Tribromoneopentyl (meth) acrylate based copolymers and lenses made therefrom

A plastic lens is potentially advantageous over a glass lens in view of its reduced weight and relative safety. These properties render such lenses particularly suitable for use in the manufacture of spectacles, safety glasses and other similar optical devices. Numerous attempts were reported in the art for preparing plastic lenses. These attempts met with the difficulty of combining together the desired optical and mechanical properties that need to be satisfied by the lens, e.g., transparency, low dispersion (expressed by high Abbe number) , high refractive index, surface hardness and high impact resistance.

Tribromoneopentyl acrylate (chemical name: 3-bromo-2,2- bis (bromomethyl) propyl acrylate) and tribromoneopentyl methacrylate (chemical name: 3-bromo-2,2- bis (bromomethyl) propyl methacrylate), which, independently or collectively, are sometimes referred to herein as tribromoneopentyl (meth) acrylate, are represented by the structure of formula I :

CH ₂ Br wherein R is hydrogen or methyl, respectively. The aforementioned compounds were suggested in the art as monomers for the preparation of plastic lenses.

US 4,578,445 teaches that the aforementioned monomers and structurally similar compounds need to be copolymerized with a halogen-substituted aromatic monomer, wherein an oxygen atom is attached to the aromatic nucleus of said monomer, in order to arrive at a suitable lens material .

JP 62-080602 discloses various lenses containing tribromoneopentyl (meth) acrylate . The specifically illustrated polymers containing the aforementioned compounds were as follows (described in terms of weight percent of the relevant monomers and cross-linking agents relative to the total weight of the polymer) :

(i) 60% tribromoneopentyl acrylate

25% dibromostyrene

15% divinylbenzene,

(ii) 75% tribromoneopentylmethacrylate

10% pentabromophenoxyethylacrylate

15% divinylbenzene

(iii) 60% tribromoneopentylmethacrylate

10% dibromostyrene

30% 2,2-bis (4-methacryloxyethoxy-3, 5-dibromophenyl) sulfone .

US 4,721,377 discloses a lens made of a polymeric composition containing 40 parts by weight of dibromostyrene, 20 parts by weight of divinylbenzene and 40 parts by weight of tribromoneopentyl acrylate. According to the description of the patent, increasing the amount of a vinyl monomer such as tribromoneopentyl (meth) acrylate is expected to

negatively alter the mechanical properties of the resulting polymer.

It has now been found that it is possible to copolymerize (i) tribromoneopentyl (meth) acrylate with (ii) one or more brominated aromatic monomers and (iii) one or more multifunctional acrylates in the presence of thermally- activated free radical initiator to form copolymers which may be suitably used for the production of lenses in view of their optical and mechanical properties.

Thus, in a first aspect, the present invention provides a copolymerizable mixture, comprising (i)one or more monomers selected from the group consisting of tribromoneopentyl acrylate and tribromoneopentyl methacrylate and (ii)one or more brominated aromatic monomers and (iii) one or more multi-functional acrylate compounds and a thermally- activated free radical initiator.

The brominated aromatic monomer is a compound having an aromatic nucleus to which one or more, and preferably three or more, bromine atoms are directly attached, which aromatic nucleus further carries a polymerizable group. The aromatic nucleus may be either a C ₅ _ ₇ ring (e.g., a benzene ring) or a fused aromatic system (e.g., naphthalene).

Especially preferred brominated aromatic monomers which have been found to be copolymerizable with tribromoneopentyl (meth) acrylate, and are hence suitable for use according to the present invention, are selected from the group consisting of pentabromobenzyl acrylate, pentabromobenzyl

methacrylate, tribromophenyl maleimide, brominated acrylolmethyl naphthalenes and mono, di, tri or tetra-bromo xylylyene diacrylate. These specific, preferred brominated aromatic monomers will now be described in more detail.

Pentabromobenzyl acrylate (chemical name: 2,3,4,5,6 pentabromobenzyl acrylate) and pentabromobenzyl methacrylate (chemical name: 2,3,4,5,6 pentabromobenzyl methacrylate), which, independently or collectively, are sometimes referred to herein as pentabromobenzyl (meth) acrylate, are represented by the structure of formula II-a:

Br Br

(II-a) wherein R is hydrogen or methyl, respectively. The synthesis of pentabromobenzyl (meth) acrylate is known in the art and may be carried out, for example, by procedures similar to those described in GB 1544022. These monomers are also commercially available (Israeli Chemicals Ltd.).

