MACROMER FOR USE IN POLYMER POLYOLS Field of the invention The present invention relates to a process for preparing a mixture comprising a macromer; to a process for preparing a polymer polyol using said mixture comprising a macromer; to a polymer polyol obtainable by said process; to a process for preparing a polyurethane foam using said polymer polyol; to a polyurethane foam obtainable by said process; and to a shaped article comprising said polyurethane foam. Background of the invention Polymer polyols (also called “graft polyols”) are commonly used for the manufacture of flexible polyurethane foams. Flexible polyurethane foams are widely used in numerous applications. Polymer polyols are dispersions of a solid polymer in a base polyol and may be prepared by polymerizing one or more ethylenically unsaturated monomers in the presence of such base polyol. Examples of said ethylenically unsaturated monomers are styrene and acrylonitrile. In addition to such low molecular weight, ethylenically unsaturated monomers, a so-called macromer may also be used when preparing polymer polyols. Macromers also contain an ethylenic unsaturation but their molecular weight is relatively high. Such macromers may be obtained by reacting a polyether polyol having a relatively high functionality, or a derivative thereof, with a compound containing an ethylenic unsaturation. Macromers participate in the above-mentioned polymerization reaction through their ethylenic unsaturation, and they are intended to stabilize the resulting polymer polyols. However, a problem that may be encountered in the manufacture of polymer polyols using a macromer and one or more ethylenically unsaturated monomers, is that the resulting polymer polyol may have a high viscosity and/or poor filterability, thereby making processing and use of the polymer polyol difficult. A decreased filterability may result from less stabilization of the polymer polyol and/or from fouling of the internal surface of the wall of a reactor wherein such polymer polyol is made or used. Furthermore, a relatively high viscosity of the polymer polyol may result in that the solids content (i.e. polymer content) of the polymer polyol should be kept relatively low, which complicates the preparation of polymer polyols with high solids content which are desired in a number of applications. US20190202970 discloses a process for preparing a macromer for use in preparing polymer polyols. In particular, according to US20190202970, by increasing the relative amount of the ethylenic unsaturation in the macromer, improvements in terms of filterability could be achieved. In specific, US20190202970 discloses a process for preparing a macromer by reacting a hexafunctional polyol P with at least one unsaturated isocyanate compound V which contains at least one isocyanate group, preferably 1,1-dimethyl-meta- isopropenylbenzyl isocyanate (TMI), using 1.1 to 1.8 mol, preferably 1.5 to 1.8 mol, of the unsaturated isocyanate compound V based on the end product. It is an object of the present invention to provide a macromer to be used in a process for preparing a polymer polyol, in such way that the resulting polymer polyol may have a reduced viscosity and/or an improved filterability. Summary of the invention Surprisingly it was found that the above-mentioned reduced viscosity and/or improved filterability of a polymer polyol prepared in a process comprising polymerizing one or more ethylenically unsaturated monomers and a macromer in the presence of a base polyol, may be achieved by mixing a macromer, which is prepared from a polyether polyol and another compound which contains an ethylenic unsaturation, and wherein the relative amount of the ethylenic unsaturation is of from greater than 0.6 to less than 1.8 mol per mol of macromer, with a polyether polyol in a weight ratio of from 1:99 to 99:1, and using the resulting mixture comprising the macromer and polyether polyol in the polymer polyol preparation process. Accordingly, the present invention relates to a process for preparing a mixture comprising a macromer, said process comprising: providing a macromer which is prepared from a polyether polyol P1, wherein said macromer additionally comprises a moiety which contains an ethylenic unsaturation and which is attached to the oxygen atom of a hydroxyl group of polyether polyol P1, wherein the relative amount of the ethylenic unsaturation is of from greater than 0.6 to less than 1.8 mol per mol of macromer; and mixing the macromer with a diluent in a weight ratio of macromer to diluent of from 1:99 to 99:1. Further, the present invention relates to a process for preparing a polymer polyol using the above-described mixture comprising a macromer, and to a polymer polyol obtainable by said process. Still further, the present invention relates to a process for preparing a polyurethane foam using the above-described polymer polyol, to a polyurethane foam obtainable by said process, and to a shaped article comprising said polyurethane foam. Detailed description of the invention While the processes and compositions of the present invention may be described in terms of “comprising”, “containing” or “including” one or more various described steps and components, respectively, they can also “consist essentially of” or “consist of” said one or more various described steps and components, respectively. In the context of the present invention, in a case where a composition comprises two or more components, these components are to be selected in an overall amount not to exceed 100 wt.