The present invention relates to a method for the preparation of low-viscosity polymers suitable as base fluids for lubricants. The method comprises polymerizing specific (meth)acrylic acid esters in the presence of a cobalt catalytic chain transfer agent.

The present invention relates to the field of lubrication. Lubricants typically contain a base fluid and variable amounts of additives. A good lubricant should posses a high boiling and low freezing point, a high viscosity index, good thermal stability, low susceptibility to corrosion, and a high resistance to oxidation. These properties are significantly determined by the additives used. Therefore, a base fluid that can support a broad variety of additives is needed to improve the overall performance of the lubricant.

According to the American Petroleum Institute (API) lubricant base fluids are subdivided into different groups. Groups I to III encompass different mineral oils distinguished by their degree of saturation, sulphur content, and viscosity index. Group IV encompasses polyalphaolefins. Group V encompasses all other base fluids including napthenics, polyalkylene glycol oils, and esters. Base fluids for lubricants may especially be oils having a kinematic viscosity in the range of 3 to 100 mm ² /s, preferably 13 to 65 mm ² /s measured at 40°C according to ASTM D 445.

The term mineral oil commonly refers to oils derived from crude oil fractions. Mineral oils of groups I to III are therefore regarded as native oils. In contrast, base fluids of groups IV and V are regarded as synthetic base fluids. Synthetic base fluids are growing in interest and are preferred over mineral oils due to their greater oxidative and chemical stability, improved viscosity index and reduced pour point. Further, their properties may be systematically controlled during synthesis to optimize the structure-property profile of the base fluids. Due to their synthesis from relatively pure raw material, synthetic base fluids also contain fewer unwanted by-products with deleterious effects.

Ester oils are group V base fluids that may have superior solubility, additive compatibility and viscosimetrics compared to other group IV and V base fluids. A particular example of ester oils are polymers of (meth)acrylic acid esters. Polymers of (meth)acrylic acid esters can be prepared by radical polymerization, in particular by catalytic chain transfer polymerization. Catalytic chain transfer (CCT) is a process which involves adding a catalytic chain transfer reagent to a radical polymerization reaction to achieve greater control over the length of the resulting polymers. It is known that cobalt porphyrins can be used as catalytic chain transfer reagents in the polymerization of methyl- methacrylate to reduce the molecular weight of the resulting poly-methyl- methacrylate (N. S. Enikolopyan et al., Journal of Polymer Science: Polymer Chemistry Edition, 1981 , vol. 19, pp. 879-889).

US 2009/0012231 A1 discloses macromonomers synthesized by cobalt-catalyzed chain transfer free radical polymerizations of (meth)acrylic monomers. It further discloses the preparation of a pigment dispersion from the reaction of said macromonomers with monomeric or oligomeric amines. However, US 2009/0012231 A1 does not relate to the synthesis of low-viscosity polymers from methacrylic acid esters. US 4,680,352 discloses the use of different Co(ll) chelates as catalytic chain transfer agents for controlling the molecular weight of homopolymers and copolymers produced in free radical polymerization processes. In particular, US 4,680,352 relates to the polymerization of methacrylic acid ester monomers and styrene monomers.

The present invention aims at providing an improved method for the preparation of low viscosity polymers from (meth)acrylic acid esters. The polymers prepared have a kinematic viscosity of less than 100 mm ² /s measured at 100°C according to ASTM D 445. Further, the method should require low amounts of free radical initiator. Additionally, the polymers should have superior viscosity indices and comparable volatilities and much lower sulphur content, when compared to state of the art base fluids.

In the context of the present invention, the term "(meth)acrylic" refers to either acrylic or to methacrylic, or mixtures of acrylic and methacrylic. Correspondingly, the term "(meth)acrylate" refers to either acrylate or to methacrylate, or mixtures of acrylate and methacrylate.