Tribromophenyl maleimide (chemical name: l-(2,4,6- tribromophenyl) - lH-Pyrrole-2, 5-dione) is represented by the structure of Formula II-b:

(II-b)

Tribromophenyl maleimide may be synthesized by reacting tribromoaniline with maleic anhydride as described, for example, in JP 9-194458. This monomer is also commercially available (Israel Chemicals Ltd.).

Brominated acrylolmethyl naphthalenes are represented by the structure of Formula II-c :

wherein R is hydrogen or methyl, n is an integer in the range of 0 to 4 and m is an integer in the range of 1 to 3. Especially preferred is the l-bromo-4-acrylolmethyl naphthalene derivative. This monomer may be obtained by brominating l-bromo-4-methyl naphthalene in an organic solvent using bromosuccinamide as the brominating agent, following which the l-bromo-4-bromomethylnapthalene intermediate thus obtained is isolated from the reaction mixture and esterified with acrylic acid to give the desired monomer, l-bromo-4-acrylolmethyl naphthalene. A detailed preparative procedure is given hereinbelow.

Another preferred class of brominated aromatic monomers that may be suitably copolymerized with tribromoneopentyl (meth) acrylate according to the present invention are brominated xylylene diacrylates of the following structure:

wherein R is hydrogen or methyl, n is an integer in the range of 1 to 4 and the two ester groups attached to the benzene ring may be in the ortho, meta or para relation. This class of monomers, of which tetrabromoxylylene (para) diacrylate is especially preferred, may be synthesized according to the procedures described in GB 1,544,022.

The term "second monomer" when used herein refers to the one or more brominated aromatic monomers (such as those described above) , in order to distinguish said monomers from the other monomeric components of the present invention.

The polymerizable mixture provided by the present invention also comprises at least one multi-functional acrylate compound. By the term 'multi-functional acrylate compound' is meant a compound carrying between two to six acrylate groups. It has been found that this class of compounds may serve as cross-linking agents in the tribromoneopentyl

(meth) acrylate and brominated aromatic monomer based copolymers of the present invention, said multi-functional acrylate compounds being capable of modifying the mechanical properties of the resulting copolymer without interfering with the copolymerization process and without diminishing the optical properties thereof. Particularly suitable cross- linking agents are selected from the group of epoxy acrylates, ethoxylated acrylates and urethane acrylates. Especially preferred are ethoxylated acrylates, which are compounds containing two or more structural units of the following formula:

wherein in each structural unit independently, p is an integer from 1 to 3. Commercially available ethoxylated acrylates that may be practiced according to the present invention are selected from the group consisting of diacrylates of ethoxylated bis phenol A such as SR601 and SR349 from Sartonαer.

The present invention also encompasses the polymerization product of the aforementioned monomers mixture, obtainable upon polymerizing a solution comprising tribromoneopentyl (meth) acrylate, one or more brominated aromatic monomers, at least one multi-functional acrylate compound and a thermally-activated free radical initiator, which is most preferably a peroxide initiator, such as benzoyl peroxide or lauroyl Peroxide.

Compositionally, the copolymers provided by the present invention preferably contain not less than 20%, and more preferably not less than 30%, and possibly not less than 50 % and even not less than 70% tribromoneopentyl (meth) acrylate, relative to the total weight of the polymer.

For example, it has been observed that the solubility of pentabromobenzyl (meth) acrylate in tribromoneopentyl

(meth) acrylate is limited to about 10% by weight at room temperature. However, copolymers having higher content of said pentabromobenzyl (meth) acrylate may be readily prepared according to the present invention, if desired, by heating the mixture comprising the first and second monomers to a temperature of about 30°C to 60°C, to achieve full dissolution. At this temperature it is possible to filter the solution to obtain a clear filtrate suitable for the polymerization. It should be noted that the solution thus prepared may be stored for a long time before polymerizing the same, if desired, since although the pentabromobenzyl

(meth) acrylate will gradually precipitate therefrom, optical clarity can be readily reestablished by gently warming the mixture. Thus, the weight concentration of pentabromobenzyl

(meth) acrylate in the polymers provided by the present invention is most preferably in the range of 5 to 60% (w/w relative to the total weight of the polymer) .

