WO2021228858A1

WO2021228858A1 - Composition comprising at least two organic peroxides for the polymerization of ethylenically unsaturated monomers

Info

Publication number: WO2021228858A1
Application number: PCT/EP2021/062489
Authority: WO
Inventors: Xiaoqian Yu
Original assignee: Arkema France
Priority date: 2020-05-15
Filing date: 2021-05-11
Publication date: 2021-11-18
Also published as: FR3110169B1; CN115551901A; JP2023526257A; TW202202480A; FR3110169A1; EP4149981A1

Abstract

The present invention relates to a composition comprising from 5 to 40% by weight relative to the total weight of the composition of a first organic peroxide having a one hour half-life temperature comprised between 50°C and less than 85°C, the first organic peroxide being a peroxyester according to formula (I): wherein R and R' are the same or different and are selected from a linear or branched C₄-C₂₀ alkyl radical; and from 60 to 95% by weight relative to the total weight of the composition of a second organic peroxide having a one hour half-life temperature comprised between 85°C and 120°C for the radical polymerization of one or more ethylenically unsaturated monomers, in particular vinyl esters monomers, and preferably vinyl acetate. The invention also concerns the use of such a composition as initiator for the radical polymerization of one or more ethylenically unsaturated monomers and in particular vinyl esters monomers, preferably vinyl acetate.

Description

DESCRIPTION

TITLErComposition comprising at least two organic peroxides for the polymerization of ethylenically unsaturated monomers

The present invention relates to a composition comprising a first organic peroxide having a one hour half-life temperature comprised between 50°C and less than 85°C and a second organic peroxide having a one hour half-life temperature comprised between 85°C and 120°C.

The invention also concerns the use of such a composition as initiator for the radical polymerization of one or more ethylenically unsaturated monomers and in particular vinyl esters monomers, preferably vinyl acetate monomers.

Finally, the invention namely deals with a process for preparing a polyolefin, especially polyvinyl polymers such as polyvinyl acetate (PVA) and polyvinyl alcohol (PVOH) polymers, comprising at least one step of radical polymerization of one or more ethylenically unsaturated monomers in the presence of an effective amount of a composition as previously defined.

Polyvinyl alcohols (PVOH) are well known in commerce and widely used as protective colloid for the manufacture of polymer dispersions in emulsion, as thickening or stabilizing agent, as size in textile industries, in the manufacture of paper and as a starting material for synthetic fiber. Polyvinyl alcohols may also be used as adhesives, films, binding agents, and the like.

These polyolefins are traditionally manufactured in a two-step process.

The first step involves a free radical polymerization of vinyl acetate (VA) carried out in the presence of one or more initiators, such as organic peroxides, azo compounds such as azobisisobutyronitrile (AIBN), or a mixture thereof, in an organic solvent, advantageously monohydric aliphatic alcohols having 1 to 4 carbon atoms such as methyl alcohol, ethyl alcohol and the various isomeric types of propyl and butyl alcohol. Vinyl acetate monomers are polymerized at a temperature depending on the temperature of decomposition of the initiators (temperature at which the initiators generate free radicals), preferably ranging from 45°C and 130°C, until they are completely converted or, in many cases, converted only to a certain extent into polyvinyl acetate (PVA).

The second step implements a conversion of said polyvinyl acetate into polyvinyl alcohol (PVOH). The conversion may be performed by carrying out a hydrolysis, alcoholysis or saponification reaction of the polyvinyl acetate. Especially, when the polyvinyl acetate obtained with alkaline hydrolysis, is subjected to an alcoholysis reaction, polyvinyl alcohol and methyl acetate are obtained. The polyvinyl acetate (PVA) comprises in its structure at least one repeat unit corresponding to the following formula:

In the above formula, n represents the degree of polymerization. Once converted, the polyvinyl alcohol (PVOH) comprises in its structure at least one repeat unit corresponding to the following formula:

