2,964,508 United States Patent Office Patented Dec. 13, 1960 2 diiodomethane, iodotribromomethane, chlorotriiodometh 2,964,508 ane, dichlorodiiodomethane, iodotrichloromethane, fluo rotriiodomethane, difluorodiiodomethane, and iodotri PROCESS FOR CONTROLLING THE MOLECU. fluoromethane. LAR WEIGHT OF VENYLDENE CHLORIDE 5 The amount of the perhalomethane that may be em POLYMERS ployed in the process of this invention may be varied Marion R. Rector and William E. Cohrs, Midland, Mich., from 0.05 to 5.0 percent by weight based on the weight assignors to The Dow Chemical Company, Midland, of the monomer. The preferred concentration is from Mich., a corporation of Delaware 1 to 3 percent by weight based on the weight of the No Drawing. 0 monomer. When less than 0.05 percent is used the Filed June 23, 1955, Ser. No. 517,663 amount of molecular weight lowering is not usually great 6 Claims. (C. 260-87.7) enough for practical purposes. When more than 5.0 percent is used no additional benefits are realized. When the compounds are used in these concentrations a sub This invention relates to a process for controlling the 5 stantial lowering of molecular weight of the polymers molecular weight of polymers. More particularly, it re occurs without the necessity of altering polymerization lates to a process for preparing vinylidene chloride poly conditions, and without significant decrease in polymeri mers and copolymers having lower molecular weights zation rate. than similar polymers prepared at the same tempera Any of the known methods of polymerization may be tures but using prior known processes. 20 employed in this process (mass, aqueous emulsion, aque Vinylidene chloride polymers and copolymers of viny ous suspension, or solution), although especially ad lidene chloride with another copolymerizable monomer vantageous results are obtained when the common non such as vinyl chloride have been fabricated into a wide emulsified aqueous suspension method is used. In such variety of useful articles. One of the most common method a water-dispersible granulating agent is usually methods of fabricating such articles is to melt the poly 25 employed and the effectiveness of that agent varies with mer and express it through a suitably shaped die orifice. the temperature. In addition, such methods employ oil The ease with which the molten polymer may be forced soluble catalysts having a relatively narrow temperature through the die orifice is dependent upon the flow vis range of catalytic effectiveness. Thus, the temperature cosity of the molten polymer. In general, the lower the at which a suspension polymerization may be conducted flow viscosity for a given polymer the easier it will be 30 is within a relatively narrow range. Those requirements to force it through the die orifice. For linear polymers are not nearly so rigid when emulsion polymerization the flow viscosity is directly related to molecular weight. techniques are employed, although the process of this in It is likewise true that for linear polymers the physical vention may likewise be used. properties of articles produced from the polymer are The perhalomethane may be added to the polymeriza related to and dependent upon the molecular weight 35 tion charge in any manner, although to secure the most of the polymer. For example, the tensile strength of a uniform results when polymerizing in an aqueous film or filament prepared from a linear polymer will medium it is preferred to add it to the monomer before usually be greater with high molecular weight polymers dispersing the monomer in the aqueous phase. In that than with low molecular weight polymers. Consequently, way the perhalomethane will be evenly distributed a compromise must be found as to the optimum molec 40 throughout monomer droplets. ular weight desired. As a further complication the The perhalomethanes as defined above are effective in molecular weight of a linear polymer is dependent upon producing lower molecular weight vinylidene chloride the particular polymerization conditions under which it polymers and are particularly effective with polymers is made. Thus, the polymerization conditions which are comprising at least 70 percent vinylidene chloride and the most economically favorable will not always pro 45 the remainder as vinyl chloride, and do not appreciably vide a polymer with the optimum molecular weight for alter the rate of polymerization. With polymers con fabrication. In prior methods of controlling the molec taining more than 30 percent vinyl chloride, the rate of ular weight of polymers various compounds having polymerization is retarded. This was most surprising chain transfer activity have been added to the polymeriza in view of the results obtained with perhalomethanes tion charge. Without exception, however, those com 50 containing no bromine or iodine and with compounds pounds having such activity have retarded the rate of such as chloroform or bromoform which contain a hydro polymerization to the extent that they were not practi gen atom. In compounds of the latter two classes there cal for commercial production