As to the other brominated monomers that are copolymerizable with tribromoneopentyl (meth) acrylate according to the invention, namely, those monomers represented by Formulas II-b, II-c and II-d depicted above, they may be used in an amount of from 2 to 30%, more preferably 5 to 15%, by weight

(relative to the total weight of the copolymer) .

The weight concentration of the multi-functional acrylate in the copolymer of the present invention is preferably in the range of about 20% to 40% (relative to the total weight of the copolymer) , and more preferably in the range of 20 to 30%.

The concentration of the thermally-activated free radical initiator in the polymerizable mixture is in the range of O.Olpph to 0.8pph, and preferably 0.03pph to O.βpph.

The polymerizable mixture according to the present invention may additionally include cross linkers that contain two or more vinyl functionalities, and especially divinylbenzene, in an amount of 1 to 50%, and preferably 5 to 20% by weight. Benzyl acrylate may also be used in an amount from 1 to 20%, and preferably 5 to 10% by weight.

Accordingly, the invention provides a copolymer which contains :

(i) structural units derived from tribromoneopentyl

(meth) acrylate;

(ii) structural units derived from at least one brominated aromatic compound, which is most preferably identified by Formulas II-a, II-b, II-c or II-d;

(iii) structural units derived from one or more ethoxylated acrylates ( as illustrated by Formula III) ; and

(iv) optionally structural units derived from divinylbenzene .

The polymers according to the present invention are obtained by mixing together the first monomer (that is, either tribromoneopentyl acrylate or tribromoneopentyl methacrylate, or a mixture thereof) , the second monomer (which is the brominated aromatic monomer, and more specifically, the monomer identified by Formulas II-a, II-b, II-c and II-d) , the multi-functional acrylate cross-linker and a thermally activated free radical initiator, and possibly also the optional components mentioned above (e.g., divinylbenzene and benzyl acrylate) to form a clear liquid composition, and polymerizing said liquid composition under heating. Although the various components may be introduced into the reaction vessel according to any desired order, in practice it is convenient to first dissolve the brominated aromatic monomer (s) either in the tribromoneopentyl (meth) acrylate or in divinylbenzene, when present, and then to introduce into the resulting solution the thermally activated free radical initiator, the multi-functional acrylate compound and any other component, as required.

Most suitably, the copolymerization is carried out as a cast copolymerization, wherein the liquid monomer mixture is placed in a suitable mold allowing the formation of the copolymer in a desired form (e.g., a lens) . The mold typically consists of curved glass plates separated by semirigid plastic spacer. The copolymerization may be carried out under an inert atmosphere, wherein the closed reaction vessel or mold is gradually heated to several distinct temperatures in the range of 30 to 100 ⁰ C, typically at intervals of about 5 to 3O ⁰ C, and is maintained at each of said temperatures for a period of about 2 to 48 hours. The reaction vessel is allowed to cool to room temperature,

following which the polymer is removed therefrom. An annealing period of 2 hours at 110- 120 ⁰ C is then applied to remove any strains from the polymeric mass after which the polymer is allowed to cool slowly to room temperature inside the oven .

The present invention further provides a copolymerization process, which comprises mixing one or more monomers selected from the group consisting of tribromoneopentyl acrylate and tribromoneopentyl methacrylate together with one or more brominated aromatic monomers (as defined for example by Formulas II-a, II-b, II-c and II-d above) and one or more multifunctional acrylate cross-linking agents to form a solution, and heating said solution in the presence of a thermally-activated free-radical initiator, whereby a copolymer is formed. According to a preferred embodiment, divinylbenzene is also present in the solution.

The resulting polymer is transparent, mechanically strong and may be easily polished. Accordingly, the polymers of the present invention may be suitably used in the preparation of, inter alia, lenses for various optical devices, such as spectacles, camera lenses, optical readers and the like.

Examples

All percentages, unless otherwise indicated, are weight percent.

