Abstract: Repulpable PSA compositions and constructions that exhibit high adhesive performance on a wide range of substrates are provided. In one embodiment, a PSA composition comprises a blend of two emulsion acrylic copolymers. The first type is a repulpable, emulsion acrylic copolymer that passes TAPPI UM 213. Preferably, the copolymer is prepared by sequential polymerization. The second type of copolymer is a non-repulpable, gpp or removable, repositionable, emulsion acrylic copolymer, also preferably prepared by sequential polymerization. The PSA compositions are inherently tacky and can be used in labels and tapes. In addition, they can be added to other PSAs as external tackifiers.

Abstract: This invention concerns comb-like branched, water-soluble cationic acrylamide copolymers, prepared from 1-40 mole % of a water-soluble cationic prepolymer backbone of formula I--[A--B--NR].sub.n -- Iwhere A is an unsaturated ammonium compound or acrylamide and B--NR-- is selected from the group consisting of olefinically unsaturated, functional amine-group-containing monomers, triethanolamine, and diethanolalkylamine, and the amount of B--NR is 1 mole % to 50 mole % and n represents a degree of polymerization which leads to molecular weights in the range from 1000 to 200,000 g/mole, and 60 to 99 mole % acrylamide by graft polymerization of the acrylamide onto the amine-group-containing cationic prepolymer backbone with a water-in-oil emulsion technique, and the comb-like branched acrylamide copolymers have a molecular weight of >1 million.

Abstract: The invention relates to a method of surface sizing cellulose based products such as paper, board and paper board. The method comprises applying to the surface of said products an aqueous dispersion of a copolymer obtainable by free-radical emulsion polymerization of a monomers mixture comprising at least one monomer selected from the group consisting of styrene and derivatives thereof; at least one monomer selected from the group consisting of esters of ethylenically unsaturated carboxylic acids and alkanols; and at least one monomer selected from the group consisting of ethylenically unsaturated carboxylic and sulfonic acids and salt thereof; optionally in combination with other ethylenically unsaturated copolymerizable monomers.

Abstract: Functionally gradient polymeric materials are formed by a process that utilizes an ascending polymerization front. A polymerization front is initiated in a reaction vessel containing a monomer solution. Additional monomers or resins are added on top of the polymerization front to maintain a substantially constant level. The composition of the monomers and/or additives are varied as they are added to the reaction vessel to form a functionally gradient polymeric material.

Abstract: The present invention is a free radical bulk polymerization process for the preparation of high molecular weight polymers from vinyl aromatic monomers, characterized in that the polymerization is conducted in the presence of a vinyl acid containing either a sulfur or phosphorus atom within the acid functional group.

Abstract: A process for making hydroxy-functional resins based on acrylate, hydroxyalkylacrylate, and allyl monomers is disclosed. The key to the process is to add the hydroxyalkylacrylate monomer in an amount and manner effective to produce a resin that contains, without vacuum stripping, less than about 1 wt. % of residual unreacted allyl monomers. Eliminating the need to vacuum strip the allyl monomer reduces process costs and enables the synthesis of new resins. The hydroxy-functional resins are readily crosslinked with melamines, polyisocyanates, or epoxies to give thermoset polymers.

Abstract: An improved process for making vinyl aromatic/allylic alcohol copolymers is disclosed. Gradual addition of a free-radical initiator during the course of the polymerization results in a substantial increase in the polymer yield obtained compared with the yield from conventional processes in which all of the free-radical initiator is charged initially. High yields are obtained when the monomer mole ratios are adjusted to produce styrene-allyl alcohol copolymers having a hydroxyl number in the 50-150 mg KOH/g range. Copolymers made by the process are valuable coating resins.

Abstract: A process of preparing an elastomeric copolymer composition of mono-vinyl aromatic hydrocarbon and conjugated diene is disclosed. The composition produced by this process has preferred properties, i.e. the product possesses the characteristics of enhancing ultimate elongation up to 900%, keeping tensile strength in a suitable range and enhancing transparency.

Abstract: The synthesis and characterization of novel linear polymers and multi-arm star polymers comprising polyisobutylene arms connected to a well-defined calixarene core are described. The polymers are directly telechelic. They synthesis has been achieved using the "core first" method wherein multifunctional calix?n!arene (where n=4 to 16) derivatives or their monofunctional analogues are used as initiators which, in conjunction with certain Freidel-Crafts acids as co-initiators, induce the living polymerization of isobutylene or a similar carbocationic polymerizable monomer. Novel initiators suitable for inducing the polymerization are also described.

