WO2013092732A1

WO2013092732A1 - Process for preparing a vinyl chloride polymer

Info

Publication number: WO2013092732A1
Application number: PCT/EP2012/076179
Authority: WO
Inventors: Séverine ROCHARD; François BIRAL; Frédéric BOSCHET
Original assignee: Solvay Sa
Priority date: 2011-12-22
Filing date: 2012-12-19
Publication date: 2013-06-27
Also published as: FR2984893B1; FR2984893A1; TW201341409A

Abstract

Process for preparing a vinyl chloride polymer, comprising a step of polymerization of at least vinyl chloride, performed in aqueous dispersion in the presence of at least one liposoluble radical initiator and of an activating system comprising a water-soluble transition metal salt and a complexing agent, and continued up to the point of reduction of the autogenous pressure of the vinyl chloride, the water-soluble salt being a zinc salt, and a mixture of the water-soluble zinc salt and of the complexing agent being introduced from the start of the polymerization step and at the very latest up to the said pressure reduction.

Description

Process for preparing a vinyl chloride polymer

This application claims priority to French application No. 1162284 filed on December 22, 2011, the whole content of this application being incorporated herein by reference for all purposes.

The present invention relates to a process for preparing a vinyl chloride polymer (PVC). The invention relates more particularly to a process for preparing such a polymer, including a polymerization step performed in aqueous dispersion, usually as an aqueous microsuspension. The invention also relates to the vinyl chloride polymers obtained via this process.

It is known practice to prepare vinyl chloride polymers via a process known conventionally as "microsuspension polymerization". This process includes a step during which droplets of at least one monomer, which is vinyl chloride (VC), are finely and homogeneously dispersed in an aqueous medium, in the presence of liposoluble radical initiators (also known more simply as liposoluble initiators), by means of powerful stirring and the presence of emulsifiers, such as alkali metal or ammonium carboxylates and alkylsulfonates, optionally in combination with liposoluble cosurfactants, such as long-chain alcohols. This process, which leads to polymer particles with a diameter of between 0.05 and 5 microns approximately, is particularly suitable for the manufacture of PVC plastisols.

However, the microsuspension polymerization of VC has a drawback: the start of the polymerization is relatively slow, which has the consequences of lengthening the polymer production cycles and of using relatively higher doses of liposoluble initiators. In addition, this polymerization of exothermic nature is frequently performed in at least one reactor of tank type with mechanical stirring. The heat exchange required for the thermal control (temperature regulation) then takes place advantageously by means of a jacket inside which circulates a heat- exchange fluid (water), generally counter-currentwise. Given the slow kinetics of the polymerization, the cooling capacity of the jacket is not used optimally during part of this polymerization.

It has already been proposed (see documents US-A-4 091 197 (I) and US- A-4 331 788 (II), but also EP 0 826 703 Al (III)) to improve the VC

polymerization kinetics via the "seeded microsuspension" method (i.e. a microsuspension polymerization in which the reaction medium contains a "seed" which is in the form of an aqueous dispersion of PVC particles containing all of the liposoluble initiator necessary for the polymerization), via the addition, to the polymerization medium, of an activating system. This activating system is an organosoluble metal complex formed by reaction of a water-soluble metal salt with a complexing agent.

The complexing agent should be capable of modifying the water-soluble form of the metal salt into a VC-soluble form and of not having any inhibitory action on the polymerization or on the activation of the initiator by the metal. Complexing agents that satisfy these conditions are monocarboxylic acids that are sparingly water-soluble; polycarboxylic acids and the corresponding anhydrides thereof; alkylphosphoric acids; lactones; ketones bearing, in the a or β position, groups that activate the carbonyl function; and carbazones. For the practical implementation of these inventions, used is made of ascorbic acid, dihydroxymaleic acid, succinic acid, citric acid, tartaric acid, naphthenic acid or sulfosalicylic acid. The complexing agent is usually introduced gradually throughout the polymerization or over a part thereof.

The salt may be introduced into the reaction zone before or during the polymerization, and in the latter case it is in admixture with the complexing agent. The water-soluble metal salt usually used is copper sulfate.

The Applicant has, for its part, found that the effect, on certain properties of the polymer obtained, of the activating systems described in documents (I) and (II) and used according to the introduction methods described in these documents could vary to a large extent. Thus, the Applicant has observed, for example, that certain metal salts have the drawback of affecting the properties of polymers obtained via their intervention, for instance of reducing their thermal stability and of increasing their residual monomer content.

A need consequently remains to obtain a process that has the advantage of making it possible to achieve an advantageous compromise between the polymerization kinetics and the properties of the polymer obtained.

The present invention is thus directed towards providing a process for preparing PVC, which does not have the drawbacks mentioned above and which makes it possible to achieve the abovementioned compromise, by means of a particular mode of introduction, into the polymerization medium, of certain zinc salts, mixed with a complexing agent, which allows better control of the polymerization kinetics and produces a PVC that is characterized by better properties. The present invention thus relates mainly to a process for preparing a vinyl chloride polymer, comprising a step of polymerization of at least vinyl chloride, performed in aqueous dispersion in the presence of at least one liposoluble radical initiator and of an activating system comprising a water-soluble transition metal salt and a complexing agent, and continued up to the point of reduction of the autogenous pressure of the vinyl chloride, the water-soluble salt being a zinc salt, and a mixture of the water-soluble zinc salt and of the complexing agent being introduced from the start of the polymerization step and at the very latest up to the said pressure reduction.

In the present description, the terms "monomer" and "polymer" are used indiscriminantly in the singular and in the plural. The liposoluble radical initiator will also be referred to more simply and without preference as the "liposoluble initiator" or the "initiator".

The polymer prepared according to the invention is a vinyl chloride polymer. In the present description, the term "vinyl chloride polymer", or

"polymer" for short, is intended to denote any polymer containing at least 50% by weight, preferably at least 60% by weight, particularly preferably at least 70% by weight and most particularly preferably at least 85% by weight of monomer units derived from vinyl chloride (monomer), and thus both vinyl chloride homopolymers (containing 100% by weight of monomer units derived from vinyl chloride) and copolymers of vinyl chloride with one or more ethylenically unsaturated monomers. As examples of ethylenically unsaturated monomers that can be copolymerized with vinyl chloride, mention may be made of chlorinated monomers such as vinylidene chloride, fluorinated monomers such as vinylidene fluoride, monomers containing both chlorine and fluorine such as

chlorotrifluoroethylene, vinyl esters such as vinyl acetate, vinyl ethers such as methyl vinyl ether, dialkyl maleates such as dibutyl maleate, (meth)acrylic monomers such as n-butyl acrylate and methyl methacrylate, styrene monomers such as styrene, and olefinic monomers such as ethylene, propylene and butadiene. Among all the vinyl chloride polymers mentioned above, preference is given to vinyl chloride homopolymers.

