A subject-matter of the invention is novel polymer compositions for the manufacture of optical lenses with a high refractive index and the optical lenses, in particular ophthalmic lenses, obtained from these compositions. 
The term xe2x80x9coptical lens with a high refractive indexxe2x80x9d means an optical lens with a refractive index of greater than or equal to 1.55 and preferably greater than 1.57. 
The novel polymerizable compositions according to the invention are preferably of use in the manufacture of spectacle glasses. 
An ophthalmic lens with a high refractive index must have all the following characteristics:
a high transparency (transmission generally of greater than 85% and preferably of greater than or equal to 90%), with an absence of or optionally a very low light scattering,
a high Abbe number of greater than or equal to 30 and preferably of greater than or equal to 35, in order to avoid chromatic aberrations,
a low yellowing index and an absence of yellowing over time,
a good impact strength (in particular according to the CEN and FDA standards),
a good suitability for various treatments (shock-proof primer, anti-reflective or hard coating deposition, and the like) and in particular good suitability for colouring,
a glass transition temperature value preferably of greater than or equal to 80xc2x0 C. and better still of greater than 90xc2x0 C., preferably of between 90 and 110xc2x0 C.
In addition, the polymerizable compositions must also be easy to process industrially. 
In particular, it is desirable for the compositions to be able to be photopolymerized, this being because this type of very fast polymerization makes it possible to reduce the cycle times for the manufacture of the lenses. 
Another point to be taken into consideration is the number and the nature of the monomers used in the composition for the production of the organic glass. 
These monomers must be readily available commercially, in large amounts, at low costs and with uniform quality. 
On this latter point, monomers manufactured in industrial amounts exhibit substantially different characteristics, depending on the industrial manufacturing parameters, which can, to a certain extent, influence the properties of the final polymer and thus of the optical lens. 
It is therefore desirable for the polymerizable composition to comprise a reduced number of precursor monomers.  In addition, this limits the problems of storage and of logistics. 
Polymerizable compositions for the production of polymers with a high refractive index are already known in the state of the art. 
Patent Application EP 394,495 discloses polymerizable compositions based on 4,4xe2x80x2-bis-(methacryloylthio)diphenyl sulphide (denoted by BMTPS) and on another vinyl monomer copolymerizable with BMTPS and a polythiol represented by the general formula R(SH)n, in which R is a polyvalent organic group and n represents an integer equal to 2 or more. 
Thus, Example 27 of this document describes a polymerizable composition with three constituents comprising, as parts by weight, 5 parts of BMTPS, 4 parts of a diethoxybisphenol A diacrylate and 1 part of an aromatic polythiol of formula: 
The documents JP 61-23855, JP 61-23856 and JP 61-23858 disclose the production of spectacle glasses from compositions comprising 4 to 6 monomer constituents, including, in particular, BMTPS. 
Furthermore, these compositions comprise aromatic vinyl derivatives which are not suited to photopolymerization. 
Patent Application EP 609,454 relates to a process for the production of sulphur-comprising polymers with a high refractive index exhibiting a decreased colouring, a high transmission power, a resistance to yellowing (during ageing of the glass) and a low relative density. 
The polymerizable compositions disclosed in this document comprise from 0.1 to 7% by weight of at least one compound chosen from chain-transfer agents and compounds for scavenging free radicals. 
Mono- or polyfunctional mercapto compounds are mentioned among these compounds.  It is specified, in this application, that, beyond 7% by weight, the mechanical properties of the polymer deteriorate or the excess compound escapes from the polymer. 
Patent Application FR 95 006443, on behalf of the Company Essilor, discloses polymerizable compositions for the manufacture of polymers with a high index comprising bis(2-methacryloylthioethyl) sulphide (denoted by BMTES) or a BMTES/BMTPS mixture with a derivative of the polyethoxybisphenol A polymethacrylate type and an aromatic or cyclanic mono(meth)acrylate monomer. 
In practice, the polymerizable compositions disclosed in this patent application comprise at least 4 to 6 polymerizable monomer constituents. 
Furthermore, although the polymerizable compositions disclosed in this application are satisfactory, they can still be improved, in particular with regard to the yellowing index and the behaviour towards ageing. 
An object of the invention is thus to provide simple polymerizable compositions with three base monomer consituents having improved properties with regard to the prior art. 