In the above formula, n represents the degree of polymerization. The degree of polymerization, the molecular weight and the conversion rate reached for both mentioned polymers mostly depend on the type and amount of initiators used during the polymerization of the vinyl acetate monomers. The tedious aspect of the two-stage process as previously described lies in the fact that the degree of polymerization, the molecular weight and the conversion rate of both polymers, polyvinyl acetate and polyvinyl alcohol, are completely intertwined. Indeed, when a high molecular weight polyvinyl alcohol is to be produced, a polyvinyl acetate having the corresponding degree of polymerization must be used and therefore obtained in the first step of the process. As a result, the production of a high molecular weight polyvinyl alcohol can turn out to be difficult if the conversion rate or the degree of polymerization of polyvinyl acetate is not satisfactory during the first step of the process.

In particular, AIBN is classically used to generate free radicals of 2-cyanopropyl, in order to initiate free radical polymerization of vinyl acetate monomers. AIBN has the advantage of being soluble in organic solvent, especially in monohydric aliphatic alcohols.

However, the presence of unreacted portion of AIBN in the final product may lead to the discoloration of the obtained polymer, especially to yellowing the obtained polyvinyl alcohol.

Moreover, tetramethylsuccinonitrile (TMSN), a decomposition by-product of AIBN, is highly toxic and remains in the final product which will impede the properties of the obtained polyvinyl alcohol.

Several attempts have already been developed in order to provide new and environmental friendly organic peroxides to replace AIBN, and more generally azo compounds, used as an initiator. Hence, in order to fulfill this purpose, peroxides, especially peroxyesters such as tert-butyl peroxypivalate sold under the name Luperox® 11M75 by Arkema, have been used to initiate the free radical polymerization of vinyl acetate monomers instead of AIBN, especially to produce low molecular weight polyvinyl alcohol. This kind of peroxides presents the benefit to be more reactive than AIBN, making it possible to consider using a lower amount of this compound, for example around 50-70% less than AIBN. Furthermore, tert-butyl peroxypivalate tends to decompose in smaller molecules than AIBN which are easier to remove from the final product. Nevertheless, the main flaw of tert-butyl peroxypivalate is the high energy of the generated free radicals, which may lead during the polymerization to polyvinyl acetate being branched and non-linear. More particularly, during the polymerization, a hydrogen atom from the alpha, beta-position or methyl carbon of the acetate group may react with another vinyl acetate monomer leading to the formation of several branch chains. Such branched polyvinyl acetate contains at least one repeat unit corresponding to the following formula:

C ^. o

CH_j

In the above formula, a, n and x represents integers, wherein n>l, x³l, and a>0, with a<n.

Accordingly if the polyvinyl acetate obtained at the end of the first step is branched and non-linear, it will also impede on the degree of polymerization of polyvinyl alcohol during the conversion reaction in the second step of the process. Because of these sub-reactions, the degree of polymerization of PVOH decreases and is even lower than the one obtained with AIBN.

It means that the tert-butyl peroxypivalate is less satisfactory than AIBN in terms of degree of polymerization, conversion rate and therefore obtained molecular weight.

In addition, further attempts have also been developed with other peroxyesters, especially tert-butylperoxy-2-ethylhexanoate sold under the name Luperox® 26 and tert-amyl peroxy-2-ethylhexanoate sold under the name Luperox® 575. However, the conversion rate of vinyl acetate monomers to polyvinyl acetate will be lower with those compounds.

As a result, it remains very difficult to replace AIBN as an initiator with peroxyesters such as Luperox® 11M75, Luperox® 26 or Luperox® 575 in order to produce polyolefins, especially polyvinyl acetate and polyvinyl alcohol, having a high degree of polymerization and a high molecular weight.

Furthermore, it is worth noticing that the above drawbacks encountered for the polymerization of vinyl acetate monomers also arises more broadly for the radical polymerization of ethylenically unsaturated monomers, especially vinyl monomers such as vinyl esters monomers and (meth)acrylic acid esters monomers.