The polymerization step included in the process for preparing the VC polymer according to the invention is performed in aqueous dispersion in a stirred reactor. In the present description, the expression "polymerization... in aqueous dispersion" is intended to denote polymerizations performed according to a radical mechanism in dispersed medium with the intervention of at least one liposoluble initiator. These polymerizations include not only the polymerization conventionally known as "suspension" polymerization, but also polymerizations known as "microsuspension" and "seeded microsuspension" polymerization. Preferably, the polymerization step is performed in microsuspension or in seeded microsuspension.

The term "suspension polymerization" is intended to denote any polymerization process that is performed with stirring in an aqueous medium in the presence of at least one dispersant and of at least one liposoluble initiator.

The term "microsuspension polymerization" is intended to denote any polymerization process (already mentioned hereinabove) in which is used at least one liposoluble initiator and in which is prepared an emulsion, also known as a "fine dispersion", of monomer droplets by means of powerful mechanical stirring and the presence of at least one emulsifier, the nature of which will be specified later in the present description.

The mechanical stirring may be produced by a suitable mechanical means, for instance a colloidal mill, a rapid pump, a vibrating agitator, an ultrasonic generator, a high-pressure homogenizer, etc.

The term "seeded microsuspension polymerization" is understood to denote any microsuspension polymerization process performed in the presence of at least one "seeding product", also called "seed", which may be, as mentioned hereinabove, a dispersion of particles of vinyl chloride polymer with a diameter advantageously between 0.01 and 1 micron and preferably between 0.05 and 0.2 micron. This seed may itself be prepared by polymerization, for example by using water, VC and an optional comonomer, at least one emulsifier and the initiator.

In the present description, the term "medium" is intended to define the contents of the reactor, excluding the monomer(s) introduced and the polymer formed.

Constituents that are common to the media in which the polymerization step is performed are:

- water,

- VC and optionally at least one of the monomers mentioned above,

- at least one liposoluble initiator, and

- an activating system.

When the polymerization is performed in suspension, the medium also contains at least one dispersant. When the polymerization is performed in microsuspension or in seeded microsuspension, the respective media also contain at least one emulsifier. These media may also optionally contain at least one liposoluble cosurfactant. The medium in which the seeded microsuspension polymerization is performed also contains a seeding product (seed). The weight of the seeding product relative to the total weight of the monomer(s) is at least 1% and preferably at least 3%.

The weight of the seeding polymer(s) relative to the total weight of the monomer(s) is not more than 25% and preferably not more than 10%.

In the polymerization medium, the ratio of the weight of water to the weight of the monomer(s) is advantageously at least 0.4 and preferably at least 0.8.

In the polymerization medium, the ratio of the weight of water to the weight of the monomer(s) is advantageously not more than 2 and preferably not more than 1.5.

The media in which the polymerizations are performed may optionally contain additives other than the constituents mentioned above. They are then conventional additives, which make it possible, in a known manner, to improve the implementation of the process and/or the characteristics of the resulting polymer. Examples of such additives are chain-transfer agents, for instance chloroform, trichlorofluoromethane and C2-C5 dialkyl carbonates; chain extenders, for instance diallyl maleate and dialkyl phthalate; anticaking agents; antistatic agents; antifoams; cosolvents; and pH regulators, for instance ammonia, buffer salts, for example sodium phosphate, polyphosphate and hydrogen carbonate, and alkali metal carbonates, for example sodium carbonate, advantageously added to the polymerization medium at the start of the said polymerization.

The polymerization step included in the process for preparing the VC polymer according to the invention is performed with the intervention of at least one liposoluble initiator. These liposoluble initiators are advantageously organic peroxide compounds or liposoluble diazo compounds.

Examples of organic peroxide compounds that may be mentioned include peroxides such as dilauryl peroxide, di-ieri-butyl peroxide or dibenzoyl peroxide; hydroperoxides such as ferf-butyl hydroperoxide; peresters such as ferf-butyl perpivalate, ferf-butyl peroxy-2-ethylhexanoate and ferf-butyl perneodecanoate; percarbonates such as diethyl, diisopropyl, diethylhexyl and dimyristyl peroxydicarbonate. Examples of diazo compounds that may be mentioned include azobisisobutyronitrile and 2,2'-azobis(methoxy-2,4- dimethylvaleronitrile). Preferred organic peroxide compounds are, in particular, dilauryl peroxide and percarbonates, in particular dimyristyl peroxydicarbonate. The amount of liposoluble radical initiator used ranges advantageously between 0.2%o and 3.5%o by weight and preferably between 0.8%o and 3%o by weight relative to the weight of monomer(s) used.

When the polymerization step included in the process for preparing the VC polymer according to the invention is performed in suspension, the

polymerization medium also contains at least one dispersant. Examples of dispersants that may be mentioned include water-soluble cellulose ethers and partially saponified polyvinyl alcohol, and mixtures thereof. Along with the dispersants, surfactants may also be used. The amount of dispersant used ranges advantageously between 0.7%o and 2.0%o by weight relative to the weight of monomer(s) used.

When the polymerization step included in the process for preparing the VC polymer according to the invention is performed in microsuspension or in seeded microsuspension, the polymerization medium also contains at least one emulsifier. These emulsifiers are advantageously ionic emulsifiers chosen from anionic emulsifiers, cationic emulsifiers and amphoteric emulsifiers. Preferably, these emulsifiers are chosen from anionic emulsifiers. Particularly preferably, these emulsifiers are chosen from the following anionic emulsifiers: alkyl sulfates, alkyl sulfonates, alkylaryl sulfonates, dialkyl sulfosuccinates and alkyl carboxylates. The salts may optionally be ethoxylated and may comprise, as counterion, a sodium, potassium, lithium, caesium or ammonium cation. These emulsifiers are most particularly preferably chosen from the following non- ethoxylated sodium salts: alkyl sulfates, for instance sodium dodecyl sulfate, alkyl sulfonates, for instance primary or secondary sodium alkyl sulfonates, alkylaryl sulfonates, for instance sodium dodecylbenzenesulfonate, dialkyl sulfosuccinates, for instance dioctyl sulfosuccinate, and alkyl carboxylates, for instance sodium ammonium myristates.

The amount of emulsifier used ranges advantageously between 0.1% and 3% by weight relative to the weight of monomer(s) used.