Abstract: Monodisperse crosslinked polymers prepared by seed/feed polymerization have less tendency to tack when a microencapsulated monodisperse crosslinked polymer has been used as seed. Compared with the seed used, the seed/feed polymer obtained exhibits substantially less broadening of the particle size distribution when microencapsulated seed has been used.

Abstract: Process with two or more steps for the preparation in suspension of mixtures of ethylene-propylene (EPM) elastomer homopolymers or ethylene-propylene-diene (EPDM) elastomer terpolymers in the presence of a catalytic system essentially consisting of a compound of Vanadium and a cocatalyst essentially consisting of an Aluminum alkyl and in the presence of an activator, the above process being characterized in that:a) in the first step a first polymerization of the monomers is carried out in the presence of the Vanadium based catalyst;b) in the second or subsequent steps further polymerizable monomers and activator are added to the suspension of the polymer obtained in step (a).

Abstract: Process for preparing (co)polychloroprene rubber, comprising:a first step (a) in which a portion of a polymerizable composition is partially polymerized according to a batchwise process,a second step (b) in which the residual polymerizable composition is sequentially fed to the reaction mixture coming from the step (a),a possible third step (c) in which the reaction mixture coming from the step (b) is polymerized up to the desired conversion rate.

Abstract: An aqueous polymer dispersion is prepared by free radical aqueous emulsion polymerization of a mixture of monomers having at least one ethylenically unsaturated group, which comprises at least 30% by weight of monomers having two conjugated ethylenically unsaturated double bonds, by a process in which the polymerization temperature is varied in the course of the free radical aqueous emulsion polymerization.

Abstract: A free radical polymerization process for the preparation of a polymer comprising heating a mixture of a free radical initiator, a stable free radical agent, a metal catalyst, or a metal stable free radical complex catalyst, and at least one polymerizable monomer; cooling said mixture; and optionally isolating said polymer.

Abstract: A vinyl chloride polymer is prepared through suspension polymerization of a vinyl chloride monomer in the presence of a polymerization initiator in a reactor. The reactor is charged with up to 4/5 of the overall monomer charge to initiate polymerization. Polymerization is carried out under a certain saturated vapor pressure of P1 kgf/cm.sup.2 at a preset polymerization temperature. When the internal pressure of the reactor declines to P2 kgf/cm.sup.2, the remainder of the overall monomer charge is continuously fed to the reactor so as to maintain the internal pressure within the range of (P1-.DELTA.P) wherein .DELTA.P=P1-P2 ranges from 0.1 to 2.0 kgf/cm.sup.2. A vinyl chloride polymer having a high degree of polymerization is efficiently produced while alleviating the load of heat removal.

Abstract: This invention relates to block polymers containing both crystalline and elastomeric blocks, the block copolymer having an A block and a B block. The A block is a polyethylene or an ethylene polymer optionally containing an alpha-olefin and is generally crystalline. The B block contains an ethylene, .alpha.-olefin copolymer segment and optionally contain an ethylene, .alpha.-olefin, non-conjugated diene terpolymer segment. If the B block contains a diene containing segment, the diene containing segment is furthest from the junction of the A and the B blocks. Also disclosed is a process for manufacture of the block copolymers and methods for coupling the block copolymers. Coupled block copolymers are useful as thermoplastic elastomers exhibiting physical properties approaching those of crosslinked EP or EPDM elastomers, but showing thermal processability after coupling.

Abstract: An saturated nitrile-conjugated diene copolymer obtained by copolymerizing acrylonitrile with butadiene in the presence of a free radical initiator and an alkylthiol compound as a molecular weight modifier. A part of the monomers is incorporated in a monomer charge before polymerization, and the remainder thereof is added in a polymerization mixture during polymerization. The alkylthiol compound has 12 to 16 carbon atoms, which include at least three tertiary carbon atoms, and further has a sulfur atom directly bound to at least one of the tertiary carbon atoms. The copolymer has a Mooney viscosity of 15 to 450 and a bound nitrile content of 10 to 60% by weight. The breadth (.DELTA.AN) of compositional distribution of the unsaturated nitrile in the copolymer is 3 to 20. The copolymer is used in combination with a sulfur-containing vulcanizer.

