GB1596689A

GB1596689A - Preparation of polyvinyl chloride resins

Info

Publication number: GB1596689A
Application number: GB6777/78A
Authority: GB
Original assignee: Shin Etsu Chemical Co Ltd
Current assignee: Shin Etsu Chemical Co Ltd
Priority date: 1977-02-24
Filing date: 1978-02-21
Publication date: 1981-08-26
Also published as: SE440784B; SU841592A3; IT7820524A0; SE7801919L; BE864149A; HU178740B; NL181659B; FR2381792B1; DE2807180A1; PT67691A; JPS5533804B2; SE440784C; DK80978A; NL7802025A; NO780590L; PT67691B; NO152754B; JPS53104684A; BR7801118A; IT1094154B

Description

(54) PREPARATION OF POLYVINYL CHLORIDE RESINS (71) We, SHIN-ETSU CHEMICAL CO. LTD., a Japanese Company, of 6-1 Otemachi 2-chome, Chiyoda-ku, Tokyo, Japan, do hereby declare the invention, for which we pray that a patent may be granted to and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to a method for the preparation of vinyl chloride polymers and, in particular, to a method for the preparation of vinyl chloride polymers with relatively low degree of polymerization.

It is well known in the art to introduce a chain transfer agent into the polymerization mixture with the purpose to obtain a vinyl chloride polymer with relatively low degree of polymerization. Several classes of compounds are known as the chain transfer agent suitable for the purpose including, for example, (1) saturated hydrocarbon compounds such as n-pentane and n-hexane, (2) saturated and unsaturated chlorinated hydrocarbon compounds such as carbon tetrachloride, trichloroethylene and perchloroethylene, and (3) aldehyde compounds such as propionaldehyde and n-butyraldehyde. These chain transfer agents, however, have their respective defects. For example, the saturated hydrocarbon compounds as a chain transfer agent must be introduced into the polymerization mixture in an amount as large as 8 to 10 % by weight based on the amount of the vinyl chloride monomer when the polymerization is conducted to obtain vinyl chloride polymers with an average degree of polymerization of about 700 in an aqueous suspension at a polymerization temperature of about 60 "C. In this case, a considerable amount of the chain transfer agent is emitted to the atmosphere during the course of polymerization and the saturated hydrocarbon compound remaining absorbed in the polyvinyl chloride product gradually dissipates during storage and processing of the polymer, thus bringing about undesirable pollution.

The amount of the chain transfer agent can be reduced to about 0.7 to 1.0 % by weight when the polymerization is carried out under the same conditions as above but with a saturated or unsaturated chlorinated hydrocarbon compound as the chain transfer agent.

These chlorinated hydrocarbon compounds are, however, inherently toxic. The aldehyde compounds can be effective in a reduced amount of 0.2 % to 0.5 % by weight but they are disadvantageous irrespective of their toxicity, owing to the retarding effect on the polymerization velocity induced by the thermal decomposition products of the aldehyde produced in the course of polymerization.

British Patent Specification No. 579,353 discloses various mercaptans as chain transfer agents. British Patent Specification No. 1,506,354 discloses and claims a method for polymerizing vinyl chloride monomer, or a monomer mixture comprising more than 50 % by weight of vinyl chloride, dispersed in an aqueous medium, in which the aqueous medium contains, as a dispersing agent, at least one water-soluble polymeric substance and at least one organic compound having at least one mercapto group and at least one hydroxy or carboxyl group in the molecule. as a chain transfer agent.

The method of British Patent Specification No. 1,506,354 has the advantages (1) that, owing to the activity of the chain transfer agent, the amount thereof can be low; (2) that the rate of polymerization is not retarded; (3) that the chain transfer agent is not absorbed in, and thereby does not affect the quality of, the vinyl chloride copolymer; (4) that the polymer products have good porosity, giving good workability; and (5) that the unreacted monomer is easily separated and removed from the polymer after completion of the polymerization reaction. Unfortunately, it has been found that the dispersion of the monomer in the aqueous medium is unsatisfactory or unstable, leading to polymer products with a coarse particle size distribution. This is especially serious when the amount of vinyl chloride monomer charged into the polymerization reactor is increased, in order to obtain higher productivity.