Therefore, there is a real need to provide compositions which are able to reduce the side products while maintaining the degree of polymerization, the molecular weight and/or the conversion rate of a polyolefin obtained by free radical polymerization of one or more ethylenically unsaturated monomers, especially of vinyl monomers and more preferably vinyl esters monomers and (meth)acrylic acid esters monomers, as compared to the azo-initiators of the prior art.

The present invention relates to a composition comprising: from 5 to 40% by weight relative to the total weight of the composition of a first organic peroxide having a one hour half-life temperature comprised between 50°C and less than 85°C, more preferably comprised between 54°C and 77°C and even more preferably comprised between 60°C and 70°C, the first organic peroxide being a peroxyester according to formula (I):

R - C — O — O — R'

1!

° (I) wherein R and R’ are the same or different and are selected from a linear or branched C4-C20 alkyl radical; and from 60 to 95% by weight relative to the total weight of the composition of a second organic peroxide having a one hour half-life temperature comprised between 85°C and 120°C, more preferably comprised between 88°C and 120°C and even more preferably comprised between 90°C and 95°C.

The composition as previously defined allows a conversion kinetic of polyolefins, especially vinyl polymers, obtained by free radical polymerization of one or more ethylenically unsaturated monomers, comparable with the one obtained with AIBN, without the drawbacks associated to its use.

Furthermore, the composition of the invention does not lead to the colouration of vinyl polymers, in particular polyvinyl alcohols, especially to their yellowing, unlike the use of AIBN. Indeed, the decomposition of peroxyesters of the invention, leads to small molecules which may be easily eliminated.

Especially, the decomposition of peroxyesters used in the composition generates small molecules that are less toxic than TMSN during the free radical polymerization of ethylenically unsaturated monomers.

Preferably, the composition according to the invention is liquid at ambient temperature and highly soluble in the organic solvent used during the free radical polymerization of one or more ethylenically unsaturated monomers, especially of vinyl monomers and more preferably vinyl esters monomer. In particular, the composition is more soluble in the organic solvent than AIBN which will ease its elimination from the final product.

Another object of the present invention relates to the use of the composition as previously defined as initiator for the radical polymerization of one or more ethylenically unsaturated monomers, in particular vinyl monomers, preferably vinyl esters monomers, even more preferably vinyl acetate.

The ethylenically unsaturated monomer(s) suitable to undergo the radical polymerization may be identical or different in order to manufacture a polyolefin.

It means that the term “polymerization” encompasses both homo- and copolymerization of one or more ethylenically unsaturated monomers. The present invention also deals with a process for preparing a polyolefin, especially polyvinyl polymers such as polyvinyl acetate (PVA) and polyvinyl alcohol (PVOH), comprising at least one step of radical polymerization of one or more ethylenically unsaturated monomers, in particular vinyl monomers, in the presence of an effective amount of a composition as previously defined.

The process according to the invention enables to lead to polyolefins, especially polyvinyl acetate (PVA) and polyvinyl alcohol (PVOH), having either a low molecular or a high molecular weight.

In particular, the process is able to lead to high molecular weight polyolefins, polyvinyl acetate (PVA) and polyvinyl alcohol (PVOH), as efficiently as azo compounds such as AIBN. The process is also able to lead to low molecular weight polyolefins, especially polyvinyl acetate (PVA) and polyvinyl alcohol (PVOH), as efficiently as azo compounds such as AIBN.

Other subjects and characteristics, aspects and advantages of the invention will emerge even more clearly on reading the description and the example that follows.

In the text herein below, and unless otherwise indicated, the limits of a range of values are included in that range, in particular in the expressions “between” and “ranging from ... to ... ”.

Moreover, the expression “at least one” used in the present description is equivalent to the expression “one or more”.

In the text herein below, the terms “perester” and “peroxyester” are equivalent.