The polymerization medium intended for suspension, microsuspension or seeded microsuspension polymerization is heated under the autogenous pressure to a temperature determined by the molecular weight that it is desired to obtain for the polymer. The polymerization temperature is advantageously between 30 and 100°C, preferably between 30 and 90°C and more particularly between 45 and

85 °C. The polymerization is advantageously performed at a pressure between 0.3 and 2.5 MPa and preferably between 0.5 and 1.5 MPa.

The polymerization step is advantageously continued until 60% to 98% by weight and preferably 80% to 95% by weight of the monomer(s) are converted, with concomitant reduction of the autogenous pressure of VC in the reactor.

The content of solid polymer in the aqueous dispersion obtained at the end of the polymerization step is advantageously between 20% and 55% by weight and preferably between 40% and 50% by weight.

As a consequence of incomplete conversion of the monomer, the amount thereof that remains in the aqueous dispersion obtained at the end of the polymerization step must be removed.

This removal may be performed conventionally by degassing the dispersion, which is usually performed in a depressurization tank,

advantageously followed by a distillation operation, steam entrainment of the residual monomer or, preferably, boiling under vacuum.

The solid polymer or the aqueous polymer dispersion derived from the abovementioned separation treatment may then be subjected to a final drying operation performed in any drying device known for this purpose. In the case of aqueous microsuspension or seeded aqueous microsuspension leading to the production of an aqueous dispersion (commonly known as a latex), the aqueous dispersion may be stored and used in this form without being dried.

The polymer synthesized by suspension polymerization is in the form of particles with a diameter advantageously between 50 and 150 microns.

The polymer synthesized by microsuspension or seeded microsuspension polymerization is in the form of elemental particles with a diameter

advantageously between 0.1 and 5 microns before drying and of particles with a diameter advantageously between 30 and 100 microns after drying.

The medium in which the polymerization step included in the process for preparing the vinyl chloride polymer according to the invention is performed contains, during at least part of this polymerization (as will be specified later), an activating system comprising a water-soluble transition metal salt and a complexing agent. According to the invention, the water-soluble transition metal salt, which is the first constituent of the activating system, is a zinc salt. Any water-soluble zinc salt may be used as constituent of the activating system. These salts may be mineral or organic. Among the water-soluble mineral zinc salts that may be mentioned are the sulfate, the chlorate, the chloride and the nitrate of this metal. Among the water-soluble organic zinc salts that may be mentioned is the acetate of this metal. Mineral zinc salts are preferred as constituents of the activating system, and, among these, zinc sulfate is particularly preferred. The zinc salt that is thus particularly preferred is zinc sulfate.

The second constituent of the activating system is a complexing agent. In the present description, the term "complexing agent" is intended to denote any chemical compound that is capable of changing zinc from its water-soluble form to the form of a complex that is soluble in VC, without exerting any inhibitory action on the polymerization or on the activation exerted by the zinc on the liposoluble initiator.

Complexing agents that satisfy these conditions may be chosen especially from monocarboxylic acids, polycarboxylic acids, alkylphosphoric acids, lactones, ketones and carbazones.

As monocarboxylic acids that may be used as complexing agents, mention may be made of those that are sparingly water-soluble, such as perfluorobutyric acid, a-bromolauric acid, sulfosalicylic acid, naphthenic acid and octanoic acid.

As polycarboxylic acids that may be used as complexing agents, mention may be made of succinic acid, tartaric acid, maleic acid and hydroxymaleic acid.

As alkylphosphoric acids that may be used as complexing agents, mention may be made of bis(2-ethyl)hexylphosphoric acid.

As lactones that may be used as complexing agents, mention may be made of ascorbic acid, its stereoisomer erythorbic acid, and esters thereof, and also γ- butyrolactone.

As ketones that may be used as complexing agents, mention may be made of ketones bearing, in the γ or β position, groups that activate the carbonyl function, such as acetylacetone, 1,3-dihydroxyacetone and benzoin.

As carbazones that may be used as complexing agents, mention may be made of diphenylthiocarbazone.

The complexing agent is preferably chosen from monocarboxylic acids, polycarboxylic acids and lactones. The complexing agent is particularly preferably chosen from lactones. The complexing agent is most particularly preferably chosen from ascorbic acid, its stereoisomer erythorbic acid, and esters thereof. The complexing agent is really most particularly preferably ascorbic acid.

The presence of several water-soluble zinc salts and/or of several complexing agents in the same activating system is not at all excluded from the scope of the invention.

An activating system comprising a zinc salt as water-soluble transition metal salt and ascorbic acid as complexing agent gave excellent results.

The amounts in which the constituents of the activating system are present during the polymerization step included in the process for preparing the VC polymer according to the invention may vary to a large extent.

Advantageously, the amount of water-soluble zinc salt introduced, expressed relative to the amount of initiator present in the polymerization step, is greater than or equal to 10^~3 mol of zinc salt per mole of initiator. Preferably, the amount of water-soluble zinc salt introduced, expressed relative to the amount of initiator present in the polymerization step, is greater than or equal to 5.10^~3 mol of zinc salt per mole of initiator. Even more preferably, the amount of water- soluble zinc salt introduced, thus expressed, is greater than or equal to 10^"2 mol of zinc salt per mole of initiator.

Advantageously, the amount of water-soluble zinc salt introduced, expressed relative to the amount of initiator present in the polymerization step, is less than or equal to 50 mol of zinc salt per mole of initiator. Preferably, the amount of water-soluble zinc salt introduced, expressed relative to the amount of initiator present in the polymerization step, is less than or equal to 5 mol of zinc salt per mole of initiator. Even more preferably, the amount of water-soluble zinc salt introduced, thus expressed, is less than or equal to 0.5 mol of zinc salt per mole of initiator.

When expressed relative to the amount of VC present in the polymerization step, the amount of water-soluble zinc salt introduced is advantageously greater than or equal to 1 ppm and preferably greater than or equal to 5 ppm.

When expressed relative to the amount of VC present in the polymerization step, the amount of water-soluble zinc salt introduced is advantageously less than or equal to 300 ppm and preferably less than or equal to 250 ppm.

Advantageously, the amount of complexing agent introduced, expressed relative to the amount of water-soluble zinc salt introduced into the

polymerization step, is greater than or equal to 0.05 mol of complexing agent per mole of zinc salt. Preferably, the amount of complexing agent introduced, expressed relative to the amount of water-soluble zinc salt introduced into the polymerization step, is greater than or equal to 0.1 mol of complexing agent per mole of zinc salt. Even more preferably, the amount of complexing agent introduced, thus expressed, is greater than or equal to 1 mol of complexing agent per mole of zinc salt.

polymerization step, is less than or equal to 50 mol of complexing agent per mole of zinc salt. Preferably, the amount of complexing agent introduced, expressed relative to the amount of water-soluble zinc salt introduced into the polymerization step, is less than or equal to 20 mol of complexing agent per mole of zinc salt. Even more preferably, the amount of complexing agent introduced, thus expressed, is less than or equal to 5 mol of complexing agent per mole of zinc salt.