The present invention relates to a method for preparing a polymer composition having a kinematic viscosity of less than 100 mm ² /s measured at 100°C according to ASTM D 445 from (meth)acrylic acid esters. This method comprises the steps of

a) preparing a reaction mixture comprising a (meth)acrylic acid ester according to formula (I) or a mixture of (meth)acrylic acid esters of formula (I)

wherein R ¹ represents a hydrogen atom or a methyl group, and R ² represents a Ci to C30 alkyl group;

b) adding a Co(ll) complex as a catalytic chain transfer agent to the reaction mixture;

c) adding a radical initiator; and

d) reacting the reaction mixture to obtain the polymer composition,

characterized in that

said reaction mixture comprises

i) from 0 to 60% by weight of the (meth)acrylic acid esters according to formula (I) based on the total weight of (meth) acrylic acid esters according to formula (I) have a Ci to C6 alkyl group as substituent R ² , and ii) from 40 to 100% by weight of the (meth)acrylic acid esters according to formula (I) based on the total weight of (meth) acrylic acid esters according to formula (I) have a C ₇ to C30 alkyl group as substituent R ² ,

from which at least 0.5% by weight of the (meth)acrylic acid esters according to formula (I), based on the total weight of (meth) acrylic acid esters according to formula (I), have a C12 to C30 alkyl group as substituent R ² , and wherein the total amount of the radical initiator added to the reaction mixture is at least 0.05% by weight, based on the total weight of (meth)acrylic acid esters.

In the context of the present invention, the (meth)acrylic acid esters according to formula (I) may also be referred to as "C _n (meth)acrylic acid esters" or "C _n (meth)acrylates". These terms refer to compounds according to formula (I), wherein R ² represents a C _n alkyl group.

Non-limiting examples of compounds of formula (I) include methyl-(meth)acrylate, ethyl-(meth)acrylate, n-propyl-(meth)acrylate, / ^' so-propyl-(meth)acrylate, n-butyl- (meth)acrylate, teff-butyl-(meth)acrylate, pentyl-(meth)acrylate, cyclopentyl- (meth)acrylate, hexyl-(meth)acrylate, 2-ethylhexyl-(meth)acrylate, heptyl- (meth)acrylate, 2-terf-butylheptyl-(meth)acrylate, octyl-(meth)acrylate, 3-isopropyl- heptyl-(meth)acrylate, nonyl-(meth)acrylate, decyl-(meth)acrylate, undecyl- (meth)acrylate, 5-methylundecyl-(meth)acrylate, dodecyl-(meth)acrylate, 2-methyldodecyl-(meth)acrylate, tridecyl-(meth)acrylate, 5-methyltridecyl- (meth)acrylate, tetradecyl-(meth)acrylate, pentadecyl-(meth)acrylate, oleyl- (meth)acrylate, cyclohexyl-(meth)acrylate, hexadecyl-(meth)acrylate, 2-methyl- hexadecyl-(meth)acrylate, heptadecyl-(meth)acrylate, 5-isopropylheptadecyl- (meth)acrylate, 4-fe/t-butyloctadecyl-(meth)acrylate, 5-ethyloctadecyl- (meth)acrylate, 3-isopropyloctadecyl-(meth)acrylate, octadecyl-(meth)acrylate, nonadecyl-(meth)acrylate, eicosyl-(meth)acrylate, cetyleicosyl-(meth)acrylate, stearyleicosyl-(meth)acrylate, docosyl-(meth)acrylate, eicosyltetratriacontyl- (meth)acrylate, and 2,3,4, 5-tetra-terf-butylcyclohexyl-(meth)acrylate. In the present invention, it is essential that the monomer reaction mixture comprises from 40% to 100% by weight of (meth)acrylic acid esters according to formula (I) having a C ₇ to C30 alkyl group, based on the total weight of (meth)acrylic acid esters according to formula (I), in order to get oil-soluble polymer compositions suitable as lubricant formulations.

In a preferred embodiment at least 20% by weight, more preferably at least 40% by weight, most preferably 100% by weight of the (meth)acrylic acid esters according to formula (I), based on the total weight of (meth) acrylic acid esters according to formula (I), have a C12 to C30 alkyl group as substituent R ² .

In another preferred embodiment at least 0.5% by weight, more preferably at least 20% by weight, even more preferably at least 40% by weight, most preferably 100% by weight of the (meth)acrylic acid esters according to formula (I), based on the total weight of (meth) acrylic acid esters according to formula (I), have a CM to C30 alkyl group as substituent R ² .