According to the present invention, in an improvement of the method claimed in British Patent Specification No. 1,506,354 for the suspension polymerization of vinyl chloride monomer, or a monomer mixture comprising at least 50 % by weight of vinyl chloride, at least 50% by weight of the organic compound having at least one mercapto group and at least one hydroxy or carboxyl group is introduced into the polymerization mixture while the monomer conversion is within the range from 1 to 30% by weight. This method permits a relatively low degree of polymerisation by suspension polymerisation in an aqueous medium while avoiding or reducing the above described problems in the prior art, especially with respect to the particle size distribution.

It is possible to conduct the method while maintaining the suspension of the monomer in the aqueous medium as satisfactorily as in the absence of the chain transfer agent. Thus coarsening of the particle size distribution can be inhibited or prevented without having to make special adjustment of the intensity of agitation of the polymerisation mixture or the pH value of the aqueous medium. Moreover, the finished articles manufactured with the vinyl chloride polymers prepared by the inventive method have good heat stability.

In one method. all the transfer agent is added while the conversion is within the stated range. In another method. it is found particularly advantageous to introduce the chain transfer agent into the polymerisation mixture divided into two stages, i.e. within a period before the monomer conversion reaches 1% by weight and within a period when the monomer conversion is in the range of 1 to 30C/c by weight, the amount of the chain transfer agent added at the former stage being 1 to 50% by weight of the total amount of the chain transfer agent. This two-stage addition generally results in better particle size distribution of the polymer products even when the amount of the vinyl chloride monomer charged into the polymerisation reactor is increased.

The organic compound used as the chain transfer agent in the present invention is, as defined above, a hydrophilic organic compound having, in a molecule, at least one mercapto group (-SH) and at least one hydroxy group (-OH) or carboxyl group (-COOH) simultaneously. Among the compounds falling within the above definition. those compounds having 2 to 7 carbon atoms, preferably 2 to 4 carbon atoms, in a molecule are especially suitable for the purpose. as exemplified by thioalcohols such as 2-mercaptoethanol, 2-mercaptopropanol, 2-hydroxypropylmercaptan and thioglycerin and mercaptocontaining carboxylic acids such as thioglycolic acid, 3-mercaptopropanoic acid, thiolactic acid and thiomalic acid. Those compounds having more than 7 carbon atoms in a molecule are less preferable because of their lower activity as a chain transfer agent.

The amount of the chain transfer agent to be added to the polymerization mixture is usually in the range from 0.001 to 0.5, or preferably from 0.005 to 0.3, % by weight based on the amount of the monomer or monomer mixture in the polymerization mixture. The actual amount used will depend on the polymerization conditions, e.g. the polymerization temperature. and the desired properties of the polymer product, e.g. the average degree of polymerization. If desired, a mixture of two or more chain transfer agents may be used in the method of the invention.

If all the chain transfer agent is introduced into the aqueous polymerization mixture when the monomer conversion is lower than 1 Xc by weight. no beneficial effect on particle size distribution in the product can be obtained, because the suspended monomer droplets have not been sufficiently stabilised at the time of introduction. Conversely, if the chain transfer agent is introduced too late, i.e. after the monomer conversion has exceeded 3()r; by weight. it is difficult to obtain a polymer product with the desired low average degree of polymerization because the chance for the introduced compound to act as a chain transfer agent has largely been lost. Preferably. at least 50% by weight of the chain transfer agent is introduced into the polymerization mixture while the monomer conversion is from 2 to 20% by weight.

The mode of introduction of the chain transfer agent is not critical. It may be added all at once, continuously or in several portions.