According to the invention, a mixture of the water-soluble zinc salt and of the complexing agent is introduced from the start of the polymerization step and at the very latest up to the reduction of the autogenous pressure of vinyl chloride.

For the purposes of the present description, the expression "start of the polymerization step" should be understood as meaning the moment (referred to as to) at which the polymerization temperature is reached (to within + 1°C).

For the purposes of the present description, the expression "from the start of the polymerization step" should be understood as meaning from the moment ¾ at which the polymerization temperature defined above is reached (to within + 1°C).

The expression "reduction of the VC autogenous pressure" is intended to denote the pressure reduction generated taking into account the disappearance of VC in the gaseous phase (since it is consumed by the polymerization reaction). This reduction in the VC autogenous pressure is advantageously not induced by a reduction in the polymerization temperature. This reduction of the VC autogenous pressure indicates advantageously that the polymerization is close to completion and is advantageously followed by the phenomenon conventionally defined by the term "self-heating", which corresponds to an increase in temperature at the end of polymerization (the heat being supplied mainly by the heat of the polymerization itself). If we call t_x the moment at which the reduction of the vinyl chloride autogenous pressure starts, the mixture of the water-soluble zinc salt and of the complexing agent is thus introduced from ¾ and at the very latest up to t_x.

The moment t_x is detected by a pressure reduction advantageously of the order of at least 0.1 bar and preferably of the order of at least 0.2 bar, relative to the mean pressure at which the polymerization step is performed.

If we call x the time elapsed between the start of the polymerization step (moment ¾ at which the polymerization temperature is reached (to within + 1 °C)) and the moment t_x at which the reduction of the VC autogenous pressure starts, the mixture of the water-soluble zinc salt and of the complexing agent is advantageously introduced

- in a single batch at any fraction of x between ¾ and t_x, including addition in a single batch at to, preferably between ¾ and (t_x- (x/4)), and particularly preferably between ¾ and (t_x- (x/3)),

- in several successive batches of the same weight or of uniformly decreasing weight distributed over the duration x,

- continuously over x at a constant rate,

- continuously over x at a decreasing rate,

- continuously over a fraction of x of between ¾ and t_x at a constant rate, or - continuously over a fraction of x of between ¾ and t_x at a decreasing rate.

The mixture of the water-soluble zinc salt and of the complexing agent is preferably introduced continuously, over x or over a fraction of x between ¾ and t_x, at a constant rate or at a decreasing rate.

The mixture of the water-soluble zinc salt and of the complexing agent may be introduced into the polymerization step in the form of an aqueous solution of its two constituents or alternatively by mixing two aqueous solutions each containing one of the two constituents. An emulsifying agent may be added to these solutions or added in the form of a separate aqueous solution. The mixture of the water-soluble zinc salt and of the complexing agent is preferably introduced in the form of an aqueous solution of its two constituents.

The process according to the invention is advantageously not limited only to the introduction of the mixture of a water-soluble zinc salt and of the complexing agent from the start of the polymerization step and at the very latest up to the reduction of the autogenous pressure of vinyl chloride. Thus, other modes of introduction, combined with that described above, form the subject of variants of the process according to the invention. According to a first variant of the process according to the invention, besides the introduction of a mixture of the water-soluble zinc salt and of the complexing agent from the start of the polymerization step and at the very latest up to the said pressure reduction, the water-soluble zinc salt is introduced at the very latest at the start of the polymerization step.

For the purposes of the present description, the expression "introduced at the very latest at the start of the polymerization step" should be understood as meaning introduced at a moment between the introduction of the first of the constituents of the medium in which the polymerization step is performed and the moment ¾ defined above.

The water-soluble zinc salt is thus advantageously introduced at any moment between the introduction of the first of the constituents of the medium in which the polymerization step is performed and the moment ¾ at which the polymerization temperature is reached (to within + 1 °C) (commonly referred to as "initial"). Preferably, it is introduced with the other constituents of the medium in which the polymerization step is performed before raising the temperature.

According to this first variant, the zinc salt may be introduced into the polymerization step in solid form, in the form of an aqueous solution or as a mixture with an emulsifier solution. The zinc salt is preferably introduced in solid form.

In the case of a seeded microsuspension polymerization, the water-soluble zinc salt may be introduced via the "seed".

According to a second variant of the process according to the invention, besides the introduction of a mixture of the water-soluble zinc salt and of the complexing agent from the start of the polymerization step and at the very latest up to the said pressure reduction, a mixture of the water-soluble zinc salt and of the complexing agent is introduced at the very latest at the start of the polymerization step.

The said mixture is thus advantageously introduced at any moment between the introduction of the first of the constituents of the medium in which the polymerization step is performed and the moment ¾ at which the

polymerization temperature is reached (to within + 1 °C) (commonly referred to as "initial"). Preferably, it is introduced with the other constituents of the medium in which the polymerization step is performed before raising the temperature. According to this second variant of the process according to the invention, the mixture of the water-soluble zinc salt and of the complexing agent introduced at the very latest at the start of the polymerization step may be introduced in the form of an aqueous solution of its two constituents or alternatively by mixing two aqueous solutions each containing one of the two constituents. An emulsifying agent may be added to these solutions or added in the form of a separate aqueous solution. The said mixture is preferably introduced in the form of an aqueous solution of its two constituents.

The process according to the invention is nevertheless preferably characterized by the sole introduction of the mixture of the water-soluble zinc salt and of the complexing agent from the start of the polymerization step and at the very latest up to the reduction of the autogenous pressure of vinyl chloride.

A subject of the present invention is also the vinyl chloride polymers obtained via a process comprising a step of polymerization of at least vinyl chloride, performed in aqueous dispersion in the presence of at least one liposoluble radical initiator and of an activating system comprising a water- soluble transition metal salt and a complexing agent, and continued up to the point of reduction of the autogenous pressure of the vinyl chloride, the said water-soluble salt being a zinc salt, and a mixture of the water-soluble zinc salt and of the complexing agent being introduced from the start of the

polymerization step and at the very latest up to the said pressure reduction.

The process according to the invention has the advantage of achieving a compromise of polymerization kinetics/properties of the polymer obtained (thermal stability, residual monomer) that is more advantageous than in the case of the processes according to the prior art.