According to the invention, the polymerizable composition comprises, as base copolymerizable monomer constituents, at least 85% by weight, with respect to the total weight of the copolymerizable monomers present in the composition:
of a mixture M consisting, per 100 parts by weight of the mixture, of:
(A) 20 to 80 parts of one or more nonaromatic compounds (I), preferably a single nonaromatic compound (I), comprising two thio(meth)acrylate functional groups 
where R4=H or CH3, and
(B) 80 to 20 parts of a compound (II), or of a mixture of these compounds, of formula: 
in which R1 and R2 represent, independently of one another, H or CH3 and m+n has a mean value of 0 to 20 inclusive; and
3 to 30% by weight, with respect to the weight of the mixture M, of a polythiol, preferably 5 to 20%;
the composition comprising less than 5% by weight of other aromatic thio(meth)acrylate, aromatic or cyclanic mono(meth)acrylate and aromatic vinyl copolymerizable monomers. 
The compositions according to the invention preferably comprise 90%, more preferably 95%, better still 98% and, as far as possible, 100% by weight of the base copolymerizable monomer constituents. 
More preferably, the compositions according to the invention do not comprise di- and/or polyvinyl monomers.  In the present application, the term xe2x80x9cdi- and polyvinyl monomersxe2x80x9d is understood to mean monomers in which two or more CH2=CHxe2x80x94 groups are bonded directly to hydrocarbonaceous fragments of the monomers, such as, for example, di- and polyvinyl hydrocarbons, divinylbenzene and divinylnaphthalene. 
The polymerizable compositions according to the invention are preferably photopolymerizable compositions. 
The nonaromatic compounds (I) comprising two thio(meth)acrylate functional groups of the mixture M of the polymerizable compositions according to the invention are compounds which are known in the art. 
The compound (I) preferably corresponds to the formula: 
in which R3 represents a linear or branched divalent aliphatic hydrocarbonaceous radical which can optionally comprise, in its chain, one or more groups chosen from the divalent groups xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94 or 
preferably xe2x80x94Oxe2x80x94or xe2x80x94Sxe2x80x94, and better still the divalent group xe2x80x94Sxe2x80x94, and R4 represents H or CH3. 
Mention may be made, among monomers of formula (I) recommended in the present invention, of: 
S-2-acryloyloxyethyl thio(meth)acrylate, 
S-2-methacryloyloxyethyl thio(meth)acrylate, 
1,2-bis[(meth)acryloylthio]ethane, 
1,2-bis[(meth)acryloylthio]propane, 
1,3-bis[(meth)acryloylthio]propane, 
1,4-bis[(meth)acryloylthio]butane, 
bis[2-(meth)acryloylthioethyl] ether, 
bis[2-(meth)acryloylthioethyl] sulphide (BMTES), 
bis[2-(meth)acryloylthioethoxy]methane, 
bis[2-(meth)acryloylthioethylthio]methane, 
1,2-bis[2-(meth)acryloylthioethoxy]ethane, 
1,2-bis[2-(meth)acryloylthioethylthio]ethane, 
bis[2-(2-(meth)acryloylthioethoxy)ethyl] ether, 
bis[2-(2-(meth)acryloylthioethylthio)ethyl] sulphide, in which R4 is defined as above. 
The particularly recommended compound (I) is BMTES. 
The second monomer constituent of the mixture M is a compound of formula (II): 
in which R1 and R2 represent, independently of one another, H or CH3 and m+n has a mean value of 0 to 20 inclusive. 
The compound (II) is preferably a mixture of fractions having a mean value of m+n of greater than or equal to 4. 
These compounds (II) are well known in the art and can be obtained commercially. 
The third polymerizable monomer constituent of the compositions according to the present invention is a polythiol of formula R5(SH)n, in which n is an integer of 2 or more, preferably 2 to 5, and R5 is an aliphatic or aromatic radical. 
The polythiol compound of the compositions of the invention is preferably a dithiol or trithiol compound or alternatively a polythiol with a high Abbe number. 
These polythiol compounds are well known in the art and are disclosed, inter alia, in Document EP 394,495. 
