carbon atoms and R3 and R4 are hydrogen or a radical con-t~ning ~rom 1 to 12 carbon atoms and more preferably an alkyl

-5-~L~596~8 group containing ~rom 1 to 8 carbon atoffl~ ~ncluding acrylamide and methacrylamide, ~-propyl acrylamid~, diacetone ~crylamlde and the like. Pre~erably, the copolymers have a softening point above the temperature o~ polymerlzation., In carrying out the polymerization reaction there is u~ed the u~ual catalyst or initlator~ which normally are ~ree radical forming ineluding organlc peroxide~ and aliphatic ~-8ZO compounds. Such materials include, ~or exam~le, ~
azodii~obutyronltrile and the like; organic peroxides includ-ing diacyl peroxides ~uch as acetyl pero~ide in dimethyl phth~late, benzoyl peroxide, 2,4-dichlorobenzoyl p~roxlde~
lauroyl peroxide, pelargonyl peroxide; p~roxy~st~r~ ~uch as ester-butyl peroYyacetate, ter-butyl peroxyisobutyrat6, tert-but~l peroxyisobutyrate, tert-butyl peroxgpivalate, tert-butyl -pero~y(2-et~yl;hexanoate), alkyl peroxides ~uch a~ ~,a'-bis-(t-butylperoxy)dl~opropylben~ene~ n-buty~-4,4-bis~tert- :
butylp~ro ~ )vale~ate, di~.tert-~myl pero~lde, dicumyl per~ide, 2,5-dimethyl-2,5-bl~(tert b~tylperoxy)hexyne 3; hydroperoxide~ .
~uch a~ tert-butyl hydroperoxide~ 1,1,3,3-tetrame~hyl butyl hydroperoxlde, cumene hydroperoxide, 2,5-dlmethylhexan0_2,5 dihydroperoxide, dii~opropylbenzene hydropercxide; ketone per~
oxides such a~ met~yl ethyl ketone peroxides~ sul~onyl acyl peroxldes 8uch a3 ~cetyl cyclohexyl ~ul~onyl peroxide; ~cetyl sec-heptylsulfonyl peroxide; peroxy carbonat~s such a~ tert-butylperoxy i80p~pyl carbonate; peroxy dic&rbonate~ such as bis(4-t-butylcyclohexyl3pero~y dicarbonate, dicyclohexyl per-oxydicarbonate, dli~opxopyl perox~dicarbonate; tertiary alkyl perketals ~uch as 2,2-bi~(tert-butylper~xy3bu~n~; mixtures thereor and the like. Fou~d use~ul are lauroyl per~xide, 30 di-(2-~thyl hexyl)peroxydlc~rbonate, dl-ethyl peroxydicarbonate~ -di (n-propyl )pero~rdlcarbonate, dii~opropyl p~roxydicarbonate, di(sec-butyl)p~roxydicarbonAte and acetyl cycloh~x~ne sul~o~lyl _6- :~