In particular, when compared with the process in which a zinc salt, in particular zinc sulfate, is introduced before the start of the polymerization step and the complexing agent is introduced from the start of this step (at the moment at which the polymerization temperature is reached) and relative to the process according to the prior art in which the water-soluble transition metal salt and the complexing agent are both introduced before the start of the polymerization step, the process according to the invention has the advantage of being characterized by improved polymerization kinetics (shorter polymerization time, higher ΔΤ maximum) and of producing a polymer latex that is characterized by a markedly lower residual monomer content. Finally, when compared with the process according to the prior art according to which a copper salt, in particular copper sulfate, is introduced, just like the complexing agent, from the start of the polymerization step (from the moment at which the polymerization temperature is reached), the process according to the invention achieves a better compromise in terms of polymerization kinetics/properties of the polymer obtained (greater thermal stability and lower residual monomer content).

Should the disclosure of any patents, patent applications, and publications which are incorporated herein by reference conflict with the description of the present application to the extent that it may render a term unclear, the present description shall take precedence.

The examples that follow are intended to illustrate the invention without, however, limiting its scope.

Example 1R

This example is given for comparative purposes.

Preparation of the seed latex (seed latex S)

A PVC latex (seed latex S) was prepared via an emulsion polymerization step performed conventionally (emulsifier: solution of dodecylbenzenesulfonate at 180 g/kg; water-soluble initiator: aqueous ammonium persulfate solution at 66.5 g/L) in a 300 L reactor equipped with a stirrer and a jacket.

The latex was emptied from the reactor. The latex was filtered through a screen with a mesh size of 1 mm. The latex was placed in a storage tank.

A sample of latex was taken from the storage tank and its solids content was measured by densimetry: the solids content of the seed latex S was 34.1%.

The distribution of the elementary polymer particles of the seed latex was also determined by photosedimentometry using a CPS machine formed from a centrifugation unit, a detector and a control/analysis device: the distribution of the elementary polymer particles of the seed latex S was unimodal; the mean diameter of these elementary particles was 105 nm.

Preparation of the fine dispersion (1st part)

47.28 kg of demineralized water were first placed in a 300 L mixing autoclave equipped with a stirrer and a jacket, and maintained at 17°C. 0.529 kg of an aqueous sodium dioctylsulfosuccinate solution at 717.3 g/kg, 90.90 g of dilauroyl peroxide (PL) (99.4% pure), 138.1 g of dimyristyl peroxydicarbonate (MYPC) (97.5% pure), 142.8 g of dioctyl adipate and 0.542 g of butylhydroxyanisole were then introduced. The mixing autoclave was closed and the stirrer switched on. The mixing autoclave was then placed under vacuum.

Loading the reagents into the reactor (1st part)

78.75 kg of demineralized water, 0.983 kg of a solution of sodium dioctylsulfosuccinate at 717.3 g/kg and 4.875 kg of the seed latex S (at 370.7 g/kg of PVC in water) were successively introduced into a 300 L reactor equipped with a stirrer and a jacket, and maintained at 17°C. The reactor was closed and the stirrer switched on. The reactor was then placed under vacuum.

Preparation of the fine dispersion (2nd part)

31.62 kg of VC were placed in the mixing autoclave and vigorous stirring was maintained in order to constitute a homogeneous aqueous dispersion of vinyl chloride droplets comprising the mixture of liposoluble initiators and the peak suppressant (butylhydroxyanisole).

Loading the reagents into the reactor (2nd part)

58.73 kg of VC were placed in the reactor.

Preparation of the fine dispersion (3rd part) and loading of the reagents into the reactor (3rd part)

A high-pressure homogenizer connecting the mixing autoclave to the reactor was switched on. The homogenization pressure was adjusted. The contents of the mixing autoclave were transferred into the reactor via this homogenizer. The operating conditions of the homogenizer were such that a fine aqueous dispersion of VC droplets comprising the liposoluble initiators and the peak suppressant was obtained at its outlet.

Polymerization

The contents of the reactor were brought to 46°C. Once this temperature was reached (to), 0.146 kg of an aqueous ammonia solution at 222.5 g/kg was introduced into the reactor.

During the polymerization, 2 x 9.036 kg of VC were introduced into the reactor.

After time t_x (in this case corresponding to a pressure reduction (ΔΡ) of 0.2 bar relative to the average pressure at which the polymerization step was performed), once a pressure reduction (ΔΡ) of 0.5 bar was detected, the contents of the reactor were brought to a higher temperature (self-heating), the polymerization time elapsed from t_Q up to this point (ΔΡ = 0.5 bar) was recorded, and 45.5 g of a commercial solution of antifoam (Tego^® KS 53 sold by Evonik) were added. End operations

A purification treatment of the residual VC was performed.

The latex was emptied out and the reactor cleaned.

The wet cake present inside the reactor, especially on the walls and on the blades of the stirrer, was collected. After weighing, the wet cake was dried in an oven. The dry cake was in turn weighed.

The latex was filtered through a screen with a mesh size of 1 mm. The wet lumps retained on this screen were collected. After weighing, the wet lumps were dried in an oven. The dry lumps were in turn weighed.

A sample of latex was collected, the solids content was measured by densitometry and the distribution of the elementary particles was measured by sedimentometry.

Drying of the latex and recovery of the resin

The PVC latex was dried by atomization. The dry PVC was recovered and ground.

Evaluation of the yellow index

A plastisol was manufactured by mixing 300 g of PVC resin, 120 g of diisononyl phthalate plasticizer, 36 g of benzyl butyl phthalate plasticizer, 9 g of a viscosity reducer (Viscobyk^® 5050) and 6 g of a stabilizer (Irgastab^® BZ505).

This plastisol was then coated as a layer 0.5 mm thick onto transfer paper, which was placed in a Werner-Mathis coating oven maintained at 180°C, for 1.5 minutes.

The yellow index obtained on the film was measured using a Luci 100 spectrocolorimeter from the company DR Lange GmbH, using the illuminant D65 and the 10° observation angle. The values measured in the defined colorimetric space were x, y and Y. The yellow index (YI) was defined by the formula YI = 100*((Cx*X)-Cz*Z)/Y with:

X = Y * x / y

Z = ((l-x-y)/y)*Y

Cx=1.3013

Cz=1.1498

Example 2 (according to the invention)

The procedure followed was identical to that of Example 1 , except that an aqueous solution containing a mixture of zinc sulfate monohydrate (at a rate of 0.3 g per kg of VC used) (the mole ratio of zinc sulfate to the mixture of initiators was thus 0.309) and of ascorbic acid (at a rate of 0.5 g per kg of VC used) (the mole ratio of ascorbic acid to the mixture of initiators (PL and MYPC) was thus about 2) was introduced continuously between (to and to+4h).