Mention may be made, among dithiols of use in the present invention, of 9,10-anthracenedimethane-thiol, 1,11-undecanedithiol, 4-ethylbenzene-1,3-dithiol, 1,2-ethanedithiol, 1,8-octanedithiol, 1,18-octadecanedithiol, 2,5-dichlorobenzene-1,3-dithiol, 1,3-(4-chlorophenyl)propane-2,2-dithiol, 1,1-cyclohexanedithiol, 1,2-cyclohexanedithiol, 1,4-cyclohexanedithiol, 1,1-cycloheptanedithiol, 1,1-cyclopentanedithiol, 4,8-dithiaundecane-1,11-dithiol, dithiopentaerythritol, dithiothreitol, 1,3-diphenylpropane-2,2-dithiol, 1,3-dihydroxy-2-propyl 2xe2x80x2,3xe2x80x2-dimercaptopropyl ether, 2,3-dihydroxypropyl 2xe2x80x2,3xe2x80x2-dimercaptopropyl ether, 2,6-dimethyloctane-2,6-dithiol, 2,6-dimethyloctane-3,7-dithiol, 2,4-dimethylbenzene-1,3-dithiol, 4,5-dimethylbenzene-1,3-dithiol, 3,3-dimethylbutane-2,2-dithiol, 2,2-dimethylpropane-1,3-dithiol, 1,3-di(4-methoxy-phenyl)propane-2,2-dithiol, 3,4-dimethoxybutane-1,2-dithiol, 10,11-dimercaptoundecanoic acid, 6,8-dimercaptooctanoic acid, 2,5-dimercapto-1,3,4-thia-diazole, 2,2xe2x80x2-dimercaptobiphenyl, 4,4xe2x80x2-dimercapto-biphenyl, 4,4xe2x80x2-dimercaptobibenzyl, 3,4-dimercapto-butanol, 3,4-dimercaptobutyl acetate, 2,3-dimercapto-1-propanol, 1,2-dimercapto-1,3-butanediol, 2,3-di-mercaptopropionic acid, 1,2-dimercaptopropyl methyl ether, 2,3-dimercaptopropyl 2xe2x80x2,3xe2x80x2-dimethoxypropyl ether, 3,4-thiophenedithiol, 1,10-decanedithiol, 1,12-dodecanedithiol, 3,5,5-trimethylhexane-1,1-dithiol, 2,5-toluenedithiol, 3,4-toluenedithiol, 1,4-naphthalenedithiol, 1,5-naphthalenedithiol, 2,6-naphthalenedithiol, 1,9-nonanedithiol, norbornene-2,3-dithiol, bis(2-mercaptoisopropyl) ether, bis(11-mercaptoundecyl) sulphide, bis(2-mercaptoethyl) ether, bis(2-mercaptoethyl) sulphide, bis(18-mercapto-octadecyl) sulphide, bis(8-mercaptooctyl) sulphide, bis(12-mercaptodecyl) sulphide, bis(9-mercaptononyl) sulphide, bis(4-mercaptobutyl) sulphide, bis(3-mercaptopropyl) ether, bis(3-mercaptopropyl) sulphide, bis(6-mercaptohexyl) sulphide, bis(7-mercaptoheptyl) sulphide, bis(5-mercaptopentyl) sulphide, 2,2-bis(mercaptomethyl)acetic acid, 1,1-bis(mercaptomethyl)cyclohexane, bis(mercapto-methyl)durene, phenylmethane-1,1-dithiol, 1,2-butane-dithiol, 1,4-butanedithiol, 2,3-butanedithiol, 2,2-butanedithiol, 1,2-propanedithiol, 1,3-propane-dithiol, 2,2-propanedithiol, 1,2-hexanedithiol, 1,6-hexanedithiol, 2,5-hexanedithiol, 1,7-heptane-dithiol, 2,6-heptanedithiol, 1,5-pentanedithiol, 2,4-pentanedithiol, 3,3-pentanedithiol, 7,8-hepta-decanedithiol, 1,2-benzenedithiol, 1,3-benzenedithiol, 1,4-benzenedithiol, 2-methylcyclohexane-1,1-dithiol, 2-methylbutane-2,3-dithiol, ethylene glycol dithio-glycolate or ethylene glycol bis(3-mercaptopropionate).  Mention may be made, among trithiols, of 1,2,3-propanetrithiol, 1,2,4-butanetrithiol, trimethylolpropane trithioglycolate, trimethylolpropane tris(3-mercaptopropionate), pentaerythritol trithioglycolate, pentaerythritol tris(3-mercapto-propionate), 1,3,5-benzenetrithiol and 2,4,6-mesitylenetrithiol. 
Mention may also be made, among polythiols of use in the compositions of the present invention, of neopentanetetrathiol, 2,2-bis(mercaptomethyl)-1,3-propanedithiol, pentaerythritol tetrakis(3-mercapto-propionate), 1,3,5-benzenetrithiol, 2,4,6-toluene-trithiol, 2,4,6-methylenetrithiol and polythiols corresponding to the formulae: 
(HSCH2CH2COOCH2)3CC2H5 (TMTP), and 4-mercaptomethyl-3,6-dithia-1,8-octanedithiol. 