Abstract: A method for producing branched polymerizates by radical polymerization of monomers in two stages comprisingin a first stage, adding 10 to 500 parts by weight of at least one monomer M.sub.1 and 0.1 to 20 parts by weight of a mercaptan with an olefinic, radically polymerizable double bond RM as a molecular weight regulator, and polymerizing said at least one monomer M.sub.1 mercaptan RM to form a copolymer MM.sub.1 in the presence of at least one polymerization initiator and 0 to 100 parts by weight of an inert, non-aqueous solvent, which completely dissolves the monomer M.sub.1, the mercaptan RM and the copolymer MM.sub.1, where the polymerizable double bonds of M.sub.1 and RM have a reactivity, expressed by a copolymerization parameter r, between 0.1 and 10, andin a second stage, adding 10 to 2000 parts by weight of a monomer M.sub.2 and 0.

Abstract: A process for the preparation of a synthetic resin dispersion stabilized by a protective colloid employing at least one initiator and based on a polymer of (a) a first monomer of at least one ester of acrylic acid or methacrylic acid and an alcohol containing 1 to 22 carbon atoms; and optionally, (b) a second monomer which can be copolymerized with the first monomers. The process comprises introducing at least 50% of the monomers based on the total weight of the monomers, at least 50% of the protective colloid based on the total weight of the protective colloid, and up to 50% of the initiator based on the total weight of the initiator into a reaction vessel to form an aqueous emulsion. Any remaining monomers and protective colloid and at least 50% of the initiator based on the total weight of the initiator are then added to the reaction vessel at a temperature of 30.degree. to 100.degree. C.

Abstract: An improved process for preparing core/shell MBS modifiers is disclosed, wherein by adding feeds of the monomers which form the second and/or third stage polymers prior to completion of polymerization of the earlier stages, a process with substantially reduced polymerization time results with no loss in properties of the product. The process further includes agglomeration of the rubbery polymer to increase particle size during the polymerization process.

Abstract: An improved gas-phase copolymerization process for producing copolymers of propylene and ethylene in multiple reactors using high activity Ti/Al catalysts is provided. The improvement consists of developing a control parameter derived from eight integral regions of the copolymer .sup.13 C NMR spectrum, defining the limits of the control parameter for the process, and operating the process to maintain the parameter within the defined limits. This is accomplished by adjusting the amount of ethylene in the primary and/or secondary reactor and adjusting other process variables as required.

Abstract: The preparation of terpolymers by free radical polymerization of an unsaturated dicarboxylic acid with an unsaturated monocarxylic acid and a further saturated monomer which contains no free carboxyl groups, in an aqueous medium, is carried out, according to the invention, in the presence of surfactant compounds.

Abstract: Low-molecular-weight, hydroxy-functional acrylate resins are disclosed. The resins are copolymers derived from an allylic alcohol or a propoxylated allylic alcohol, an acrylate or methacrylate monomer, and optionally, one or more additional ethylenic monomers. The resins are uniquely prepared without a chain-transfer agent or reaction solvent, and are useful in many thermoset polymer applications, including thermoset polyesters, polyurethanes, crosslinked polymeric resins, melamines, alkyds, uralkyds, and epoxy thermosets.

Abstract: An improved process for preparing core/shell MBS modifiers is disclosed, wherein by adding feeds of the monomers which form the second and/or third stage polymers prior to completion of polymerization of the earlier stages, a process with substantially reduced polymerization time results with no loss in properties of the product. The process further includes agglomeration of the rubbery polymer to increase particle size during the polymerization process.

Abstract: This invention discloses a process for preparing a latex which is particularly beneficial for utilization in manufacturing high performance masking tape, which includes the sequential steps of (1) terpolymerizing a vinyl aromatic monomer, an alkyl acrylate monomer, and an alkyl propenoic acid monomer in an aqueous polymerization medium by free radical polymerization to produce a seed polymer latex; (2) neutralizing the seed polymer latex to a pH of about 6 to about 9 by the addition of an alkali; (3) adding additional vinyl aromatic monomer and alkyl acrylate monomer to the neutralized seed polymer latex; and (4) allowing the additional vinyl aromatic monomer and alkyl acrylate monomer to polymerize in a second polymerization step to a solids content of about 30% to about 50% to produce the latex which is particularly beneficial for utilization in manufacturing high performance masking tape.