The table below collates the results of Examples 1R and 2.

Table

(*) The ΔΤ maximum corresponds to the largest temperature difference observed between the jacket and the reaction medium. It is representative of the exothermicity of the reaction, and thus of the polymerization rate.

(§) The residual VC content was measured on a Thermo Finnigan brand gas chromatograph (Trace GC) equipped with a flame ionization detector and an automatic headspace injector of the same brand (Triplus), having a stirring function and also a system for acquisition and processing of the chromatographic data (ChromCard). The assay was performed by the external calibration method (calibration with VC samples of known concentration in N,N-dimethylacetamide (DMA)).

The latex sample (1 ml) to be analysed dispersed in a water/DMA mixture

(40:60 by weight, 5 ml) contained in a hermetically sealed penicillin-type flask was placed on the chromatograph sampler and subjected to incubation with stirring for 30 minutes at a temperature of 70°C. After this equilibration, the headspace was analysed by chromatography on a packed semicapillary column, divinylbenzene type (20 μιη), commercial name RT-Q Bond, 30 m long and 0.53 μιη inside diameter of the abovementioned chromatograph.

The residual VC content is expressed in mg/kg resin (ppm). The results collated in the table show the advantages obtained by means of the process according to the invention (large reduction in the residual VC content of the polymers; significant decrease in polymerization time obtained by means of the improvement in the kinetics of this polymerization). The content of cakes in the reactor after the polymerization was also lower. As regards the yellow index, it was equivalent, if not better.

Example 3R (comparative)

Preparation of the seed latex (seed latex S)

The process was performed as described in Example 1R. The solids content of the seed latex S was 38.0%

Preparation of the fine dispersion (1st part)

51.3 kg of demineralized water were first placed in a 300 L mixing autoclave equipped with a stirrer and a jacket. The following were then placed in the mixing autoclave: 1.4 kg of a commercial solution of sodium

dodecylbenzenesulfonate at 31.95 % in water, a mixture of liposoluble initiators comprising 78.1 g of dilauryl peroxide (PL) and 111.2 g of dimyristyl peroxydicarbonate (MYPC), 168.0 g of dioctyl adipate and 0.56 g of

butylhydroxyanisole. The mixing autoclave was closed and the stirrer switched on. The mixing autoclave was then placed under vacuum.

Loading the reagents into the reactor (1st part)

63.3 kg of demineralized water, 2.1 kg of a commercial solution of sodium dodecylbenzenesulfonate at 31.95% in water, 12.27 kg of the seed latex S containing 38.0% solids and 34.0 g of sodium carbonate were successively introduced into a 300 L reactor equipped with a stirrer and a jacket. The reactor was closed and the stirrer switched on. The reactor was then placed under vacuum.

Preparation of the fine dispersion (2nd part)

The process was performed as described in Example 1R, introducing 37.33 kg of VC into the mixing autoclave.

Loading the reagents into the reactor (2nd part)

56.0 kg of VC were placed in the reactor.

The process was performed as described in Example 1R. Polymerization

The contents of the reactor were brought to 51°C. Once this temperature was reached (to), 18.66 kg of VC were introduced into the reactor during the polymerization.

After time t_x (in this case corresponding to a pressure reduction (ΔΡ) of 0.2 bar relative to the average pressure at which the polymerization step was performed), once a pressure reduction (ΔΡ) of 1.0 bar was detected, the contents of the reactor were brought to a higher temperature (self-heating), the polymerization time elapsed from t_D up to this point (ΔΡ = 1.0 bar) was recorded, and 56.4 g of a commercial solution of antifoam (Tego^® KS 53 sold by Evonik) were added.

End operations

The process was performed as described in Example 1R.

Drying of the latex and recovery of the resin

The process was performed as described in Example 1R.

Evaluation of the thermal stability

10 g of the PVC resin obtained were mixed with 4 g of diisononyl phthalate plasticizer. The thermal stability of the paste (0.5 g) was evaluated using a Thermomat PVC 763 constructed by the company Metrohm.

The PVC sample heated to 180°C breaks down with evolution of HC1, which is entrained by a gas stream (7 L/h of N₂) into a measuring cell where it is absorbed by ultrapure water. The HC1 concentration of this water is measured continuously by conductimetry.

The conventional thermal stability was defined as the induction time of the dehydrochlorination reaction at a temperature of 180°C leading to an increase in conductimetry of 5 μ8/αη relative to the initial value. It is expressed in minutes and seconds.

Evaluation of the yellow index

The process was performed as described in Example 1R.

Determination of the residual monomer in the latex

The process was performed as described in Example 1R.

In this example:

- the polymerization lasted 363 minutes;

- the ΔΤ maximum was 24.0°C;

- the amount of dry lumps and cakes (expressed as a percentage of the VC introduced) was 0.31; - the yellow index was 1.3 after 90 seconds of gelation at 180°C;

- the thermal stability of the paste sample obtained was 16 minutes 48 seconds;

- the residual monomer content in the latex was 34.1 ppm.

Example 4R (comparative)

Example 3R was repeated, except that:

- in the step of loading the reagents into the reactor (1st part), zinc sulfate monohydrate was added at a rate of 0.050 g per kg of VC used; (the mole ratio of the zinc sulfate to the mixture of initiators was thus about 0.076);

- in the polymerization step, 0.0875 g, per kg of VC used, of ascorbic acid in aqueous solution form was also added continuously at a constant rate between

(to and to+4h) (the mole ratio between ascorbic acid and copper sulfate was thus about 1.8).

The results of this example were as follows:

- the polymerization lasted 252 minutes;

- the ΔΤ maximum was 40.0°C;

- the amount of dry lumps and cakes (expressed as a percentage of the VC introduced) was 0.37;

- the yellow index was 1.5 after 90 seconds of gelation at 180°C;

- the thermal stability of the paste sample obtained was 9 minutes;

- the residual monomer content in the latex was 15.7 ppm.

Example 5 (according to the invention)

Example 3R was repeated, except that:

- in the polymerization step, a mixture containing, respectively, 0.045 and 0.0875 g, per kg of VC used, of zinc sulfate monohydrate and of ascorbic acid in aqueous solution form was also added continuously at a constant rate between (to and to+4h) (the mole ratio between ascorbic acid and zinc sulfate was thus about 2) (the mole ratio between the zinc sulfate and the initiators is about 0.069).