Composition

The composition of the present invention comprises at least two organic peroxides having: a one hour half-life temperature comprised between 50°C and less than 85°C, more preferably comprised between 54°C and 77°C and even more preferably comprised between 60°C and 70°C, and a one hour half-life temperature comprised between 85°C and 120°C, more preferably comprised between 88°C and 120°C and even more preferably comprised between 90°C and 95°C respectively.

The “one hour half life temperature” is the temperature at which half of the peroxide has decomposed in one hour. In other words, it is the temperature at which a loss of one-half of the peroxide’s active oxygen content occurs after 1 hour. Classically, the half-life temperature at 1 hour of the organic peroxides is determined by measuring the decomposition rate in n- decane or n-dodecane.

The first organic peroxide having a one hour half-life temperature comprised between 50°C and less than 85°C, is a peroxyester according to formula (I):

wherein R and R’ are the same or different and are selected from a linear or branched C4-C20 alkyl radical.

In other words, R and R’ may represent, independently of each other, a linear or branched C4-C20 alkyl radical.

Especially, R and R’ may represent, independently of each other, a linear or branched C4-C20 alkyl radical wherein R contains a higher number of carbon atoms than R’ .

Preferably, R and R’ represent, independently of each other, a branched C4-C20 alkyl radical.

Advantageously, R represents a branched C4-C10 alkyl radical, preferably C7-C9 alkyl radical and R’ represents a branched C4-C9 alkyl radical_, preferably C4-C5 alkyl radical.

More preferably, R represents a branched C₆-Cs alkyl radical, more preferably a C7 alkyl radical and R’ represents a branched C4-C5 alkyl radical. According to a preferred embodiment, R represents a branched

C9 alkyl radical and R’ represents a branched C4 alkyl radical.

Preferably, the first organic peroxide is selected from the group consisting of 1, 1,3,3-tetramethylbutylperoxyneodecanoate (sold under the name Luperox® 810), tert-amylperoxyneodecanoate (sold under the name Luperox® 546), tert-butylperoxyneodecanoate (sold under the name Luperox® 10), 1, 1,3,3-tetramethylbutylperoxypivalate, tert- butylperoxyneoheptanoate (sold under the name Luperox® 701), tert- amylperoxypivalate (sold under the name Luperox® 554), tert- butylperoxypivalate (sold under the name Luperox® 11) and mixtures thereof, more preferably, the first organic peroxide is at least one tert- butylperoxyneodecanoate.

The first organic peroxide is present in an amount ranging from 5 to 40% by weight, preferably from 10 to 30% by weight, more preferably from 15 to 25% by weight relative to the total weight of said composition.

Preferably, the second organic peroxide having a one hour half- life temperature comprised between 85°C and 120°C is a peroxyester, preferably a peroxyester according to formula (II):

Ri— C— O— O— R₂

O (P) wherein Ri and R₂ are the same or different and are selected from a linear or branched C4-C20 alkyl radical.

In other words, Ri and R₂ may represent, independently of each other, a linear or branched C4-C20 alkyl radical.

Especially, Ri and R₂ may represent, independently of each other, a linear or branched C₄-C₂₀ alkyl radical wherein R contains a higher number of carbon atoms than R₂.

Preferably, Ri and R₂ represent, independently of each other, a branched C4-C20 alkyl radical.

Advantageously, Ri represents a branched C4-C10 alkyl radical, preferably C₆-Cs alkyl radical and R2 represents a branched C4-C9 alkyl radical_, preferably a branched C4-C5 alkyl radical, even preferably a branched C5 alkyl radical.

More preferably, Ri represents a branched C₆-Cs alkyl radical, more preferably a C7 alkyl radical and R2 represents a branched C4-C5 alkyl radical.

According to a preferred embodiment, Ri represents a branched C7 alkyl radical and R2 represents a branched C5 alkyl radical.