Although some of the chain transfer agent may be added to the polymerization mixture before the monomer conversion reaches 1C/c by weight. e.g. at the beginning of the polymerization reaction, it is essential that at least 50% by weight of the total amount of the chain transfer agent is introduced into the polymerization mixture during the period when the monomer conversion is in the specified range of 1 to 30% by weight.

This method of divided addition of the chain transfer agent, i.e. addition of 1 to 50% by weight of the chain transfer agent before the monomer conversion reaches 1% by weight or, preferably, at or before the beginning of the polymerisation and the remainder when the monomer conversion is in the range from 1 to 30% by weight resulted unexpectedly in the still smaller amount of the residual monomer in the polymer products. Such divided addition of the chain transfer agent is especially advantageous in improving the particle size distribution when an increase is desired in the amount of the vinyl chloride monomer charged into the polymerisation reactor. In this case, however, it should be kept in mind that too small an amount of the chain transfer agent added at an early stage, i.e. before 1% monomer conversion, will not produce such a desirable effect while larger amounts of the chain transfer agent added at this early stage involve the danger of an adverse effect on the particle size distribution. It is therefore recommended that the amount of the chain transfer agent added at this early stage is in the range from 0.0005 to 0.028% by weight based on the amount of the monomer or monomer mixture, the remainder of the chain transfer agent being added at the later stage, i.e. when the monomer conversion is in the range of 1 to 30%.

The water-soluble polymeric substance used in the method of the invention as a suspending agent may be for example, partially saponified polyvinyl alcohol, methylcellulose, polyvinyl pyrrolidone, vinyl acetate-maleic anhydride copolymers, starch and gelatine, optionally in combination with anionic or nonionic surface active agents. The polymerization reaction may be conducted in conventional manner using a polymerization initiator which is usually a monomer-soluble one exemplified by organic peroxides such as diisopropylperoxy dicarbonate, acetylcyclohexylsulfonyl peroxide, tert-butylperoxy pivalate and lauroyl peroxide, and azo compounds such as azobisisobutyronitrile, a,a'-azobis2,4-dimethylvaleronitrile and ara'-azobis-4-methoxy-2,4-dimethylvaleronitrile.

The method of the present invention may be used in the suspension polymerization of a mixture of monomers comprising at least 50C/c by weight of vinyl chloride. Comonomers which can be copolymerised with vinyl chloride to give a copolymer include vinyl esters such as vinyl acetate, vinyl ethers, acrylic acid, methacrylic acid or esters thereof, maleic acid. fumaric acid and anhydride or esters thereof, aromatic vinyl compounds such as styrene, unsaturated nitrile compounds such as acrylonitrile, vinylidene halides such as vinylidene fluoride and vinylidene chloride, and olefins such as ethylene and propylene.

The following Examples illustrate the method of the present invention in further detail, or are comparative. All percentages and parts are by weight. The procedures for the determination of the average molecular weight, the amount of the residual vinyl chloride monomer the dioctyl phthalate absorption and the heat stability were as follows.

Average molecular weight: by the method as specified in JIS K6721.

Residual vinyl chloride monomer: gas chromatographic analysis for the solution of tg of the polymer in 25 ml of tetrahvdrofuran.

Dioctyl phthalate absorption: 20 g of dioctyl phthalate was added to 10 g of the vinyl chloride polymer obtained in the example to form a blend which was kept standing at room temperature for 1 hour followed by centrifugal separation to remove the excess of dioctyl phthalate remaining unabsorbed in the polymer. The dioctyl phthalate absorbed in the polymer thus determined was expressed in % by weight based on the amount of the polymer.

Heat stability: a blend of 100 g of the vinyl chloride polymer obtained in the example, 0.5 g of an organic mercaptide (TUS-8831. product of Nitto Chemical Co.. Japan) and 0.5 g of stearic acid was milled for 5 minutes in a roller blender with the temperature of the rolls at 1600C. and shaped into a sheet of ().8 mm thick. The sheet thus prepared was kept in an oven at 1700C for 40 minutes and the appearance of the sample taken out of the oven was examined visually, the results being recorded in 5 grades as follows.