The results of this example were as follows:

- the polymerization lasted 217 minutes;

- the ΔΤ maximum was 50.0°C;

- the amount of dry lumps and cakes (expressed as a percentage of the VC introduced) was 0.09;

- the yellow index was 1.8 after 90 seconds of gelation at 180°C;

- the thermal stability of the paste sample obtained was 7 minutes 48 seconds;

- the residual monomer content in the latex was 10.9 ppm. Comparison of the results of Example 5 with those of Examples 3R and 4R shows a significant decrease in the polymerization time and an equally significant increase in the ΔΤ maximum, illustrating the improvement afforded by the process according to the invention on the polymerization kinetics relative to the case where the zinc salt is introduced before the start of the polymerization step and the complexing agent is introduced from the start of this step (at the moment at which the polymerization temperature is reached). In addition, it is seen from this comparison that the content of residual monomer in the latex obtained by means of the activating system according to the invention is much lower than that observed in the case of the comparative examples.

Example 6R (comparative)

Preparation of the seed latex (seed latex S)

The process was performed as described in Example 1R. The solids content of the seed latex S was 37.0%

Preparation of the fine dispersion (1st part)

The process was performed as described in Example 3R, except that 50.1 kg of demineralized water and 3.135 kg of a commercial sodium

dodecylbenzenesulfonate solution at 32.15% in water were used.

Loading the reagents into the reactor (1st part)

The process was performed as described in Example 3R, except that 64.23 kg of demineralized water, 0.35 kg of a commercial sodium

dodecylbenzenesulfonate solution at 32.15% in water, and 12.60 kg of the seed latex S at 37.0% solids were used.

Preparation of the fine dispersion (2nd part)

The process was performed as described in Example 3R.

Loading the reagents into the reactor (2nd part)

The process was performed as described in Example 3R.

Polymerization

After the time t_x (in this case corresponding to a pressure reduction (ΔΡ) of

0.2 bar relative to the average pressure at which the polymerization step was performed), once a pressure reduction (ΔΡ) of 0.4 bar was detected, the contents of the reactor were brought, 1 hour after this time, to a higher temperature (self- heating), the polymerization time elapsed from ¾ up to this moment (ΔΡ = 0.4 bar + 1 hour) was recorded, and 56.4 g of a commercial antifoam solution (Tego^® KS53 sold by Evonik) were added.

End operations

The process was performed as described in Example 3R.

Drying of the latex and recovery of the resin

Latex was coagulated, filtered and dried in an oven under vacuum at 50°C for evaluation of the thermal stability.

Evaluation of the thermal stability

The thermal stability of the dried latex (0.5 g) was evaluated using a Thermomat PVC 763 constructed by the company Metrohm.

Determination of the residual monomer in the latex

The process was performed as described in Example 1R.

In this example:

- the polymerization lasted 286 minutes;

- the ΔΤ maximum was 25.0°C;

- the amount of dry lumps and cakes (expressed as a percentage of the VC introduced) was 0.46;

- the thermal stability of the resin sample obtained was 7 minutes 12 seconds;

- the residual monomer content in the latex was 119.6 ppm.

Example 7R (comparative)

Example 6R was repeated, except that:

- in the step of loading the reagents into the reactor (1st part), copper sulfate heptahydrate was added at a rate of 0.097 g per kg of VC used (the mole ratio of copper sulfate to the initiator mixture used was thus about 0.108) and 0.0875 g, per kg of VC used, of ascorbic acid in aqueous solution form was added (the mole ratio between ascorbic acid and copper sulfate was thus about 1.27); and

- in the step of polymerization, after the time t_Xi it was once a pressure reduction (ΔΡ) of 1.0 bar was detected, that the contents of the reactor were brought to a higher temperature (self -heating), that the polymerization time elapsed from ¾ up to this moment (ΔΡ = 1.0 bar) was recorded and that 56.4 g of a commercial antifoam solution (Tego^® KS53 sold by Evonik) were added. The results of this example were as follows:

- the polymerization lasted 295 minutes;

- the ΔΤ maximum was 29.0°C;

- the amount of dry lumps and cakes (expressed as a percentage of the VC introduced) was 0.16;

- the thermal stability of the resin sample obtained was 2 minutes 48 seconds;

- the residual monomer content in the latex was 669.9 ppm.

Example 8R (comparative)

Example 6R was repeated, except that:

- in the step of loading the reagents into the reactor (1st part), zinc sulfate monohydrate was added at a rate of 0.0486 g per kg of VC used (the mole ratio of zinc sulfate to the initiator mixture used was thus about 0.075) and 0.0875 g, per kg of VC used, of ascorbic acid in aqueous solution form was added (the mole ratio between ascorbic acid and zinc sulfate was thus about 1.83); and

- in the step of polymerization, after the time tx, it was once a pressure

reduction (ΔΡ) of 1.0 bar was detected, that the contents of the reactor were brought to a higher temperature (self -heating), that the polymerization time elapsed from tO up to this moment (ΔΡ = 1.0 bar) was recorded and that 56.4 g of a commercial antifoam solution (Tego® KS53 sold by Evonik) were added.

The results of this example were as follows:

- the polymerization lasted 436 minutes;

- the ΔΤ maximum was 23.0°C;

- the amount of dry lumps and cakes (expressed as a percentage of the VC introduced) was 0.86;

- the thermal stability of the resin sample obtained was 4 minutes 12 seconds;

- the residual monomer content in the latex was 43.8 ppm. Comparison of the results of Examples 6R, 7R and 8R with those of Example 5 shows the advantages obtained by means of the process according to the invention, namely improved polymerization kinetics (shorter polymerization time, higher ΔΤ maximum) and the production of a polymer latex that is characterized by a markedly lower residual monomer content, when compared with the process according to the prior art in which the water-soluble transition metal salt and the complexing agent are introduced before the start of the polymerization step.

Example 9R (comparative)

Preparation of the seed latex (seed latex S)

The process was performed as described in Example 1R. The solids content of the seed latex S was 37.9%.

Preparation of the fine dispersion (1st part)

The process was performed as described in Example 3R, except that 50.0 kg of demineralized water and 3.2 kg of a commercial sodium

dodecylbenzenesulfonate solution at 31.25% in water were used.

Loading the reagents into the reactor (1st part)

The process was performed as described in Example 3R, except that 64.5 kg of demineralized water, 0.36 kg of a commercial sodium

dodecylbenzenesulfonate solution at 31.25% in water, and 12.3 kg of the seed latex S at 38.0% solids were used.

Preparation of the fine dispersion (2nd part)

Loading the reagents into the reactor (2nd part)

56.0 kg of VC were placed in the reactor.

The process was performed as described in Example 1R.