It has surprisingly been found that with the inventive method, polymers of a kinematic viscosity of less than 100 mm ² /s, preferably less than 90 mm ² /s, more preferably less than 80 mm ² /s measured at 100°C according to ASTM D 445 can be prepared. Additionally, the polymer composition prepared by the inventive method are sulphur free.

It has also surprisingly been found that by varying the degree of linearity of the alkyl group R ² , the viscosity of the polymers can be adjusted. In particular, a higher degree of linearity of the alkyl group R ² results in a lower viscosity.

In the context of the present invention, the term "degree of linearity" refers to the amount of (meth)acrylic acid esters according to formula (I) having a linear alkyl group as substituent R ² , based on the total weight of (meth)acrylic acid esters according to formula (I). Preferably, the degree of linearity of the (meth)acrylic acid esters according to formula (I) is at least 30%, preferably at least 70%, most preferably 100%. That means that at least 30% by weight, preferably at least 70% by weight, most preferably 100% by weight of the (meth)acrylic acid esters according to formula (I), based on the total weight of (meth)acrylic acid esters according to formula (I), have a linear alkyl group as substituent R ² . In a particularly preferred example, R ² in formula (I) represents a linear alkyl group. This means that the degree of linearity is 100%.

It is particularly preferred that at least 20% by weight, more preferably at least 40% by weight, most preferably 100% by weight of the (meth)acrylic acid esters according to formula (I), based on the total weight of (meth) acrylic acid esters according to formula (I), have a linear C12 to C30 alkyl group as substituent R ² . Even more preferably, 100% by weight of the (meth)acrylic acid esters according to formula (I), based on the total weight of (meth) acrylic acid esters according to formula (I), have a linear C16 to C20 alkyl group as substituent R ² .

The reaction mixture prepared in step a) may additionally comprise a solvent. The solvent may be selected according to the polarity of the monomers used. Suitable solvents include, for example, aromatic hydrocarbons such as, for example, benzene, toluene, and xylenes; ethers such as, for example, tetrahydrofuran, diethyl ether, ethylene glycol and polyethylene glycol monoalkyl and dialkyl ethers; alkyl esters of acetic, propionic and butyric acids; mixed ester-ethers, such as, for example, monoalkyl ether-monoalkanoate esters of ethylene glycol; ketones such as, for example, acetone, butanone, pentanone and hexanone; alcohols such as, for example, methanol, ethanol, propanol and butanol. Oils such as, for example, hydrocracked oil, petroleum oil, polyalphaolefins, esters or polymers of the present invention may also be used.

The Co(ll) complex used in the inventive method acts as a catalytic chain transfer agent. By using a cobalt based catalytic chain transfer agent it has surprisingly been found that polymer compositions of extremely low viscosity can be produced. To achieve a kinematic viscosity of less than 1 10 mm ² /s measured at 100°C according to ASTM D 445, the amount of Co(ll) added to the reaction mixture in the form of a Co(ll) complex is preferably 30 to 500 ppm by weight, based on the total weight of (meth)acrylic acid esters, more preferably 30 to 100 ppm by weight, most preferably 50 to 100 ppm by weight.

Suitable examples of Co(ll) complexes of the present invention include complexes comprising Co(ll) and at least one of the ligands according to formulae (II) to (VII)

wherein each R ³ independently represents a phenyl group or a Ci to C12 alkyl group, or two R ³ on adjacent carbon atoms together represent a C ₅ to Cs alkylene group; each R ⁴ independently represents a hydrogen atom or a Ci to C12 alkyl group; each R ⁵ independently represents a hydroxyl group or an amino group; each R ⁶ independently represents a hydrogen atom, a Ci to C12 alkyl group, a phenyl group, a hydroxyphenyl group, or a Ci to C ₄ alkoxyphenyl group; and each n independently represents an integer 2 or 3. In a particularly preferred embodiment the Co(ll) complex comprises Co(ll) and a ligand of formula (VII). More preferably, the Co(ll) complex is 5,10,15,20- tetraphenyl-porphine Co(ll).

The radical initiator used in the inventive method may be any free radical initiator suitable for use in radical polymerization reactions. Such radical initiators are well known in the art. Azo compounds are particularly preferred radical initiators.