A: the same as before heating (even after extension of the heating time to 60 minutes); B: slightly discoloured; C: discoloured to brown; D: discoloured to dark brown: E: completely blackened.

In the Tables given in Examples the abbreviations DMVN, PV and LPO are"for ct,a'-azobis-2,4-dimethylvaleronitrile, tert-butyl peroxypivalate and lauroyl peroxide, respectively. Particle size distribution data were obtained using Japanese Industrial Standard mesh sizes.

Examples 1 to 10 Into a 50 litre capacity stainless steel polymerisation reactor were introduced 30 kg of deionized water. 20 g of partially saponified polyvinyl alcohol, 15 kg of vinyl chloride monomer and a polymerisation initiator as indicated in Table 1 below in an amount also as indicated and the polymerisation reaction was started by elevating the temperature. At the time when the monomer conversion reached the percentage as indicated in the Table, a mercapto-containing organic compound as indicated in the Table was introduced into the polymerisation mixture and the polymerisation was continued at the temperature given in the Table. After the overall time of polymerisation as indicated in the Table, the polymerisation reaction was stopped and, after removal of the unreacted vinyl chloride monomer, the resultant polymer in the aqueous slurry was dehydrated and dried into a finished polyvinyl chloride product.

The polyvinyl chloride product thus obtained was examined for the average degree of polymerisation, particle size distribution, amount of the residual vinyl chloride monomer, dioctyl phthalate absorption and heat stability to give the results as set out in the Table.

Examples 9 and 10 in Table 1 were undertaken for comparative purpose in which 10 g of 2-mercaptoethanol was introduced into the polymerisation mixture either at the time when the monomer conversion was about 50% (Example 9) or before the initiation of the polymerisation reaction as introduced together with the deionized water, partially saponified polyvinyl alcohol, vinyl chloride monomer and polymerisation initiator (Example 10). The other conditions for these experiments were as shown in the Table and the results of the tests undertaken of the polymer products obtained in these comparative experiments were as set out in the Table.

TABLE I Example 1 2 3 4 5 6 7 8 9 10 Chain transfer (a) (b) (c) (d) (a) (a) (b) (b) (a) (a) agent, (g) 3.8 4.5 4.0 10.0 10.0 10.0 12.0 2.0 10.0 10.0 Initiator (g) DMVN PV DMVN DMVN DMVN DMVN DMVN LPO DMVN DMVN 4.5 5.0 4.5 4.5 4.5 4.5 4.5 17.0 5.0 5.0 Polymerization 62 62 62 58 58 58 56 64 58 58 temperature ( C) % Monomer conversion when chain transfer 17 12 15 8 4 20 10 12 50 0 agent was added (hrs. from the begin- (1.0) (1.0) (1.0) (1.0) (0.5) (2.0) (1.5) (1.0) (4.0) (0) ning of polymerization) Overall polymeriza- 8 hrs. 8 hrs. 8 hrs. 9 hrs. 9 hrs. 9 hrs. 9 hrs. 8 hrs. 8 hrs. 9 hrs. tion time (hrs.) 45 mins. 30 mins. 10 mins. 40 mins. 30 mins. 30 mins.

Average degree of 680 700 710 720 680 700 690 720 830 690 polymerization Particle size distribu- 60 mesh 100.0 100.0 100.0 100.0 98.8 100.0 100.0 100.0 100.0 60.5 tion, % passed through screen of 200 mesh 1.5 2.0 1.2 2.5 3.9 1.5 1.0 0.9 2.5 2.2 Residual monomer in the 15 13 14 10 9 9 10 30 42 50 polymer (p.p.m.) DOP absorption (%) 17.5 16.5 17.0 18.5 18.0 19.0 19.5 17.0 16.2 19.5 Heat stability B A C B B C B A D D Chain transfer agent: (a) 2-mercaptoethanol; (b) 3-mercaptopropanol; (c) thioglycolic acid; (d) 2-mercaptopropanol.