Polymerization

The process was performed as described in Example 3R.

End operations

The process was performed as described in Example 1R.

Drying of the latex and recovery of the resin

The process was performed as described in Example 1R. Evaluation of the thermal stability

The process was performed as described in Example 3R.

Evaluation of the yellow index

The process was performed as described in Example 1R.

Determination of the residual monomer in the latex

The process was performed as described in Example 1R.

In this example:

- the polymerization lasted 404 minutes;

- the ΔΤ maximum was 21.0°C;

- the amount of dry lumps and cakes (expressed as a percentage of the VC introduced) was 0.15;

- the yellow index was 1.9 after 90 seconds of gelation at 180°C;

- the thermal stability of the paste sample obtained was 21 minutes 36 seconds;

- the residual monomer content in the latex was 38.7 ppm.

Example 10R (comparative)

Example 9R was repeated, except that:

- in the step of loading the reagents into the reactor (1st part), copper sulfate heptahydrate was added at a rate of 0.0245 g per kg of VC used (the mole ratio of copper sulfate to the initiator mixture used was thus about 0.027) and 0.0219 g, per kg of VC used, of ascorbic acid in aqueous solution form was added (the mole ratio between ascorbic acid and copper sulfate was thus about 1.27).

The results of this example were as follows:

- the polymerization lasted 360 minutes;

- the ΔΤ maximum was 24.0°C;

- the amount of dry lumps and cakes (expressed as a percentage of the VC introduced) was 0.41;

- the yellow index was 1.9 after 90 seconds of gelation at 180°C;

- the thermal stability of the paste sample obtained was 9 minutes 18 seconds; - the residual monomer content in the latex was 19.6 ppm.

Example 11 (according to the invention)

Example 9R was repeated, except that:

- in the step of loading the reagents into the reactor (1st part), zinc sulfate monohydrate was added at a rate of 0.012 g per kg of VC used (the mole ratio of zinc sulfate to the initiator mixture used was thus about 0.0187) and 0.0219 g, per kg of VC used, of ascorbic acid in aqueous solution form was added (the mole ratio between ascorbic acid and zinc sulfate was thus about 1.81).

The results of this example were as follows:

- the polymerization lasted 328 minutes;

- the ΔΤ maximum was 24.7°C;

- the amount of dry lumps and cakes (expressed as a percentage of the VC introduced) was 0.32;

- the yellow index was 1.9 after 90 seconds of gelation at 180°C;

- the thermal stability of the paste sample obtained was 13 minutes 48 seconds;

- the residual monomer content in the latex was 8.0 ppm.

Comparison of the results of Example 11 with those of Examples 9R and 10R reveals an improvement in the polymerization kinetics (reduction of the polymerization time and increase in the ΔΤ maximum) obtained by means of the activating system according to the invention. Comparison of the results of Example 11 with those of Example 10R also shows that the polymer obtained in Example 11 is thermodynamically more stable and has a markedly lower residual monomer content than the polymer obtained in Example 10R. The use of a zinc salt, in particular zinc sulfate, thus leads to a better compromise in terms of polymerization kinetics/properties of the polymer obtained than that of a copper salt, in particular copper sulfate.

Claims

C L A I M S

1. Process for preparing a vinyl chloride polymer, comprising a step of polymerization of at least vinyl chloride, performed in aqueous dispersion in the presence of at least one liposoluble radical initiator and of an activating system comprising a water-soluble transition metal salt and a complexing agent, and continued up to the point of reduction of the autogenous pressure of the vinyl chloride, the said process being characterized in that the water-soluble salt is a zinc salt and in that a mixture of the water-soluble zinc salt and of the complexing agent is introduced from the start of the polymerization step and at the very latest up to the said pressure reduction.

2. Process according to Claim 1 , characterized in that the polymerization step is performed in microsuspension or in seeded microsuspension.

3. Process according to Claim 1 or 2, characterized in that the zinc salt is zinc sulfate.

4. Process according to any one of Claims 1 to 3, characterized in that the complexing agent is chosen from lactones.

5. Process according to Claim 4, characterized in that the complexing agent is ascorbic acid.

6. Process according to any one of Claims 1 to 5, characterized in that the amount of water-soluble zinc salt introduced, expressed relative to the amount of initiator present in the polymerization step, is greater than or equal to 5. 10^"3 mol of zinc salt per mole of initiator.

7. Process according to any one of Claims 1 to 5, characterized in that the amount of water-soluble zinc salt introduced, expressed relative to the amount of initiator present in the polymerization step, is less than or equal to 5 mol of zinc salt per mole of initiator.

8. Process according to any one of Claims 1 to 7, characterized in that the amount of complexing agent introduced, expressed relative to the amount of water-soluble zinc salt introduced into the polymerization step, is greater than or equal to 0.1 mol of complexing agent per mole of zinc salt.

9. Process according to any one of Claims 1 to 7, characterized in that the amount of complexing agent introduced, expressed relative to the amount of water-soluble zinc salt introduced into the polymerization step, is less than or equal to 20 mol of complexing agent per mole of zinc salt.

10. Process according to any one of Claims 1 to 9, characterized in that if we call x the time elapsed between the start of the polymerization step (moment to at which the polymerization temperature is reached (to within + 1 °C)) and the moment t_x at which the reduction of the VC autogenous pressure starts, the mixture of the water-soluble zinc salt and of the complexing agent is introduced in a single batch at any fraction of x between ¾ and t_x including addition in a single batch at to, in several successive batches of the same weight or of uniformly decreasing weight distributed over the duration x, continuously over x at a constant rate, continuously over x at a decreasing rate, continuously over a fraction of x of between ¾ and t_x at a constant rate, or continuously over a fraction of x of between ¾ and t_x at a decreasing rate.

11. Process according to Claim 10, characterized in that the mixture of the water-soluble zinc salt and of the complexing agent is introduced continuously, over x or over a fraction of x between to and t_x, at a constant rate or at a decreasing rate.

12. Process according to any one of Claims 1 to 11, characterized in that the mixture of the water-soluble zinc salt and of the complexing agent is introduced in the form of an aqueous solution of its two constituents.

13. Vinyl chloride polymers obtained via a process comprising a step of polymerization of at least vinyl chloride, performed in aqueous dispersion in the presence of at least one liposoluble radical initiator and of an activating system comprising a water-soluble transition metal salt and a complexing agent, and continued up to the point of reduction of the autogenous pressure of the vinyl chloride, the said water-soluble salt being a zinc salt, and a mixture of the water- soluble zinc salt and of the complexing agent being introduced from the start of the polymerization step and at the very latest up to the said pressure reduction.