%. Where upper and lower limits are quoted for a property then a range of values defined by a combination of any of the upper limits with any of the lower limits is also implied. The term “molecular weight” (or “MW”) is used herein to refer to number average molecular weight, unless otherwise specified or context requires otherwise. The number average molecular weight of a polyol can be measured by gel permeation chromatography (GPC) or vapor pressure osmometry (VPO). The term “hydroxyl (OH) value” or “hydroxyl (OH) number” is used herein to refer to the milligrams of potassium hydroxide equivalent to the hydroxyl content in one gram of polyol determined by wet method titration. Hence, said OH value or number is expressed in mg KOH/g. The term “equivalent weight” (or “EW”) is used herein to refer to the weight of polyol per reactive site. The equivalent weight is 56100 divided by the hydroxyl value of the polyol. The term “functionality” or “hydroxyl (OH) functionality” of a polyol refers to the number of reactive hydroxyl sites per molecule of polyol. The nominal functionality (or “Fn”) of a polyol is the same as that of its starter compound (initiator). Unless indicated otherwise, functionality refers to the actual average functionality which may be lower than the nominal functionality and is determined by the number average molecular weight of the polyol divided by the equivalent weight of the polyol. The term “primary hydroxyl content” (or “PHC”) is used herein to refer to the relative proportion (in %) of primary hydroxyl groups in a polyether polyol based on total number of hydroxyl groups including primary and secondary hydroxyl groups. The terms “ethylene oxide content” and “propylene oxide content”, respectively, in relation to a polyether polyol refer to those parts of the polyol which are derived from ethylene oxide and propylene oxide, respectively. Said contents may also be referred to as oxyethylene content and oxypropylene content, respectively. Further, said contents are based herein on total alkylene oxide weight. The term “ethylenic unsaturation” means an unsaturation comprising two double bonded carbon atoms (i.e. >C=C<) that is capable of undergoing free radically induced addition polymerization reactions. The term “ethylenically unsaturated monomer” means a compound having a relatively low molecular weight, preferably at most 500 g/mol, and containing at least one ethylenic unsaturation. The term “macromer” means a compound having a relatively high molecular weight, preferably at least 5,000 g/mol, and containing at least one ethylenic unsaturation. In the process of the present invention, a macromer is mixed with a diluent, resulting in a mixture comprising the macromer and diluent (i.e. a diluted macromer), in which process the macromer is mixed with diluent in a weight ratio of macromer to diluent of from 1:99 to 99:1. Preferably, said weight ratio is of from 20:80 to 80:20, more preferably 25:75 to 75:25, most preferably 40:60 to 60:40. Further, said weight ratio is at least 1:99, preferably at least 20:80, more preferably at least 25:75, more preferably at least 40:60, more preferably at least 45:55, most preferably at least 50:50. Further, said weight ratio is at most 99:1, preferably at most 95:5, more preferably at most 90:10, more preferably at most 85:15, more preferably at most 80:20, most preferably at most 75:25. In the present invention, mixing the macromer and diluent means combining the diluent with the macromer after the macromer has been prepared. This may be achieved through addition of the macromer to the diluent or the addition of the diluent to the macromer. It is preferred that diluent is added to the macromer. Further, preferably, diluent is added to the macromer or macromer is added to the diluent continuously for a period of time (i.e. not all-at-once), which may be of from 1 to 120 minutes, suitably of from 5 to 20 minutes. It is preferred that during and subsequent to mixing the macromer and the diluent, stirring is performed, before any further use of the resulting mixture comprising macromer and diluent. The period of time for said mixing and optional stirring may be at least 1 minute or at least 1 hour or at least 2 hours or at least 4 hours or at least 8 hours or at least 12 hours or at least 14 hours. Further, the period of time for said mixing and optional stirring may be at most 48 hours or at most 24 hours or at most 20 hours or at most 18 hours. Said stirring may be performed in any way, for example by using a roller bank, an impellor, an overhead stirrer or any other mode of stirring. Other than stirring, other suitable mixing methods for use in the present invention comprise use of a microwave or an ultra-sonicator. The temperature during said mixing and optional stirring may be of from 20 to 90 °C, suitably of from 30 to 70 °C, more suitably of from 40 to 60 °C. An increase in the temperature may