Especially, the second organic peroxide may be selected from the group consisting of 1, l,3,3-tetramethylbutylperoxy-2- ethylhexanoate (sold under the name Luperox® 826), tert-amylperoxy- 2-ethylhexanoate (sold under the name Luperox® 575), tert- butylperoxy-2-ethylhexanoate (sold under the name Luperox 26®), tert- butylperoxydiethylacetate, tert-butylperoxyisobutyrate (sold under the name Luperox® 80), tert-amylperoxy -3,5,5-trimethylhexanoate (sold under the name Luperox® 570) and mixtures thereof, preferably is selected in the group consisting of tert-amyl peroxy-2-ethylhexanoate, tert-butyl peroxy-2-ethylhexanoate and mixtures thereof. More preferably, the second organic peroxide is at least one tert-amyl peroxy- 2-ethylhexanoate.

The second organic peroxide is present in an amount ranging from 60 to 95% by weight, preferably from 70 to 90% by weight, more preferably from 75 to 85% by weight relative to the total weight of said composition.

Preferably the weight ratio between the first organic peroxide and the second organic peroxide, especially between tert- butylperoxyneodecanoate and tert-amylperoxy-2-ethylhexanoate ranges from 5 :95 to 40:60, preferably from 10:90 to 30:70, more preferably from 15 :85 to 20:80.

Preferably, the first organic peroxide is tert- butylperoxyneodecanoate and the second organic peroxide is tert-amyl peroxy-2-ethylhexanoate.

The composition of the present invention may also comprise at least one additive, in particular a mineral oil.

Preferably, the composition comprises less than 20% by weight of additive(s) relative to the weight of the composition.

Preferably, the composition comprises less than 10% by weight of additive relative to the weight of the composition.

According to an embodiment, the composition does not contain any additional additives, meaning that it consists of the two organic peroxides of as previously defined.

Process

Another object of the present invention concerns a process for preparing a polyolefin comprising at least one step a) of radical polymerization of one or more ethylenically unsaturated monomers, in particular vinyl monomers, in the presence of an effective amount of the composition as previously defined. The radical polymerisation step may be performed under the conditions known classically depending on the monomers involved.

It means that the radical polymerisation step may be carried out in emulsion, suspension, bulk or solution. Especially when the polyolefin is chosen among (meth)acrylic polymers, its corresponding radical polymerisation may be carried out in emulsion, suspension, bulk or solution.

The composition as previously defined can be added to the ethylenically unsaturated monomers in batches or continuously. The radical polymerization step may be performed at a temperature ranging from 45°C to 130°C, preferably from 50°C to 90°C, preferably from 60°C to 80°C, even more preferably from 65°C to 75°C.

The radical polymerization step may be performed in the presence of at least one organic solvent which may be chosen among the group consisting of monohydric aliphatic alcohols having 1 to 4 carbon atoms such as methanol, ethanol, acetone, isopropyl alcohol, methyl acetate, ethyl acetate and mixture thereof.

Preferably, the organic solvent comprises methanol, more preferably consists of methanol. The radical polymerization step may be performed in the presence of at least one additive, preferably chosen from the group consisting of aldehyde, in particular acetaldehyde, butyraldehyde and valeraldehyde, mercaptan, in particular dodecyl mercaptan, methyl mercaptan and ethyl mercaptan, alcohol, in particular ethanol, n-butyl alcohol, isopropyl alcohol and vinylether, in particular vinyl ether and vinyl polyoxyethylene ether.

Preferably, the said at least additive represents less than 20 % by weight, relative to the total weight of the ethylenically unsaturated monomer(s), more preferably less than 10 % by weight. The amount of the composition introduced to initiate the radical polymerization is preferably ranging from 0.001 to 5% by weight, preferably from 0.01 to 2.5% by weight, more preferably from 0.01 to 0.8% by weight, and even more preferably from 0.015 to 0.5% by weight, relative to the total weight of the ethylenically unsaturated monomer(s).

The ethylenically unsaturated monomer(s) used in the process of the present invention is (are) chosen in the group consisting of vinyl monomers, especially (meth)acrylic acid esters monomers such as methyl acrylate and methyl methacrylate, styrene monomers, vinyl esters monomers and mixture thereof, more preferably in the group consisting of vinyl esters of carboxylic acids saturated monomers such as vinyl acetate or vinyl propionate, and even more preferably is vinyl acetate.