Materials

Tribromoneopentyl acrylate (hereinafter sometimes abbreviated "Trinol acrylate" or simply "TA") was prepared by the esterification of acrylic acid with tribromoneopentyl

glycol (commercially available as Trinol from Israel Chemicals Industrial Products Ltd.) in toluene, in the presence of sulfuric acid as a catalyst. The resulting crude monomer was distilled at 150°C (0.1 mm Hg) and stabilized with 50ppm methoxy phenol. Alternatively, tribromoneopentyl acrylate may be prepared by the procedure described in US 3,480,600.

Tribromoneopentyl methacrylate (hereinafter sometimes abbreviated "TMA") is commercially available from Polysciences Inc.

Pentabromobenzyl acrylate (hereinafter sometimes abbreviated "FR 1025M") is commercially available from Israel Chemicals Ltd.

Tribromophenyl maleimide (hereinafter sometimes abbreviated "FR 1033M") is commercially available from Israel Chemicals Ltd.

l-bromo-4-acryloylmethyl naphthalene (Hereinafter sometimes abbreviate "BrNAc") was obtained according to Preparation A below.

Tetrabromo-xylylene diacrylate is commercially available from Israel Chemicals Ltd.

Acrylate Cross linkers were purchased from Sartomer: SR349 - ethoxylated(3)bisphenol A diacrylate SR601 - ethoxylated(4)bisphenol A diacrylate

Benzoyl peroxide, purchased from B. D. H., was recrystallized from methanol before use.

Lauroyl Peroxide (abbreviated "L. P.") _/ purum ≥ 95% CAS.: 105- 74-8, Fluka Cat.: 61740)

Divinylbenzene (abbreviated "DVB") _/ technical grade, 80%, mixture of isomers (CAS: 1321-74-0, Aldrich, Cat. 41,456-5)

Benzyl Acrylate CAS: 2495-35-4 (Polyscience Ltd., Cat.: 01997)

Prior to polymerization, the monomers were passed through a plug of basic alumina to remove traces of stabilizers.

The refractive index (RI) of the monomer solution prior to the polymerization was measured by an Abbe refractometer at the D-line wavelength (589 nm) .

Example 1

Copolymerization of tribromoneopentyl acrylate, pentabromobenzyl acrylate and ethoxylated bis-phenol A diacrylate

To 8g of a solution containing 90% tribromoneopentyl acrylate and 10% pentabromobenzyl acrylate in a glass vial, were added 2.Og of an ethoxylated bis-phenol A diacrylate (SR349-Sartomer) as a cross-linker and 50mg benzoyl peroxide as initiator. After blanketing with argon, the vial was closed and heated in an oven for 15h at 50 ⁰ C, followed by 2 days at 7O ⁰ C. The polymerization was completed by heating for a further 2 hours at 100 ⁰ C. After removal

from the vial, the disc was annealed for two hours at 120°C in an oven and then allowed to cool slowly to room temperature. A hard clear disc was obtained with a specific gravity of 1.6.

The monomer mixture prior to polymerization had a refractive index of 1.555.

Example 2

Copolymerization of tribromoneopentyl acrylate, pentabromobenzyl acrylate and ethoxylated bis-phenol A diacrylate

The procedure according to Example 1 was repeated, but the ethoxyated acrylate cross-linker used was SR601E (Sartomer) . A similar result was obtained. The monomer mixture prior to polymerization had a refractive index of 1.553.

Example 3

Copolymerization of tribromoneopentyl acrylate (or

Tribromoneopentyl methacrylate) , tribromophenyl maleimide and ethoxylated bis-phenol A diacrylate

1.5g FR1033 were dissolved in DVB 3g (30%) from which inhibitor had been removed by passage via basic alumina column, by heating to 60°C-70°C. A yellow slightly opaque solution was obtained. Lauroyl Peroxide, (0.3pph) was dissolved in the above solution and Trinol Acrylate 3.5g (35%) was then added. The solution was stirred for a few minutes. SR 349 2g, (20%) was added to the warm solution.

After stirring to homogenize for Ih, the solution was filtered via PTFE 0.45 μm filter using a syringe to apply pressure. The solution was introduced into a 3x3 cm glass mold via a small hole in the 3mm cork gasket using a syringe and needle. The filling hole was covered with a tape and maintained uppermost. The full mold was then placed upright in a forced air oven. The sample was heated at 50-55°C for 1 hour, 60°C 16-17 hours, 80°C 24 hours and 100°C for 2 hours. The polymer obtained was slowly cooled.