Abstract: An improved process for making vinyl aromatic/allylic alcohol copolymers is disclosed. Gradual addition of a free-radical initiator during the course of the polymerization results in a substantial increase in the polymer yield obtained compared with the yield from conventional processes in which all of the free-radical initiator is charged initially. The process is particularly useful for making commercially important varieties of styrene-allyl alcohol copolymers.

Abstract: This invention relates to a short residence time polymerization reactor operating in periodic mode with multiple catalyst feeds and staged monomer feeds operating to minimize off specification product and optimize and control molecular weight and molecular weight distribution of the polymer produced. This invention contemplates the polymerization of olefins particularly alpha-olefins, particularly ethylene and propylene into homo, co and terpolymers.

Abstract: The invention is directed toward polymerizing or copolymerizing alpha-olefins either alone or in combination with one or more other alpha-olefins in the presence of a metallocene catalyst in a gas phase reactor having a fluidized bed and a fluidizing medium such that the fluidizing medium entering the reactor comprises a gas and a liquid phase.

Abstract: An improved process for making vinyl aromatic/allylic alcohol copolymers is disclosed. Gradual addition of a free-radical initiator during the course of the polymerization results in a substantial increase in the polymer yield obtained compared with the yield from conventional processes in which all of the free-radical initiator is charged initially. The process is particularly useful for making commercially important varieties of styrene-allyl alcohol copolymers.

Abstract: Simultaneous control of the molecular weight and the molecular weight distribution in the production of toner resins can be accomplished by using a starve fed emulsion polymerization process, which includes: selecting a desired molecular weight and molecular weight distribution of the polymer resin; determining a starve fed fraction to achieve the desired molecular weight and molecular weight distribution; and charging a reaction vessel with a chain transfer agent and a first monomer emulsion, and polymerizing monomer in the first monomer emulsion while simultaneously adding to the reaction vessel additional monomer, in the form of a second monomer emulsion, and additional chain transfer agent. The resins thus produced may be used in dry toners, liquid developers and inks suitable, for example, for ink jet applications.

Abstract: The invention is directed toward polymerizing or copolymerizing alpha-olefins either alone or in combination with one or more other alpha-olefins in a gas phase reactor having a fluidized bed and a fluidizing medium such that the fluidizing medium entering the reactor comprises a gas and a liquid phase.

Abstract: Copolymers of ethylene with acrylates or acrylic acid are prepared by copolymerizing 100 parts by weight of ethylene with from 1.0 to 20.0 parts by weight of tert-butyl acrylate or tert-butyl methacrylate or acrylic acid or solutions of acrylic acid in acrylates in a continuously operated, tubular polymerization system at above 800 bar and at from 130.degree. to 320.degree. C. in the presence of a free radical polymerization initiator by feeding two gas streams into the polymerization system, by a process in which the first gas stream contains exclusively ethylene with a polymerization initiator and, if required, a polymerization regulator and the second gas stream contains a mixture of ethylene and tert-butyl acrylate or tert-butyl methacrylate or a mixture of ethylene with a solution of acrylic acid in acrylates and polymerization initiators and, if required, polymerization regulators.

Abstract: The invention is directed toward polymerizing or copolymerizing alpha-olefins either alone or in combination with one or more other alpha-olefins in a gas phase reactor having a fluidized bed and a fluidizing medium such that the fluidizing medium entering the reactor comprises a gas and a liquid phase.

Abstract: A process for the preparation of a random copolymer of at least one conjugated diene and at least one vinyl aromatic compound comprising reacting the monomers with the aid of an initiator based on lithium in the presence of an essentially inert solvent wherein said solvent consists of at least 60 per cent by weight of cyclopentane and the heat of reaction is withdrawn by means of reflux cooling.

Abstract: The present invention provides a process for preparing a flame retardant styrenic resin which comprises:(1) an initiation step wherein 3 to 10 parts by weight of a bromostyrene compound, 10 to 25 parts by weight of styrene, 5 to 10 parts by weight of an unsaturated nitrile, 0.01 to 0.5 part by weight of a chain transfer agent and 0.01 to 1.0 part by weight of an initiator are introduced to a polymerization reactor and polymerized until the conversion reaches 30 to 50%;(2) a control step wherein 10 to 35 parts by weight of a bromostyrene compound, 20 to 50 parts by weight of styrene, 5 to 20 parts by weight of an unsaturated nitrile, 0.01 to 1.0 part by weight of an initiator and 0.01 to 0.5 part by weight of a chain transfer agent are introduced to the reactor in one or more divisions in accordance with the progress of said polymerization until the conversion reaches 80 to 90%; and(3) an optimization step wherein 3 to 10 parts by weight of styrene, 5 parts by weight or less of an unsaturated nitrile, 0.