Examples 11 to 13 Into the same stainless steel polymerization reactor as used in Examples 1 to 10 were introduced 30 kg of deionized water, 20 g of partially saponified polyvinyl alcohol, 15 kg of vinyl chloride monomer and a,a'-azobis-2,4-dimethylvaleronitrile in an amount as indicated in Table II below and the polymerization reactor was started by elevating the temperature. In the course of polymerization, 2-mercaptoethanol in an amount as indicated in the Table was added to the polymerization mixture at the time when the monomer conversion was in the range as indicated in the table and the polymerization reaction was continued at a temperature of 62 "C or 58 "C.

After an overall polymerization time of 8 hours or 9 hours from the beginning of the polymerization, the polymerization reaction was stopped and, after removal of the unreacted vinyl chloride monomer, the resultant polymer in the aqueous slurry was dehydrated and dried to give the finished polyvinyl chloride product. The properties of the polymer products thus obtained were as set out in the Table.

Example 10, in which 2-mercaptoethanol was introduced into the polymerization reactor together with deionized water, partially saponified polyvinyl alcohol and vinyl chloride monomer is reproduced in Table II for comparative purposes. In Example 13, also comparative. trichloroethylene was used as the chain transfer agent in place of 2-mercaptoethanol, the other polymerization conditions being the same as in Example 10.

The properties of the thus obtained comparative polymer products are given in the Table.

TABLE II Example 11 12 10 13 Chain transfer agent (g) 2-Mercapto- 2-Mercapto 2-Mercapto- Trichloroethanol ethanol ethanol ethylene (3.8) (10.0) (10.0) (300) DMVN used (g) 4.5 4.5 5.0 4.5 Polymerization ( C) 62 58 58 62 % Monomer coversion when Chain transfer agent was 15-25 11-19 0 0 added (hrs. from the beginning of polymerization (1-2) (1-3) (0) (0) Overall polymerization time 8 hrs. 9 hrs. 9 hrs. 9 hrs.

30 mins. 30 n ins.

Average degree of polymerization 700 710 690 710 Particle size distri- 60 mesh 100.0 99.9 60.5 100.0 bution, % passed through screen of 200 mesh 0.8 0.5 2.2 2.1 Residual monomer in the polymer (p.p.m.) 12 9 50 125 DOP absorption (%) 18.0 17.0 19.5 15.0 Heat stability B B D E Into the same stainless steel polymerization reactor as used in Examples 1 to 10 were introduced 30 kg of deionized water, 20 g of partially saponified polyvinyl alcohol, a monomer mixture composed of 12.7 kg of vinyl chloride monomer and 2.3 kg of vinyl acetate monomer and 7.5 g of a,ol'-azobis-2,4-dimethyl-valeronitrile and the polymerization reaction was started. At the time when the monomer conversion reached 12 %, 15.0 g of 2-mercaptoethanol were added to the polymerization mixture and the polymerization reaction was continued. After the overall polymerization time of 11 hours and 20 minutes from the beginning of the polymerization reaction, the polymerization reaction was stopped and, after removal of the unreacted monomers, the resultant copolymer in the aqueous slurry was dehydrated and dried to give a finished product of vinyl chloride-vinyl acetate copolymer. The properties of the thus prepared copolymer product were examined to give the results as set out in Table III.

Examples 15 and 16 (comparative) The procedure of Example 14 was repeated twice, except that (in Example 15) 15 g of 2-mercaptoethanol was introduced into the polymerization reactor together with the deionized water, partially saponified polyvinyl alcohol, vinyl chloride monomer and other ingredients and the overall polymerization time was extended to 12 hours and (in Example 16) 200 g of trichloroethylene was used as the chain transfer agent in place of 15 g of 2-mercaptoethanol, and the overall polymerization time was extended to 123 hours to give vinyl chloride-vinyl acetate copolymers whose properties were as set out in Table III.