1~5~69~ ' peroxide.
The nitric oxide preferably iB sub~tantially free of nltrogen dioxide, i.e., less than about 0,2% and i~ added to the polymerization reactor and quickly and thoroughly mixed throughout the polymeri~atlon ma~s, preferably being inserted at more than one point in the reaction ~ystem. While even a ~ew ppm nitric oxide have an e~ect on the reaction r~te, amounts ~rom abvut 0.005 to 0.3 weight parts per one weight part of free radical cataly~t, or fro~ 2 mill~mol to a mol ratlo o~ about 2 mol~ of nitric oxide per mol of catalyst a~
~n organo pero~y cataly~t will normally be u~ed. Whlle larger amount~ o~ nitric oxlde may be u~ed, they are not nece~sa~y and it is pre~erred that no ~cre than about 2 mole o~ nitric oxide per mol o~ organo-peroxy catalyst be charged. Largar amounts can be addad in an ~mer~sncy.
The present inve~tion ls u~e~ul in any bulk or mas~
polymerlzation ~stem, ~or ex~mple, in pipe, autoclav~ and the l~ke. It has been ~ound to be especially use ~ 1 ln ~ystems where the ~ass polymerizatlon i begun ln one ~t~ge and carried :~
to low conversion generally with high ~g~tatlon, and then tran~ferred to another stage with lo~er agitation, whsrein the mas i~ ~onrerted into a subst~ntially dry state ~nd re-mo~ed ~rom this ~tage in the ~or~ of ~i~yl chloride poly~er particles. 0~ course, there may be more thAn o~e ~tage.
Example~ o~ ~uch ~ystems are found dascribed ln detail in U.S. ~ i Patents 3,522~227, 3~562,237 and 3,687,919. . :::
In e~3~ence, the~ ~othg~8 lie in prep~rin~ m~s .-polymer~ and copolymer~ o~ vinyl chloride ln the ~rm o~ gr~n- .
ule~ or sphere3 b~ polymerlzing the monomer~ in a plurality Q~ 8tage8, gener~lly sequ~ntla~ly, whlch m ~ be und~r sim~lar or varying conditlons of pres~ure, temp~r~ture and cataly3i~
In any e~nt, there 1~ a~ lsa~t a ~lrst stage ~h0re the con- ;
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~sg69~ ' version o~ monomers is conducted to about 7 to 12%, pre~erably with high agitation or agitation of high turbulence, and then ~^
at least one additional stage dur~ng ~lch st~3e the polymeri-zation is conducted generally in the pre8ence of addit;ional and o~ten di~srent type~ catalyst and at lower agitation or absence of high turbulence to complet~on of the reaction. In 3 j562,237 the second stage ls ~ondueted ~n an autoclave pro~ :
vided with a rlbbon blade activator extending hellcally about its axis of rotation, contiguou8 to the wall8 o~ the autoclave and radially 5ps,ced ~rom ~aid a~
~ormally, there i~ no necessity ~or ~topping the polymerization of vinyl chloride in the early stage~ such as pr~or to 12 to 1~% conver~on of monomers to polymer in the ab~ence of some severe mech~ic~l breakdown or overcharge o~ :
cataly8t. ~ore usuall~, the out-o~control r~action ~ill occur in the latter ~tages ~hen the conversion i8 going up ~rcm ~-about 12 to 15% to any sub~equent degree o~ cor~ver~ion up to and including 100% conversion. I~, at any point during this part o~ the react~on, lt i6 necessary to 510w down or stop the pol~rmerization, nitric oxide i~ added at point~ in the ~yste~n to obt~in maximum r~pld mixing and contact as wlll be readily : .
evident to tho~e skilled in the art, depending mos~ly on the type of equipment that i~ being u~ed. To be most Qf`f~3CtiVe, the mass oi the nitric oxide i~ preferably disper~ed through the v~ ctiloride monomer~vin~rl chlo~ide polymer mas~. The ;~
reaction between nitric oxid& and initiator i~ rapld in ~uit-able ~y~tems with efficient d1 stributlon of nitric oxide by adequate point~ of entry ~d/or a~itation. It wlll b~ obv~ous, of course, th~t the ~vai1~bility Or increased agitatioal during the lntroduction o:~ the nltric oxide 7rould be ad~rantageou~.
It i~ pre~rred that æ subatantially o~ygen~rree polymeriz~tion syst~m be used, other than uxygen sourco ~rom ~8~
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~5~69~3 the catalyst which is minimal. Usual precautions in excluding oxygen ~rom such ~inyl chloride polymerizatlon systems are generally adequate to prevent the .~ormation of excess ~mounts o~ nitrogen dioxide which are not desired.
A~ter the polymerization the ~ass may be expelled from the rea~tion system ~nd the wnreacted nitric oxide and ~lnyl chloride vented under proper safety condltions prior to ~urther processing of the vinyl chloride polymer. When minimum amount~ of nitric oxide~ are added to only 810w the polymeri-zation to the desired degree or ~n exact equi~olar a~ount has been added to react wlth the catalyst, the polymerization may be reiniti~ted under controlled condition~ with additional desired ~ree radical catalyst~ to continue the polymerization i~ this 18 desirsd.
Enough nitric oxide may be added to a particular polymerization to produce ~ precalculated ti~e delay ln the polymerization. To determlne th~ amount o~ nitrlc o~ide ne-cessary ~or such delay or retardatlon~ the amount o~ initlator ~hat wlll decompose during the del~y i~ calculat~d, or in other word~, one c~lculat~s the amount of nitr~c ox~de required - i to react qu3ntitatively wlth ~ree radicals rormed during a ~iven p~rlod of ti~e.
~.
A qimple equatio~ bas~d on rir3t order de~ompo~itlon kinetic~ i8 u3ed ~or the~e eimple calculations and one r~quires only the lnltlal catalyst concentration, polymeri~a~ion time, nitric oxlde lnduced delay ti~e snd half-lire o~ the lnitlator in vinyl ehlorid~ at the polymeri~tion te~p~rature. mus, the a~ount u~ed depends upon the inltlator halr;li~e ~nd con-centrat~on, ~he t~mperature, the ti~e to beginnlng o~ the delay in polymeri~tion ~nd length o~ the del~y, E~h ~ol~cule o~
dceompo~ing initiator produces two free r~d~cal~ and, there~ ,~
~ore, two ~ol~ o~ nltric oxide are required ~or co~plete re~c~
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1~5~69~3 ' tlon with one mol o~ cataly~t. For control pu ~ oses of course, much less than two mol~ o~ nitric oxide may be used depend~ng upon the degree of control and t~ml3 del~y deslred.
To demonstr~te th~ proce~s of thi~ invention, poly-merizations are conducted ln 30 ga:Llon ~talnless st~e~, Jacketed pressure ~28~el8 equipped with vertical ~nchor agitator and a brine cooled condenser. A pr~ssure controller regulate~ the brine ~low to the condenser controlling ths internal reactor pres~ure. A temperature rlse in the re~ctor measured at con-~tant pre~sure ~8 used a~ an lndic~tlon o~ the beginning o~ a pre~ure drop. ~ gas chro~stograph i~ used to ~onitor con~er eion in the re~ctor. A small qu~ntity of n-butane, about one `.
part per 100, i~ used as ~ r~Perence materi~
In one ~mbodimen~ o~ the inventlon there wa~ charged to the reactor 100 weight ~arta of ~inyl chloride ~ono~er9 43 ~:
weight p~rts o~ polyvinyl chlorid~ o~ a~out 200 ~icrons di~-meter and ha~ing a vi~co~ity ~ cycldhexane o~ o.g6, 0.07 part of secondary butyl peroxydicarbonate and the system ~luæhed with 1.42 parts o~ n-but~ne. At about 35 minutes ~ter reac-.1 20 tion a~ a t ~ era~ure of 5BC., at which time the co~ver~ion of monomer to pol~mer had reached about 66%, 0.0039 weight part Or ni~ric oxide (o.436 ~ol per m~l of catalyst) ~a~ added ;~
to the polym~rl~atlon reaction and the ~onversion measured at 15 mlnute l~t~rvals thereafter. The pol~erlzation coaYa~ion re~alned con~t~nt at about 6 ~ ~or 1-3/4 hours. APter one hour and ~orty~ minutes, the polymerization react~on beg~n again and w~nt to grea~r than 80~ conv~rsion in le~s than four hours. The r~sultlng poly~ nYl chlorido wae reco~ered and ~r~ed Or mono~er ~ d dried and ~ound to be acc~pt~le quality ~or c~ ercial U80 and had a vlsco~ity o~ abou~ 0.95.
The polymerlzati~n rate b~rore adding shortstop ln thi~ reae tlon ~B 8 ~ p~r hour, a~d a~ter the t~o hours d~l~y the r~te .