The total amount of the radical initiator added to the reaction mixture is at least 0.05% by weight, based on the total weight of (meth)acrylic acid esters, preferably in the range of 0.1 to 3% by weight, based on the total weight of (meth)acrylic acid esters. It has surprisingly been found that by varying the amount of initiator, polymer compositions of different viscosity and different pour points may be produced. To achieve a particularly low viscosity, the total amount of initiator added to the reaction mixture is preferably 0.1 to 1 .5% by weight, based on the total weight of (meth)acrylic acid esters. The radical initiator may be added to the reaction mixture in a stepwise fashion to ensure that the radical initiator does not get depleted too quickly during long polymerization times. For example, a first dose of the radical initiator is added to the reaction mixture to start the polymerization reaction, then the reaction is allowed to proceed for a certain amount of time, then an additional dose of initiator is added, and so on. The total amount added in all steps, however, should not exceed the preferred total amount of radical initiator mentioned above. The time interval between the additions of the different doses of radical initiator may be in the range of 10 minutes to 5 hours, preferably 30 to 60 minutes.

Examples of suitable radical initiators include azo-compounds such as azobisisobutylonitrile (AIBN), 2,2'-Azobis(2-methylbutyronitrile), 2-(2-cyanobutan- 2-yldiazenyl)-2-methylbutanenitrile, and 1 ,1 -azobiscyclohexanecarbonitrile; peroxy compounds such as methyl-ethyl-ketone peroxide, acetylacetone peroxide, dilauryl peroxide, terf-butyl per-2-ethylhexaneoate, ketone peroxide, terf-butyl peroctoate, methyl isobutyl ketone peroxide, cyclohexanone peroxide, dibenzoyl peroxide, terf-butyl peroxybenzoate, terf-butyl peroxyisopropylcarbonate, 2,5-bis- (2-ethylhexanoylperoxy)-2,5-dimethylhexane, terf-butyl peroxy-2-ethylhexanoate, terf-butyl-peroxy-3,5,5-trimethylhexanoate, dicumyl peroxide, 1 ,1 -bis(terf- butylperoxy)cyclohexane, 1 ,1 -bis(terf-butylperoxy)-3,3,5-trimethylcyclohexane, cumyl hydroperoxide, terf-butyl hydroperoxide, and bis(4-terf-butylcyclohexyl) peroxydicarbonate; and mixtures of the aforementioned compounds.

The reaction mixture may be reacted in step d) at standard ambient pressure, reduced pressure or elevated pressure. The reaction temperature may in the range of -20°C to 200°C, preferably 50°C to 160°C, more preferably 80°C to 160°C.

In a preferred embodiment, the addition of the radical initiator in step c) and the reaction in step d) takes place in an inert gas atmosphere to prevent degradation of the radical initiator. Preferably, nitrogen gas is used as inert gas. The reaction may be allowed to proceed in step d) for up to 12 hours, preferably for 10 minutes to 12 hours, more preferably for 1 to 6 hours.

In a particularly preferred embodiment of the present invention, the method comprises the steps of:

a) preparing a reaction mixture consisting of from 0 to 60% by weight of the (meth)acrylic acid esters according to formula (I), based on the total weight of (meth) acrylic acid esters according to formula (I), having a Ci to Ce alkyl group as substituent R ² , and from 40 to 100% by weight of the (meth)acrylic acid esters according to formula (I), based on the total weight of (meth) acrylic acid esters according to formula (I), having a C ₇ to C30 alkyl group as substituent R ² , said (meth)acrylic acid esters according to formula (I) having a degree of linearity of at least 30% and comprising at least 20% by weight of (meth)acrylic acid esters according to formula (I) which have a C12 to C30 alkyl group as substituent R ² ;

b) adding Co(ll) as a catalytic chain transfer agent in the form of a complex comprising Co(ll) and a ligand according to formula (VII) at a concentration of 30 ppm to 100 ppm by weight of Co(ll), based on the total weight of (meth)acrylic acid esters;

c) adding 0.1 to 3% by weight, based on the total weight of (meth)acrylic acid esters of a radical initiator in a step-wise fashion; and

d) reacting the reaction mixture at a temperature of 80°C to 160°C for 1 to 6 hours.