Abstract: Novel ethylene homopolymers and ethylene copolymers which are suitable for film production and have a density from 925 to 940 kg /m.sup.3, a melt flow index (MFI) of <1 g/10 min at 190.degree. C. and at an applied force of 2.16 kp can be prepared by free radical polymerization of the monomers at from 500 to 5000 bar and reaction temperatures T of from 40.degree. to 320.degree. C. by means of an initiator with virtually complete exclusion of oxygen in at least n=3 polymerization stages. In the first stage, the total amount or the predominant part of the monomers is subjected to the polymerization by means of part of the required initiator until the polymerization virtually comes to a stop. A further part of the initiator and, if required, of the monomers is then added to the mixture cooled to 20.degree.-60.degree. C. This process is repeated in the subsequent stages until the n th stage, at least the initiators used in the n th stage having a half-life temperature of from 90.degree. to 260.degree. C.

Abstract: There is provided inherently tacky, emulsion pressure-sensitive adhesive polymers comprising about 35 to about 65 percent by weight alkyl acrylates, about 15 to about 35 percent vinyl esters, about 20 to about 35 percent by weight diesters of a dicarboxylic acid, and up to about 5 percent by weight of an unsaturated carboxylic acid. There is preferably included in the monomers a reactive surfactant and a chelating monomer, with or without a chain transfer agent. The preferred polymers have a glass transition temperature less than about -30.degree. C. and a gel content of about 50 to 70 percent by weight.

Abstract: The invention relates to copolymers containing carboxyl groups and having an acid number of 20 to 150 mg of KOH/g and a number average molecular weight of 2000 to 8000, which can preferably be prepared from copolymers containing hydroxyl groups by reaction with carboxylic anhydrides, to a process for their preparation and their use in coating agents. The copolymers contain a vinyl ester, a vinyl aromatic, a hydroxyalkyl ester of an .alpha.,.beta.-unsaturated carboxylic acid, and optionally other monomers that may contain acid groups.

Abstract: The present invention is directed to a product and method of incrementally adding comonomers to co-polytetrafluoroethylene (PTFE) by an improved process, resulting in a PTFE product independent and/or much less dependent upon drying temperatures. The improved PTFE exhibits increased green strength at substantially constant extrusion pressures and an extrusion pressure that is substantially independent of drying temperatures of the wet PTFE.

Abstract: Copolymerization is started by reacts in and acrylamide represented by the general formula ##STR1## wherein R.sub.1 represents a hydrogen atom or a methyl group and R.sub.2 represents a hydrogen atom, an alkyl group, an aryl group or an aralkyl group, with an amount of methyl methacrylate in pure methanol or methanol containing Water in an amount of not more than 30% by weight based on the total weight of methanol and water, and, an additional amount of the methyl methacrylate is added at periods during proceeding of the polymerization. The obtained copolymer is heated at 80.degree. to 150.degree. C. in the same solvent and in the presence of an alkaline catalyst, to an imidized acrylic resin containing glutarimide groups.The imidized acrylic resins obtained by the process of the present invention have excellent thermal stability, transparency and color shade, and contain little acid or acid anhydride.

Abstract: A process for emulsion polymerization to form a latex having a particle size greater than about 0.5 micrometers by charging to a reaction vessel under agitation a reaction mixture including water, emulsifiers at an initial concentration slightly greater than or equal to the critical micelle concentration, a protective colloid, ethylenically unsaturated monomers, and an ionically balanced combination of an anionic and a cationic water soluble free radical initiator or a nonionic initiator; reacting the mixture to form an emulsion of polymerized monomer; and adding additional monomer, emulsifier, and protective colloid and a water soluble chain transfer agent to the reactor while continuing to agitate the mixture under conditions such that the emulsifier concentration does not exceed that at which new polymeric nuclei are formed.