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1-~596g~ ' a~ter the reactlon began again was 36% per hour, which ls readily increased ~y ~dding additional initiator to the reac-tion.
In another demonstration of the practice o~ the invention ~n the same equlpment, 100 welght parts o~ vinyl chlorlde mono~er, 1.2 parts o~ sod:~um dodecyl benzene ~ulfonate, 0.05 weight parts o~ secondary butyl peroxyd~carbonate and 1.0 part o* n-butane were ch~rged to the reactor. In thi~ Ex-ample~ 50 pound~ o~ vinyl chlorlde monomer were u~ed. After the polymerization had been under way for 1.5 h~urg, 20.6 millimol~ (0.0027 weight parts or 0.425 mol per mol o~ initia-tor) of nitric oxide W&S charged to the poly~erization directly into the poly~erizer and polymerlzation wa~ ~mmediately stopped and remained ~topped ~or 2 hours. Therearter th~ polymeriza-tion re~umed and reached a percent conv~rsion o~ 66 a~ter about a total o~ 7 hour~. In thi~ example, the nitric oxide was added to the polymerizer v~por spare and ~mmediate st~pp~ge o~ the vinyl chloride maæ polymerization wa~ obser~ad. In addition to the constant conYersion ~igure noted during the delay period9 the stopp ~ e o~ polymerizatlon was also re~lected by the sudden reduction in coollng requ~r~ment and dropplng ~f ; t~mper~ture and pressure i~ide the poly~erizer. The nitri¢
oxide may be add~d below t~e top level o~ the reactlon mass so long ~ precautlon~ are taken to pr~vent blockin~ oP any 25 5uch en~ry point. The nitric oxlde may al~o be ln~ected into the reactor by puttlng th~ nitrl~ oxide in agit~tor se~l ~atèr .~:
and overpressuring the system ~o that th0 ~ater containing ` -the nitric oxide is introduced lnto the rea~tor. ~a~y oth~r entry points to ob~ain rapid mlxing ln the polymerizatio~ mix ~.
~111 b~ obviou~ to thnse skilled in the art.
On a l~rgsr scal~ poly~erizatlon ln ~ co~merclal plant9 i~ the roactlon oP about 20,000 pound~ o~ vinyl chlorlde .