In a second aspect, the present invention also relates to the use of a (meth)acrylic acid ester accordin to formula (VIII)

wherein R ⁷ is a hydrogen atom or a methyl group, and R ⁸ is a C12 to C30 alkyl group for the preparation of a polymer composition having a kinematic viscosity of less than 100 mm ² /s, preferably less than 90 mnn ² /s, more preferably less than 80 mm ² /s measured at 100°C according to ASTM D 445.

In a preferred embodiment of the second aspect, R is a linear alkyl group. In another preferred embodiment, R ⁸ is a CM to C30 alkyl group, even more preferably a linear C16 to C20 alkyl group. Preferably, the polymer composition is a composition of polymers of (meth)acrylic acid esters.

In a third aspect, the present invention relates to a method of reducing the kinematic viscosity of a polymer composition, said method comprising a method for the preparation of a polymer composition, said method for the preparation of a polymer composition comprising the steps of:

a) preparing a reaction mixture comprising a (meth)acrylic acid ester according to formula (IX) or a mixture of (meth)acrylic acid esters of formula (IX)

wherein R ⁹ is a hydrogen atom or a methyl group, and R ¹⁰ is a C ₇ to C30 alkyl group;

b) adding a Co(ll) complex as a catalytic chain transfer agent to the reaction mixture;

c) adding a radical initiator; and

d) reacting the reaction mixture to obtain the polymer composition,

wherein the method of reducing the kinematic viscosity is characterized in that the reaction mixture comprises

i) from 0 to 60% by weight of the (meth)acrylic acid esters of formula (IX) based on the total weight of (meth) acrylic acid esters of formula (IX) have a Ci to C6 alkyl group as substituent R ¹⁰ , and

ii) from 40 to 100% by weight of the (meth)acrylic acid esters of formula (IX) based on the total weight of (meth) acrylic acid esters of formula (IX) have a C ₇ to C30 alkyl group as substituent R ¹⁰ , from which at least 0.5% by weight, preferably at least 20% by weight, more preferably at least 40% by weight, most preferably 1 00% by weight of the (meth)acrylic acid esters according to formula (IX), based on the total weight of (meth) acrylic acid esters according to formula (IX), have a C12 to C30 alkyl group as substituent R ¹⁰ , and wherein the total amount of the radical initiator added to the reaction mixture is at least 0.05% by weight based on the total weight of (meth)acrylic acid esters.

The method for the preparation of a polymer composition is the method to prepare a polymer composition as described above. The Co(ll) complex that is used as catalyst is the Co(ll) complex as described above.

In a preferred embodiment, at least 30% by weight, preferably at least 70% by weight, most preferably 1 00% by weight of the (meth)acrylic acid esters according to formula (IX), based on the total weight of (meth) acrylic acid esters according to formula (IX), have a linear alkyl group as substituent R ¹⁰ . Even more preferably, 1 00% by weight of the (meth)acrylic acid esters according to formula (IX), based on the total weight of (meth) acrylic acid esters according to formula (IX), have a linear C16 to C20 alkyl group as substituent R ¹⁰ .

In another preferred embodiment, at least 0.5% by weight, preferably at least 20% by weight, more preferably at least 40% by weight, most preferably 1 00% by weight of the (meth)acrylic acid esters according to formula (IX), based on the total weight of (meth) acrylic acid esters according to formula (IX), have a CM to C30 alkyl group as substituent R ¹⁰ .

Examples In the following examples, ethyl-hexyl methacrylate (EHMA) is a Cs methacrylate with a 0% degree of linearity. Isodecyl-methacrylate (IDMA) is a mixture consisting of 98.7% by weight Cio methacrylate, 0.8% by weight C12 methacrylate, and 0.5% by weight CM methacrylate. The degree of linearity of IDMA is approximately 0%. Methacrylate from LIAL ^® 125 alcohol (LIMA) is a mixture consisting of 24.3% by weight C12 methacrylate, 29.4% by weight C13 methacrylate, 28.4% by weight Ci ₄ methacrylate, and 17.9% by weight C15 methacrylate. The degree of linearity of LIMA is approximately 40%. Lauryl methacrylate (LMA) is a mixture consisting of 72.2% by weight C12 methacrylate, and 27.8% by weight Ci ₄ methacrylate. The degree of linearity of LIAL is approximately 100%.