or had a deleterious ef is either no molecular weight lowering or there is a seri fect on the resulting polymer. ous retardation of the rate of polymerization. The provision of a process for controlling the molec 55 The molecular weight of the vinylidene chloride co ular weight of linear vinylidene chloride polymers is polymers, such as are prepared by the process of this in the principal object of this invention. vention, is indicated by a measurement of the solution It is a further object to provide such a process which viscosity of a 2 percent solution of copolymer in orthodi will not appreciably affect the rate of polymerization. chlorobenzene at 120° C. That method has been em The above and related objects are accomplished by 60 ployed for a long time in the vinylidene chloride copoly means of a process wherein a monomeric ingredient mer art and has been especially valuable in screening comprising at least 70 percent vinylidene chloride is poly large numbers of copolymers to check their relative merized in the presence of a perhalomethane compound molecular weight. The determination of the absolute having the general formula Y-C-X3 wherein Y is a molecular weight of any given copolymer is a difficult halogen atom selected from the group consisting of 65 task and subject to considerable error and such an ab bromine and iodine and each X may be any halogen. As solute molecular weight is important only insofar as it typical examples of useful perhalomethane compounds affects the flow viscosity of the molten polymer. The may be mentioned carbon tetrabromide, bromotrichloro molten flow viscosity or the resistance to flow of a given methane, dibromodichloromethane, chlorotribromo copolymer is the property which determines the ease or methane, bromotrifluoromethane, dibromodifluorometh 70 difficulty of extruding or molding that copolymer. In ane, fluorotribromomethane, bromochlorodifluorometh evaluating results of solution viscosity determinations ane, carbon tetraiodide, bromotriiodomethane, dibromo a minor change in value may indicate a substantial 2,964,508 3 4. change in molecular weight and consequently in flow maintaining them at that temperature until they had viscosity. reached about 80 percent conversion. The polymer was The process of the invention will be more apparent dried and solution viscosities determined with the results from the following illustrative examples in which all parts listed in Table II. - - - are by weight. Table II EXAMPLE 1. Solution Several polymerization charges were prepared by add Agent Percent Wiscosity ing to 160 parts of an aqueous phase containing 0.24 part Agent (cps.) of methyl cellulose (400 cps. grade) an oil phase consist O None---------------------------------------------------- 0.95 ing of 68 parts of vinylidene chloride, 12 parts vinyl chlo Carbon tetrabromide---------------------- 0.05 0.9 ride, 0.3 part lauroyl peroxide and varying amounts of the D0------ 0.0 0.89 Do- 0.20 0.87 perhalomethane compounds of this invention, or of pre Do- 0.50 0.8 viously known chain transfer agents. Polymerization was Do- 1.0 0.72 initiated by warming the charges to 65° C. and maintain D0------ 1.4 0.65 ing them at that temperature for 47 hours, with continu I5 ous agitation to maintain dispersion. The polymer was We claim: isolated by filtration and air dried. The molecular weight 1. A process for controlling the molecular weight of of the polymers was compared by measuring the solution vinylidene chloride polymers comprising the steps of first viscosity of a 2 percent solution of each polymer in o 20 preparing a polymerizable ingredient comprising from 95 dichlorobenzene at 120° C. in an Ostwald viscosimeter. to 99.95 percent by weight of a monomer containing at The results are tabulated in Table I. least 70 percent by weight of vinylidene chloride with the balance vinyl chloride and correspondingly from 5.0 to Table I 0.05 percent by weight of perhalomethane having the Percent Solution 25 general formula Agent Parts Con- Viscosity Agent Wesol (cps.) wherein Y is a halogen atom selected from the group None--------------------------------------------- 87.7 0.92 consisting of bromine and iodine and each X may be any Lauryl mercaptain (for comparison).----- 0.5 79.0 0.8 Carbon tetrachloride (for comparison).--- 1.5 88.0 0.9 30 halogen atom and thereafter dispersing said polymerizable Chlorodibromomethane (for compari ingredient in an aqueous phase containing a water-soluble Son).---------------------------------- 0.5 92.3 0.91 cellulose ether as granulating agent, subjecting the so Bromochloromethane (for comparison)-- 0.5 90.0 0.94 Bromoform (for comparison).----- - - 0.5 90.5 0.92 formed dispersion to thermal and catalytic conditions Carbon tetrabromide...-- 0.4 87.2 0.84 Bromotrichloronethane- 0.4 87.5 0.87 known to induce polymerization. Dibromodichloromethane- 0.4 85.6 0.87 2. The process as claimed in claim 1 wherein the per Fluorotrilbronomethane- 0.5 85.3 0.80 35 halomitehane is carbon tetrabromide, Iodotrichloromethane------------ - - - - 0.5 79.6 0.80 Dibromodifiuoromethane--------------- 5 94.2 0.87 3.