3L~59~

containing about o~o68 weight part of initiator, aft~r th~
v~nyl chloride had polymerlzed 2 hour~ and 15 minutes, which i8 wlthin the tlme range when many out-o~-control re~ction~ have occurred, 165 grams (0.002 weight part per 100 w2ight parts of vinyl chloride monomer, about o.36 mol~ol lnitiator) wa~
added to the reaction. On the addiLtlon, the reactio~ ~topped immediately. Be~ore the addition the ~utocalve ~a~ taking full eooling wa~sr and a~ soon a~ th~ nitric axide ~as added the pres~ure controller cut o~ the coollng ~a~er and was un-able to maint n the pres3ure in the re~ctor ~hich was decre~-ing rapidly. This polymerlzatlon and th~ ~toppin~ ther~o~ ~a~
repeated with only 6B gr~s o~ nltr~c ~x~d~ und again thls amount o~ nitrlc oxid~, 1 weight part p~r 100 ~e~ghg p6rts of inlti~tor, ~ more than snough to co~pl~tely stop th~ re~c tion which did not begln ag~in. The poly~er reco~red ~ro~
these reactor~, although having incr~as~d porosit~ bec~u~e of le~ than essentinlly co~plet~ c~nverslon ~a8 otherwis~ ~atis- ~;
P~ctory ror proc-~sing into usenul articles.

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Claims (16)

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

1. A process for controlling the free radical mass polymerization of monomers comprising vinyl chloride, which comprises adding nitric oxide to said vinyl chloride.

2. A process according to claim 1, wherein said polymerization comprises homopolymerization of vinylchloride.

3. A process according to claim 1, wherein said poly-merization comprises the copolymerization of a monomer com-prising vinylchloride monomer and vinylidene monomer copoly-merizable with said vinyl chloride monomer.

4. A process of claim 1, wherein nitric oxide is added to said monomers after at least 12% of the monomers have been converted to polymer.

5. A process of claim 4, wherein said nitric oxide is added in an amount of about 0.001 to 2 mols per mol of free radical initiator.

6. A process of claim 5, wherein there is at least 55% vinyl chloride with the remainder being one or more other copolymerizable vinylidene monomers copolymerizable with said vinyl chloride and containing at least one terminal methylene group.

7. A process of claim 5, wherein the free radical initiator is an organo peroxy catalyst and the nitric oxide is added after at least 30% of the monomers have been converted to polymer.

8. A process of claim 7, wherein said nitric oxide is added in an amount of about 0.005 to 0.3 weight parts per one weight part of organo peroxy-catalyst.

9, A process of claim 8, wherein the organo-peroxy catalyst is selected from the group consisting of lauryl peroxide, di(2-ethylhexyl)peroxydicarbonate, diethyl peroxydi-carbonate, di(n-propyl)peroxydicarbonate, diisopropyl peroxy-dicarbonate, di(sec-butyl)peroxydicarbonate and acetyl cyclohexane sulfonyl peroxide.

10. A process of claim 9, wherein in the mass poly-merization of the monomers the polymerization is in at least two sequential stages including a first stage in an apparatus effecting a high rate of agitation wherein a conversion of monomers is effected of about 7 to 15% and a second stage in which the polymerization may be carried out to completion controlling the polymerization of said monomers by adding to said second stage, nitric oxide in amount sufficient to stop the polymerization reaction.

11. A process of claim 10, wherein the nitric oxide is added at more than one point to the polymerizing mass and thoroughly mixed therewith.

12. A process of claim 10, wherein the molar excess of nitric oxide to catalyst is added to the polymerization mass.

13. A process for stopping the organo peroxy free radical mass polymerization of vinyl chloride comprising adding nitric oxide substantially free of nitrogen dioxide to said vinyl chloride.

14. A process of claim 13, wherein said nitric oxide is added in an amount of about 0.001 to 2 mols per mole of organo peroxy free radical initiator.

15. A process of claim 14, wherein there is at least 55%
vinyl chloride with the remainder being one or more other vinylidene monomers copolymerizable with said vinyl chloride and containing at least one terminal methylene group.

16. A process of claim 14, wherein the nitric oxide is added after at least 30% of the monomers have been converted to polymer.