DPMA is a mixture consisting of 24.6% by weight C12 methacrylate, 29.1 % by weight C13 methacrylate, 24.3% by weight Ci ₄ methacrylate, and 22.0% by weight Ci5 methacrylate. The degree of linearity of DPMA is approximately 80%.

Stearyl methacrylate (SMA) is a mixture consisting of 29.3% by weight C16 methacrylate, 69.8% by weight Cis methacrylate, and 0.8% by weight C20 methacrylate. The degree of linearity of SMA is approximately 100%.

Comparative example 1 250 g of EHMA were charged into a 500 mL 4-necked round bottom flask. 0.188 g of 5,10,15,20-Tetraphenyl Porphine Cobalt(ll) was then added to the flask. The contents of the flask were mixed using an overhead stirrer, inerted with nitrogen, and heated to 90°C. Once the mixture reached temperature and the cobalt catalyst appeared to be dissolved, 1 g of initiator solution, composed of 25% 2-(2- cyanobutan-2-yldiazenyl)-2-methylbutanenitrile (Vazo67) in diisobutyl ketone, was added to the flask using a syringe through a rubber septum. The reaction was allowed to proceed for 60 minutes. Two additional shots of 1 g of initiator solution were added 60 minutes apart. The reaction was allowed to hold for one hour after the final addition of initiator.

Example 2

250 g of IDMA were charged into a 500 ml_ 4-necked round bottom flask. 0.188 g of 5,10,15,20-Tetraphenyl Porphine Cobalt(ll) was then added to the flask. The contents of the flask were mixed using an overhead stirrer, inerted with nitrogen, and heated to 90°C. Once the mixture reached temperature and the cobalt catalyst appeared to be dissolved, 1 g of initiator solution, composed of 25% 2-(2- cyanobutan-2-yldiazenyl)-2-methylbutanenitrile (Vazo67) in diisobutyl ketone, was added to the flask using a syringe through a rubber septum. The reaction was allowed to proceed for 60 minutes. Two additional shots of 1 g of initiator solution were added 60 minutes apart. The reaction was allowed to hold for one hour after the final addition of initiator.

Example 3

250 g of LMA were charged into a 500 ml_ 4-necked round bottom flask. 0.188 g of 5,10,15,20-Tetraphenyl Porphine Cobalt(ll) was then added to the flask. The contents of the flask were mixed using an overhead stirrer, inerted with nitrogen, and heated to 90°C. Once the mixture reached temperature and the cobalt catalyst appeared to be dissolved, 1 g of initiator solution, composed of 25% 2-(2- cyanobutan-2-yldiazenyl)-2-methylbutanenitrile (Vazo67) in diisobutyl ketone, was added to the flask using a syringe through a rubber septum. The reaction was allowed to proceed for 60 minutes. Two additional shots of 1 g of initiator solution were added 60 minutes apart. The reaction was allowed to hold for one hour after the final addition of initiator. Example 4

250 g of LIMA were charged into a 500 ml_ 4-necked round bottom flask. 0.188 g of 5,10,15,20-Tetraphenyl Porphine Cobalt(ll) was then added to the flask. The contents of the flask were mixed using an overhead stirrer, inerted with nitrogen, and heated to 90°C. Once the mixture reached temperature and the cobalt catalyst appeared to be dissolved, 1 g of initiator solution, composed of 25% 2-(2- cyanobutan-2-yldiazenyl)-2-methylbutanenitrile (Vazo67) in diisobutyl ketone, was added to the flask using a syringe through a rubber septum. The reaction was allowed to proceed for 60 minutes. Two additional shots of 1 g of initiator solution were added 60 minutes apart. The reaction was allowed to hold for one hour after the final addition of initiator.

Example 5

250 g of DPMA were charged into a 500 ml_ 4-necked round bottom flask. 0.188 g of 5,10,15,20-Tetraphenyl Porphine Cobalt(ll) was then added to the flask. The contents of the flask were mixed using an overhead stirrer, inerted with nitrogen, and heated to 90°C. Once the mixture reached temperature and the cobalt catalyst appeared to be dissolved, 1 g of initiator solution, composed of 25% 2-(2- cyanobutan-2-yldiazenyl)-2-methylbutanenitrile (Vazo67) in diisobutyl ketone, was added to the flask using a syringe through a rubber septum. The reaction was allowed to proceed for 60 minutes. Two additional shots of 1 g of initiator solution were added 60 minutes apart. The reaction was allowed to hold for one hour after the final addition of initiator.