The composition and properties of the resulting polymer and of additional polymers prepared in a similar manner are given in Table 1.

Table 1

M: The monomer's mixture P: The copolymer

Preparation A

Synthesis of l-bromo-4-acryloylmethyl naphthalene

l-broπιo-4-methacryloylmethyl naphthalene

Bromination: To a 3L, three-necked flask, fitted with a dropping funnel, a mechanical stirrer, a nitrogen inlet and a reflux condenser, were added, 1-bromo-4-methyl naphthalene, 30Og (1.357 moles) (Acros) in l.δlitres chlorobenzene and 24βg N-bromosuccinamide (1.38 moles). The flask was heated on an oil bath to 60°C. Then AIBN initiator (5.4g in 200 ml chlorobenzene) was added dropwise during one hour while the temperature in the oil bath was gradually raised to 95 ⁰ C. The conversion to l-bromo-4- bromomethylnaphthalene required about 1.5 hours and was tracked by HPLC analysis. The nominal integrated peak area of the bromomethyl intermediate was greater than 90%. After cooling, the reaction products were washed twice with 1.5L water, once with a sodium bisulfite solution and again with water. After drying the organic phase over sodium sulfate, the chlorobenzene was removed by distillation under vacuum at 50°C.An off-white solid was obtained that after crystallization from methanol yielded 264g of the bromomethyl intermediate. (Yield 65 %, mp 105 ⁰ C) .

Esterification: To 9g of l-bromo-4-bromomethylnaphthalene in 37 ml chlorobenzene were added with vigorous mixing 2.2g of freshly distilled acrylic acid, 0.12g of tetrabutyl ammonium bromide and 4.6g of a 26% sodium hydroxide solution. The temperature was gradually raised from 75°C to 95 ⁰ C during six and a half hours. The product was worked up as described

above and crystallized from methanol to yield 5.5g of 1- bromo-4-acryloylmethyl naphthalene .

Heating the monomer in a DSC at 10°C/min, gave a melting point of 54 ⁰ C and a polymerization range of 220 - 28O ⁰ C.

The monomer was used in Example 4 as described below.

Example 4

Copolymerization of tribromoneopentyl acrylate,

Pentabromobenzyl acrylate, l-bromo-4-methacryloylmethyl naphthalene and ethoxylated bis-phenol A diacrylate

FR 1025M 0.56g (5.6 %) and BrNAc 1.44g (14.4%) were dissolved in DVB 1.25g (12.5 %) (after removal of stabilizer with basic alumina) by heating 60°C-70°C. A yellow opaque solution was obtained. Lauryl Peroxide 0.3pph, was dissolved in the solution obtained. Trinol Acrylate 3g (30%) and Benzyl Acrylate 1.25 g (12.5%) were added. The solution was stirred for some minutes. SR 349 2.5g (25% w/w) was then added to the warm solution. After stirring the solution for Ih to homogenize it, it was filtered via a PTFE 0.45 μm filter with the aid of a syringe. The solution was injected into 3cmx3cm glass plate mold with a 3mm cork spacer via a hole in the spacer as described in Example 3. The sample was heated in a thermostated oven at 50°C-55°C for 1 hour, 60 ⁰ C 16-17 hours, 80 ⁰ C 24 hours and 100 ⁰ C 2 hours. The polymer obtained was slowly cooled. The polymer was transparent. The refractive index and Abbe number were measured using an Abbe Refractometer at 2O ⁰ C and the results are shown in Table 2 below.

Table 2

Example 5

Copolymerization of tribromoneopentyl acrylate and tetrabronαo-4-xylyl- bis-acrylate

Using the same procedure as in Example 3, 3.5g tribromoneopentyl acrylate, tetrabromo-4-xylyl- bis-acrylate (1.5g -a laboratory sample from Israel Chemical Industries), SR 349, 2.Og, DVB, 3.Og and L. P. (0.3 pph) were dissolved in each other and homogenized. After polymerization they yielded a pale yellow copolymer with R.I. = 1.5993 and Abbe Number =40.2.