The preferred polythiols according to the present invention are ethylene glycol bis(thioglycolate), trimethylolpropane tris(3-mercaptopropionate) and pentaerythritol tetrakis(3-mercaptopropionate). 
As indicated above, the polymerizable composition according to the invention comprises 3 to 30% by weight of polythiol with respect to the total weight of the mixture M. The polythiol is preferably used at a concentration of 5 to 20% by weight, better still of greater than 7% by weight and even better still generally of between 10 and 15% by weight. 
The polymerizable compositions according to the invention can also comprise additives conventionally used in polymerizable compositions for the moulding of optical articles, in particular spectacle glasses, in conventional proportions, namely inhibitors, colorants, UV absorbers, fragrances, deodorants, antioxidants and anti-yellowing additives. 
Mention may be made, as preferred examples of antioxidizing agents, of triphenylphosphine (TPP) and Irganox(copyright) 1010 (pentaerythritol tetrakis[3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate]) (CG1010). 
Fragrances make it possible to mask the odour of the compositions, in particular during surface working operations. 
The compositions according to the invention generally comprise polymerization initiators, preferably photoinitiators or mixtures of photo-initiators and of thermal initiators, in a proportion of 0.001 to 5% by weight with respect to the total weight of polymerizable monomers present in the composition. 
Mention may in particular be made, among photoinitiators which can be used in the polymerizable compositions according to the invention, of 2,4,6-tri-methylbenzoyldiphenylphosphine oxide, 1-hydroxycyclo-hexyl phenyl ketone, 2,2-dimethoxy-1,2-diphenylethan-1-one, alkylbenzoyl ethers, the photoinitiator sold by the Company Ciba-Geigy under the name CGI 1700, which is a 25/75 mixture of a compound of formula: 
and of a compound of formula: 
and the photoinitiator CGI 1850 sold by the Company Ciba-Geigy, which is a mixture (50/50) (by weight) of compound (A) and of Irgacure(copyright) 184 of formula: 
As indicated above, the preferred polymerizable compositions according to the invention are photopolymerizable compositions comprising one or more photoinitiators. 
More preferably, the polymerizable compositions according to the invention are photo- and thermo-polymerizable compositions comprising both a polymerization photoinitiator and a polymerization thermal initiator. 
In particular, it has been found that, when the concentration of polythiol in the polymerizable composition according to the invention is of the order of approximately 10% by weight or more with respect to the total weight of the mixture M, it is preferable to carry out an initial photopolymerization for a period preferably varying from 1 to 5 minutes and then to subject the composition to a thermal polymerization at a temperature of generally between 60 and 130xc2x0 C. for a period of approximately 1 to 2 hours. 
Thermal polymerization initiators are compounds which are well known in the art and mention may be made, among them, of peroxides, such as benzoyl peroxide, cyclohexyl peroxydicarbonate or isopropyl peroxydicarbonate. 
In the following examples illustrating the present invention, all the parts and percentages, except when otherwise indicated, are expressed by weight. 
In these examples, the following abbreviations were used:
BMTES: bis[2-(methacryloylthio)ethyl] sulphide;
80 N: BPE-80N monomer, sold by the company Shin Nakamura, of formula: 
in which the mean value of m+n=2.6;
200 N: BPE-200N monomer from Shin Nakamura of formula (II) in which m+n has a mean value of 4.2;
D 121: monomer of formula (II) in which m+n has a mean value 4.8, supplied by the company Akzo;
EGTG: ethylene glycol bis(thioglycolate);
TMTP: trimethylolpropane tris(3-mercapto-propionate);
PETP: pentaerythritol tetrakis(3-mercapto-propionate);
TPP: triphenylphosphine;
UV 5411: 2-(2-hydroxy-5-(t-octyl)phenyl)benzotriazole (UV absorber);
CGI 1700: photoinitiator from the company Ciba-Geigy comprising the two compounds: 
with a ratio B/A 75/25. 
The yellowing indices of the polymers obtained by moulding and polymerization of the compositions of the examples were determined by the spectroscopic route.  For this spectroscopic route, use is made of a normalized method (ASTM D-1925-63) in which the yellowing index is defined by:       Yi    =                  (                              127.5            ⁢                          xe2x80x83                        ⁢            X                    -                      105.8            ⁢                          xe2x80x83                        ⁢            Z                          )            Y        ,
where X, Y and Z are the trichromatic coordinates of the sample, measured with a UV/visible spectrophotometer over the entire spectrum between 380 and 780 nm.  The Yi value is given before and after annealing the polymers. 
The higher the Yi value, the higher the yellowing index.