Example 6

250 g of SMA were charged into a 500 ml_ 4-necked round bottom flask. 0.188 g of 5,10,15,20-Tetraphenyl Porphine Cobalt(ll) was then added to the flask. The contents of the flask were mixed using an overhead stirrer, inerted with nitrogen, and heated to 90°C. Once the mixture reached temperature and the cobalt catalyst appeared to be dissolved, 1 g of initiator solution, composed of 25% 2-(2- cyanobutan-2-yldiazenyl)-2-methylbutanenitrile (Vazo67) in diisobutyl ketone, was added to the flask using a syringe through a rubber septum. The reaction was allowed to proceed for 60 minutes. Two additional shots of 1 g of initiator solution were added 60 minutes apart. The reaction was allowed to hold for one hour after the final addition of initiator.

Measurements of viscosity, molecular weight, and sonic shear stability

The kinematic viscosities of the polymers were measured according to ASTM D 445. The polymer molecular weights were measured by gel permeation chromatography (GPC) calibrated using PMMA standards. The sonic shear stability (SSI) was determined according to ASTM D 5621 . The pour point was determined according to ASTM D 6749. The viscosity index was determined according to ASTM D 2270.

Examples 2-6 demonstrate that the use of a cobalt based catalytic chain transfer agent for the polymerization of methacrylate monomers results in low viscosity polymers (see table 1 ), when the methacrylate monomers of the reaction mixture comprise at least 0.5 wt% (meth) acrylic acid esters according to formula (I), based on the total weight of (meth) acrylic acid esters according to formula (I), having a C12 to C30 alkyl group as substituent R ² . The molecular weight of the polymers indicates a degree of polymerization of about 10 units.

In addition, examples 4, 5 and 6 clearly demonstrate that with an increasing degree of linearity, a lower viscosity is obtainable. Example 1 shows that when using (meth) acrylic acid esters according to formula (I), having a C ₇ alkyl group as substituent R ² in the reaction mixture, then polymer compositions with a kinematic viscosity of 103 mm ² /s at 100°C and a molecular weight Mw of 3.2 kg/mol can be obtained. However, it was surprisingly observed that, even if Examples 1 and 2 show the lowest molecular weights M _w (3.2 kg/mol and 3.8 kg/mol, respectively), their kinematic viscosities are still higher than in Examples 3 to 6 prepared with (meth)acrylic acid esters according to formula (I), having a linear C12 to C30 alkyl group as substituent R ² , despite the fact that polymer compositions of Examples 3 to 6 have much higher molecular weights M _w (4.4 kg/mol, 5.4 kg/mol, 4.9 kg/mol and 4.7 kg/mol, respectively).

Table 1 : Viscosimetric data of examples 1 to 6. The amounts given are relative to the total weight of methacrylate monomers.

Example 1 2 3 4 5 6

C ₇ methacrylate [% by weight] 100

Cio methacrylate [% by weight] 98.7 0

Ci2 methacrylate [% by weight] 0.8 72.2 24.3 24.6

Ci3 methacrylate [% by weight] 29.4 29.1

On methacrylate [% by weight] 0.5 27.8 28.4 24.3

Ci ₅ methacrylate [% by weight] 17.9 22.0

Ci6 methacrylate [% by weight] 29.3

Ci8 methacrylate [% by weight] 69.8

C20 methacrylate [% by weight] 0.8

Degree of linearity [%] 0 0 100 40 80 100

Initiator [% by weight] 0.3 0.3 0.3 0.3 0.3 0.3

Co(ll) [ppm by weight] 66 66 66 66 66 66

Mw [kg/mol] 3.2 3.8 4.4 5.4 4.9 4.7

Kinematic viscosity at 100°C [mm /s] 103 81 40 75 50 38

Kinematic viscosity at 40°C [mm /s] 2560 1746 437 1058 572 ND

Viscosity Index 109 108 140 142 144 ND

SSI (ASTM D 5621 ) 0.4 0.78 0.03 0.45 0.2 ND