Preferably, the ethylenically unsaturated monomers correspond to vinyl monomers, in particular vinyl esters monomers, preferably vinyl acetate monomers. The vinyl monomers can be chosen in the group consisting of vinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate, vinyl isobutyrate, vinyl pivalate, vinyl acetate, vinyl caprylate, vinyl laurate, vinyl stearate and benzyl acetate, vinyl acetate and mixtures thereof, more preferably vinyl acetate or vinyl propionate, and even more preferably is vinyl acetate.

Accordingly, the polyolefin obtained with the process of the present invention is chosen among the group consisting of polyvinyl polymers and more preferably (meth)acrylic polymers such as (meth)acrylic homopolymers, acrylic/methacrylic styrene copolymer, (meth)acrylic/vinyl acetate and (meth)acrylic/vinyl acetate / styrene copolymers, polyvinyl acetate and polyvinyl alcohol.

Even more preferably, the polyolefin obtained with the process of the present invention is chosen among the group consisting of polyvinyl acetate and polyvinyl alcohol. More particularly, the process leads to polyvinyl acetate and polyvinyl alcohol having a degree of polymerization higher than 500, preferably between 500 to 3000, more preferably from 1100 to 2000.

More particularly, the process leads to polyvinyl alcohol having a degree of hydrolysis from 60% to 99,9%, more preferably from 90% to 99,9%, even more preferably from 96% to 99,9%.

The degree of polymerisation and of hydrolysis may be evaluated according to the standard ISO 15023-2:2003.

According to a preferred embodiment, the process according to the present invention is a process for preparing a polyvinyl polymer comprising at least one step a) of radical polymerization of one or more vinyl monomers, preferably vinyl esters monomers, especially vinyl acetate. The present invention is also relative to a process for preparing a polyvinyl alcohol comprising a step b) of conversion of said polymeric vinyl ester into polyvinyl alcohol obtained after step a).

In other words, the process for obtaining a polyvinyl alcohol is at least a two-step process, wherein the first step is a radical polymerization of one or more vinyl ester monomers as previously described and the second step is the conversion step of the polymeric vinyl ester obtained in the first step into the polyvinyl alcohol.

According to this embodiment, the polymeric vinyl ester obtained in the first step is preferably polyvinyl acetate.

At the end of the radical polymerization step a), a step a’) of removing the eventual presence of residues and the organic solvent may be performed before the step b) of conversion.

The step b) of conversion may performed by carrying out a hydrolysis, alcoholysis or saponification reaction of the polymeric vinyl ester.

Preferably, the conversion step is performed by carrying out an alcoholysis reaction of the polymeric vinyl ester.

The alcoholysis reaction is preferably performed in the presence of methanol in order to obtain said polyvinyl alcohol and methyl acetate.

The alcoholysis reaction may be performed in the presence of a alkaline catalyst or an acid catalyst.

The alkaline catalyst is preferably chosen among the group consisting of sodium hydroxide, potassium hydroxide, sodium methylate, potassium methylate and the like.

The acid catalyst is preferably chosen among the group consisting of organic and inorganic sulfonic acid, preferably is an organic sulfonic acid.

Preferably, the organic sulfonic acid is chosen from the group consisting of benzene sulfonic acid, p-toluene sulfonic acid, alkyl sulfonic acid, 2-methyl-5-propyl benzene sulfonic acid, 1,5-naphthalene sulfonic acid, 2,7-benzene disulfonic acid, methane sulfonic acid, ethane sulfonic acid, butyl sulfonic acid, octyl sulfonic acid.

Step b) of conversion may performed at a temperature comprised between 20°C and 50°C, preferably between 30°C and 40°C.