TABLE III Example 14 15 16 Chain transfer 2-Mercapto- 2-Mercapto- Trichloroagent. (g) ethanol ethanol ethylene (15.0) (15.0) (2()0) DMVN used, g 7.5 7.5 7.5 Polymerization temperature. "C 56 56 56 % Monomer conversion when chain transfer 12 0 0 agent was added (hrs. from the begin ning of polymeri- (1) ( ) (()) zation) Overall polymeri- 11 hrs. 12 hrs. 12 hrs. zation time 20 mins. 30 mins.

Average degree of polymerization 560 550 57() Particle size 60 100.0 16.5 11)0.0 distribution. mesh % passed through screen 200 2.8 1.5 3.5 of mesh Examples 17 to 26 Into a 1000 liter-capacity stainless steel polymerization reactor were introduced 500 kg of deionized water, 120 g of partially saponified polyvinyl alcohol, 60 g of hydroxypropyl methylcellulose, 350 kg of vinyl chloride monomer and the chain transfer agent and the polymerization initiator as indicated in Table IV in amounts also given in the table and the polymerization reaction was started by elevating the temperature. When the monomer conversion reached the percentage as shown in the table, an additional amount of the same chain transfer agent was added to the polymerization mixture and the polymerization reaction was continued at the same polymerization temperature until the pressure inside the polymerization reactor dropped to 7 kg/cm2G when the polymerization reaction was stopped by purging the unreacted vinyl chloride monomer.

The resultant vinyl chloride polymerizate was dehydrated and dried in a fluidizing drier operated with hot air of 75"C for 20 minutes after the temperature of the polymerizate reached 70"C.

For comparison, the same experimental procedures as above were undertaken except that all of the 2-mercaptoethanol was added before the starting of the polymerization reaction (Example 24), that part of the chain transfer agent was added before the starting of the polymerization reaction and the remainder was added when the monomer conversion was 0.5% (Example 25), or that trichloroethylene instead of the mercapto-containing chain transfer agent was added before the starting of the polymerization reaction (Example 26).

TABLE IV Example 17 18 19 20 21 22 23 24 25 26 Chain transfer (a) (a) (a) (a) (d) (d) (a) (a) (a) (e) agent First addition of chain transfer 0.010 0.015 0.005 0.010 0.007 0.015 0 0.030 0.010 0.60 agent, % of monomer Second addition of chain transfer 0.020 0.020 0.085 0.060 0.028 0.025 0.030 0 0.025 0 agent, % of monomer (at % monomer (15) (10) (25) (15) (5) (2) (15) - (0.5) conversion) Polymerization DMVN DMVN DMVN PV DMVN DMVN DMVN DMVN DMVN DMVN initiator, (0.04) (0.04) (0.05) (0.04) (0.04) (0.04) (0.04) (0.04) (0.04) (0.04) (% of monomer) Polymerization 62 62 55 57 62 63 63 63 62 63 temperature ( C) Particle size 60 mesh 100 100 100 100 100 100 100 20 23 100 distribution, 100 mesh 85 77 88 87 72 75 82 15 18 72 % passed through 200 mesh 0.2 1.8 0.7 1.5 2.2 0.8 2.5 5.5 6.2 1.2 screen of Heat stability A B B A B A C C D A Residual monomer in 1.5 1.0 1.8 0.5 1.2 0.9 50 18 23 350 the polymer (p.p.m.) Average degree of 650 670 640 660 640 640 660 650 670 660 polymerization Chain transfer agent (e): trichloroethylene Examples 27 to 30 Into the same polymerization reactor as used in Examples 17 to 26 were introduced 600 kg of deionized water, 120 g of partially saponified polyvinyl alcohol, 60 g of hydroxypropyl methylcellulose. 250 kg of vinyl chloride monomer and the chain transfer agent and the polymerization initiator as indicated in Table V and the polymerization reaction was started by elevating the temperature. When the monomer conversion reached the percentage as shown in the table, an additional amount of the same chain transfer agent was added and the polymerization reaction was continued at the same temperature until the pressure inside the polymerization reactor dropped to 7 kg/cm2G when the polymerization reaction was stopped by purging the unreacted vinyl chloride monomer. The treatment of the polymerizate after completion of the polymerization reaction was the same as in Examples 17 to 26 and the results of the experiments are summarized in the Table.