Abstract: A process for producing a styrenic copolymer for injection molding or extrusion molding by continuous mass polymerization of a styrenic monomer with a polymerizable unsaturated fatty acid, charcterized by using a polymerization apparatus comprising a circulation polymerization line (I) in which a tubular reactor with plural mixing elements fixed therein is incorporated, said mixing elements having no mobile parts, and a second non-circulation polymerization line (II) provided in succession to said circulation polymerization line (I), in which a tubular reactor with plural mixing elements fixed therein is incorporated, said mixing elements having no mobile parts; and supplementing the polymerizable unsaturated fatty acid to at least two spots of the second non-circulation polymerization line (II).

Abstract: Expandable styrene polymer of controlled particle size is prepared by polymerizing styrene and optionally polymerizable comonomers in a stirred aqueous suspension in the presence of monomer-soluble free radical initiator and dispersant to the extent that at least 70%, based on the total monomer, of the monomers are polymerized in the aqueous suspension initially to a conversion of at least 70% by weight; and then adding the remaining monomer, initiator and optional copolymerizable monomer and additives to the polymerization medium over one to three hours.

Abstract: There is provided inherently tacky, emulsion pressure-sensitive adhesive polymers comprising about 35 to about 65 percent by weight alkyl acrylates, about 15 to about 35 percent vinyl esters, about 20 to about 35 percent by weight diesters di-carboxylic acid, about 0.1 to 1 percent by weight of a reactive multifunctional monomer and up to about 10 percent by weight of an unsaturated carboxylic acids. The polymers are formed in the presence of a reactive surfactant, with or without a chain transfer agent. The preferred polymers have a glass transition temperature less than about -20.degree. C. and a gel content of at least 60 percent by weight.

Abstract: A suspension polymerization process for the preparation of a polymeric material formed from at least two monomers, a non-gaseous monomer and a gaseous monomer. The composition of the beads or particles of the polymeric material is controlled by adding gaseous monomer to the vapor phase after polymerization of about 5% to about 90% by weight of the non-gaseous monomer or by removing a portion or substantially all of the gaseous monomer in the vapor phase before polymerization of about 70% by weight of the gaseous monomer in the aqueous suspension, or a combination thereof.

Abstract: Water-soluble, high molecular weight, linear copolymers of a diallyl ammonium monomer and an acrylamide monomer are prepared by emulsion copolymerization of a major portion of the acrylamide monomer via stage-addition, and the addition of a chain transfer agent at the conclusion of said copolymerization to prevent branching and cross-linking.

Abstract: There is provided inherently tacky, emulsion pressure-sensitive adhesive polymers comprising about 35 to about 65 percent by weight alkyl acrylates, about 15 to about 35 percent vinyl esters, about 20 to about 35 percent by weight diesters of a dicarboxylic acid, and up to about 5 percent by weight of an unsaturated carboxylic acid. There is preferably included in the monomers a reactive surfactant and a chelating monomer, with or without a chain transfer agent. The preferred polymers have a glass transition temperature less than about -30.degree. C. and a gel content of about 50 to 70 percent by weight.

Abstract: Disclosed herein is a process for the production of cycloolefin random copolymers comprising copolymerizing ethylene with at least one specific cyclooloefin in a liquid phase in the presence of a catalyst and a solvent in a polymerization vessel, wherein ethylene is copolymerized with the cycloolefin while feeding the cycloolefin or a mixture of the cycloolefin and the solvent onto inside circumferential wall of the polymerization vessel above the gas-liquid interface in the polymerization vessel. The proposed process ensures a smooth copolymerization reaction and a continuous and stable running of the apparatus for a prolonged period of time, and is productive of ethylene-cycloolefin random copolymers which have a uniform quaility and are excellent in heat resistance, heat aging property and various mechanical properties.

Abstract: Water-soluble, high molecular weight, linear copolymers of a diallyl ammonium monomer and an acrylamide monomer of various cationicities are prepared by copolymerization of a major portion of the acrylamide monomer via stage-addition in a solvent solution and adding a chain transfer agent at the conclusion of said copolymerization to prevent branching and cross-linking from occurring.

Abstract: To produce a graded index polymer there is diffused into a base material formed of at least one partly polymerized monomer adapted to yield a homopolymer with refractive index N1 in the gel state an additive material formed of at least one monomer adapted to yield a homopolymer with refractive index N2 different from N1. The polymerization of the base material treated in this way is then completed. The additive material comprises a mixture of at least two monomers and the composition of this mixture is varied with time.