Use The present invention also deals with the use of the composition as previously defined as an initiator for the radical polymerization of one or more ethylenically unsaturated monomers, in particular vinyl monomers, preferably chosen among the vinyl esters monomers, and even more preferably vinyl acetate.

Product-bv-process The present invention also relates to polyolefin, in particular a polyvinyl acetate or a polyvinyl alcohol, obtainable by the process as previously defined.

Preferably, said polyolefin has a degree of polymerization higher than 500, preferably from 500 to 3000, more preferably from 1100 to 2000.

Preferably, said polyolefin has a degree of of hydrolysis from 60% to 99,9%, more preferably from 90% to 99,9%, even more preferably from 96% to 99,9%.

The examples below are given as illustrations of the present invention.

EXAMPLES F

The protocol described hereinafter for obtaining polyvinyl alcohol (see part II) has been carried out with the following initiators:

[Table 1]

II. Synthesis of polyvinyl acetate and conversion to polyvinyl alcohol 210 grams of vinyl acetate monomer from Celanese and 90 grams of methanol are introduced in a 2L glass reactor. The temperature of the reactor is then increased up to 70°C.

A certain amount for each initiator (see part I) is introduced in the reactor in order to initiate the radical polymerization. The reaction is kept for 4 hours at a temperature of 70°C.

After a period of time of 4 hours, a sample of 5 grams is collected and accurately weighted for conversion calculation.

The sample is put in a vacuum oven at 90°C for a period of time of 12 hours to remove residues and methyl alcohol. The convertion rate of the polymerization is then calculated.

After calculation, 100 grams of polyvinyl acetate are left in the reactor.

The molecular weight of polyvinyl acetate is analyzed by gel permeation chromatography (GPC) with THF used as a solvent. Polystyrene is used as reference for time.

An alcoholysis step is then carried out at 35°C by initially dissolving polyvinyl acetate in methanol and sodium hydroxide in order to convert said polyvinyl acetate into polyvinyl alcohol.

Polyvinyl alcohol is then extracted by methanol to remove salt residues and dried under vacuum at a temperature around 45°C for 8 hours.

The molecular weight of polyvinyl alcohol is analyzed by gel permeation chromatography (GPC) with water used as fluid. PEG is standard.

III. Conversion rate of polyvinyl acetate

Table 2 indicates the conversion rate in percent at 120, 180, 240 and 300 minutes of polyvinyl acetate for the initiators mentioned in Table 1. [Table 2

The compositions of the invention permit to achieve very good conversion of polyvinyl acetate in similar time as with azo initiators, with a use level of initiator that is significantly lower. Such a kinetic is not observed with peroxyesters used alone.

Furthermore, it can be noticed that the conversion of polyvinyl acetate using such compositions proceeds in a much more linear manner than with tert-butyl peroxypivalate alone (see comparative example 3). This can be of advantage to avoid runaway conditions, and the more constant need for cooling capacity. This thus allows for potentially higher load of the reactors or higher polymerisation temperature and thus faster kinetics.

Claims

1. Composition comprising:

- from 5 to 40% by weight relative to the total weight of the composition of a first organic peroxide having a one hour half-life temperature comprised between 50°C and less than 85°C, more preferably comprised between 54°C and 77°C and even more preferably comprised between 60°C and 70°C, the first organic peroxide being a peroxyester according to formula (I):

wherein R and R’ are the same or different and are selected from a linear or branched C4-C20 alkyl radical; and

- from 60 to 95% by weight relative to the total weight of the composition of a second organic peroxide having a one hour half-life temperature comprised between 85°C and 120°C, more preferably comprised between 88°C and 120°C and even more preferably comprised between 90°C and 95°C.

2. Composition according to claim 1, wherein R represents a branched C4-C10 alkyl radical, preferably C7-C9 alkyl radical and R’ represents a branched C4-C9 alkyl radical_, preferably C4-C5 alkyl radical. 3. Composition according to any one of the preceding claims, wherein the first organic peroxide is selected from the group consisting of 1, 1,3,3-tetramethylbutylperoxyneodecanoate, tert- amylperoxyneodecanoate, tert-butylperoxyneodecanoate, 1, 1,3,

3- tetramethylbutylperoxypivalate, tert-butylperoxyneoheptanoate, tert- amylperoxypivalate, tert-butylperoxypivalate and mixtures thereof, preferably the first organic peroxide is at least one tert- butylperoxyneodecanoate.