For comparison, all of the 2-mercaptoethanol was added before the starting of the polymerization reaction (Example 30), the other conditions being the same as above.

Claims

TABLE V Example 27 28 29 30 Chain transfer agent (a) (a) (a) (a) First addition of chain transfer agent, 0.010 0.005 0.015 0.030 % of monomer Second addition of chain transfer agent, 0.025 0.020 ().025 0 % of monomer (at % monomer conversion) (15) (10) (20) Polymerization initiator, DMVN DMVN DMVN DMVN (% of monomer) (0.04) (0.04) (0.04) (0.04) Polymerization temperature, "C 62 62.5 62 63 Particle size 60 mesh 100 100 100 98 distribution, 100 mesh 89 90 78 45 % passed through 200 mesh 1.5 0.8 1.5 3.5 screen of Heat stability A B C C Residual monomer in 0.8 1.5 1.5 20 the polymer (p.p.m.) Average degree of 660 680 650 66() polymerization WHAT WE CLAIM IS: 1. A method for the preparation of a polymer product by the suspension polymerisation of vinyl chloride monomer or a monomer mixture comprising at least 50coo by weight vinyl chloride in an aqueous medium containing a water-soluble polymeric dispersing agent and a chain transfer agent which is an organic compound having at least one mercapto group and at least one hydroxy or carboxyl group. in which method at least 50% by weight of the said organic compound is introduced into the polymerisation mixture while the conversion of the monomer or monomers is from 1 to 30C/c by weight.
2. A method as claimed in claim 1 wherein the said organic compound has from 2 to 7 carbon atoms.
3. A method as claimed in claim 1 wherein the said organic compound is selected from 2-mercaptoethanol, 2-mercaptopropanol, 2-hydroxypropyl mercaptan and thioglyccrin.
4. A method as claimed in claim 3 wherein the said organic compound is 2 mercaptoethanol.
5. A method as claimed in claim 1 wherein the said organic compound is selected from thioglycolic acid, 3-mercaptopropanoic acid, thiolactic acid and thiomalic acid.
6. A method as claimed in any preceding claim wherein the total amount of the said organic compound contained in the polymerisation mixture is from 0.001 to 0.5 % by weight based on the amount of the said vinyl chloride monomer or monomer mixture.
7. A method according to any preceding claim in which all the said organic compound contained in the polymerisation mixture is added while the conversion of the monomer or monomers is from 1 to 30% by weight.
8. A method according to claim 6 in which from 1 to 50% by weight of the total amount of the organic compound is added to the polymerisation mixture before the conversion of the monomer or monomers reaches 1% by weight and the remainder of the organic compound is added to the polymerisation mixture while the conversion of the monomer or monomers is in the range of from 1 to 30CHo by weight.
9. A method as claimed in claim 8 wherein the amount of the organic compound added into the polymerisation mixture before the conversion of the monomer or monomer mixture reaches 1% by weight is from 0.0005Yc to 0.028% by weight based on the amount of the said vinyl chloride monomer or monomer mixture.
10. A method according to claim 1 substantially as herein described with reference to any of Examples 1 to 8. 11. 12, 14, 17 to 23, 27. 28 and 29.
11. Polymer made by a process according to any preceding claim.