4. Composition according to any one of the preceding claims, characterized in that the first organic peroxide is present in an amount ranging from 10 to 30% by weight, more preferably from 15 to 25% by weight relative to the total weight of said composition.

5. Composition according to any one of the preceding claims, characterized in that the second organic peroxide having a one hour half- life temperature comprised between 85°C and 120°C is a peroxyester, preferably a peroxyester according to formula (II):

Ri— C— O— O— R₂ o (P) wherein Ri and R2 are the same or different and are selected from a linear or branched C4-C20 alkyl radical.

6. Composition according to any one of the preceding claims, characterized in that Ri represents a branched C4-C10 alkyl radical, preferably C₆-Cs alkyl radical and R2 represents a branched C4-C9 alkyl radical_, preferably a branched C4-C5 alkyl radical, even preferably a branched C5 alkyl radical.

7. Composition according to any one of the preceding claims, characterized in that the second organic peroxide is present in an amount ranging from 70 to 90% by weight, more preferably from 75 to 85% by weight relative to the total weight of said composition.

8. Composition according to any one of the preceding claims, characterized in that the second organic peroxide is selected from the group consisting of 1, l,3,3-tetramethylbutylperoxy-2-ethylhexanoate, tert-amylperoxy-2-ethylhexanoate, tert-butylperoxy-2-ethylhexanoate, tert-butylperoxy diethyl acetate, tert-butylperoxy isobutyrate, tert- amylperoxy -3,5,5-trimethylhexanoate and mixtures thereof, preferably is selected in the group consisting of tert-amyl peroxy-2- ethylhexanoate, tert-butyl peroxy-2-ethylhexanoate and mixtures thereof, more preferably is at least one tert-amyl peroxy-2- ethylhexanoate.

9. Process for preparing a polyvinyl polymer comprising at least one step a) of radical polymerization of one or more vinyl monomers, in the presence of an effective amount of a composition as defined according to any one of Claims 1 to 8.

10. Process according to Claim 9, characterized in that the radical polymerization step is performed at a temperature ranging from 45°C to 130°C, preferably from 50°C to 90°C, preferably from 60°C to 80°C, even more preferably from 65°C to 75°C.

11. Process according to any of Claim 9 or 10, characterized in that the radical polymerization step is performed in the presence of at least one organic solvent which may be chosen among the group consisting of monohydric aliphatic alcohols having 1 to 4 carbon atoms such as methanol, ethanol, acetone, isopropyl alcohol, methyl acetate, ethyl acetate and mixture thereof.

12. Process according to any of Claims 9 to 11, characterized in that the vinyl monomers, are vinyl esters monomers, preferably vinyl acetate monomers.

13. Process according to any of Claims 9 to 12 for preparing a polyvinyl alcohol comprising a step b) of conversion of said polymeric vinyl ester into polyvinyl alcohol obtained after step a).

14. Process according to Claim 13, wherein the step b) of conversion is performed by carrying out a hydrolysis, alcoholysis or saponification reaction of the polymeric vinyl ester.

15. Use of a composition according to any of Claims 1 to 8 as initiator for the radical polymerization of one or more vinyl monomers, more preferably vinyl esters monomers, and even more preferably vinyl acetate monomers.

16. Polyvinyl polymer obtainable by the process as defined in any of claims 9 to 14, preferably having a degree of polymerization higher than 500, preferably from 500 to 3000, more preferably from 1100 to 2000.

17. Polyvinyl polymer according to claim 16, having a degree of hydrolysis from 60% to 99,9%, more preferably from 90% to 99,9%, even more preferably from 96% to 99,9%.