shorten the required mixing and stirring time. Further, in the present invention, said macromer is a macromer which is prepared from a polyether polyol P1, wherein said macromer additionally comprises a moiety which contains an ethylenic unsaturation and which is attached to the oxygen atom of a hydroxyl group of polyether polyol P1, wherein the relative amount of the ethylenic unsaturation is of from greater than 0.6 to less than 1.8 mol per mol of macromer. Hence, in the present invention, a first step may be to prepare, and hence provide, a macromer from a polyether polyol P1, wherein said macromer additionally comprises a moiety which contains an ethylenic unsaturation and which is attached to the oxygen atom of a hydroxyl group of polyether polyol P1, wherein the relative amount of the ethylenic unsaturation is of from greater than 0.6 to less than 1.8 mol per mol of macromer. Such macromer preparation step is further described below. In the present invention, the macromer may be prepared by reacting polyether polyol P1 or a derivative of polyether polyol P1 with an unsaturated compound which contains one ethylenic unsaturation, such as to result in a macromer wherein the relative amount of the ethylenic unsaturation is of from greater than 0.6 to less than 1.8 mol per mol of macromer. Preferably, in the present invention, the amount of ethylenic unsaturation in the macromer is of from 0.8 to 1.6 mol per mol of macromer, more preferably of from 1.0 to 1.4 mol per mol of macromer, most preferably of from 1.1 to 1.3 mol per mol of macromer. In the present invention, the amount of ethylenic unsaturation in the macromer is greater than 0.6 mol per mol of macromer, preferably at least 0.8 mol per mol of macromer, more preferably at least 1.0 mol per mol of macromer, most preferably at least 1.1 mol per mol of macromer. Further, in the present invention, the amount of ethylenic unsaturation in the macromer is less than 1.8 mol per mol of macromer, preferably at most 1.6 mol per mol of macromer, more preferably at most 1.4 mol per mol of macromer, most preferably at most 1.3 mol per mol of macromer. Within the present specification, said relative amount of ethylenic unsaturation in the macromer is an average molar ratio of ethylenic unsaturation to macromer, averaging over all molecules in the macromer including reacted and any unreacted polyether polyol molecules or polyether polyol derivative molecules. Hence, said ratio does not have to be an integer and does not have to be equal to or greater than 1. Thus, in the present invention, the macromer may be prepared by reacting polyether polyol P1 or a derivative of polyether polyol P1 with an unsaturated compound which contains one ethylenic unsaturation, and wherein the relative amount of the unsaturated compound is of from greater than 0.6 to less than 1.8 mol per mol of polyether polyol P1 or derivative thereof. The above-mentioned preferences for the amount of ethylenic unsaturation in the macromer also apply to said relative amount of the unsaturated compound used in preparing the macromer. The above-mentioned unsaturated compound which contains one ethylenic unsaturation, may additionally contain a functional group with which polyether polyol P1 or a derivative of polyether polyol P1 may react. Said functional group may be an epoxide group; an isocyanate group of formula -N=C=O; an ester group of formula -CO ₂ Z where Z is a hydrocarbyl radical; an acyl halide group of formula -C(O)Y where Y is a halogen; an alkoxy group, suitably a C ₁ -C ₁₀ alkoxy group; a halide group; a carbamic acid ester group of formula -NHCO ₂ Z where Z is a hydrocarbyl radical; or an -OY group where Y is a halogen. Such functional groups are capable of reacting with a hydroxyl group on polyether polyol P1 or with another group on a derivative of polyether polyol P1, such as for example below-described derivative of polyether polyol P1 prepared by reacting polyether polyol P1 with a cyclic dicarboxylic acid anhydride. Preferably, said functional group in the above-mentioned unsaturated compound which contains one ethylenic unsaturation, is an epoxide group or an isocyanate group. In the latter cases, the unsaturated compound is a compound containing an epoxide group and one ethylenic unsaturation and a compound containing an isocyanate group and one ethylenic unsaturation, respectively. In particular, the above-mentioned unsaturated compound which contains one ethylenic unsaturation may be of the following formula: wherein: R ¹ represents a hydrogen atom, a branched or straight chain hydrocarbon group containing from 1 to 8 carbon atoms which may be saturated or unsaturated, or an aryl group containing from 6 to 12 carbon atoms, preferably a hydrogen atom; R ² represents a hydrogen atom, a branched or straight chain hydrocarbon group containing from 1 to 8 carbon atoms, or an aryl group containing from 6 to 12 carbon atoms, preferably a hydrocarbon group containing from 1 to 8 carbon atoms, more preferably a hydrocarbon group containing from 1 to 3 carbon atoms, most preferably a methyl group; A is absent or is a carboxyl moiety of formula -C(=O)O- wherein C=O is attached to the ethylenic unsaturation; R ³ represents a group (i) which contains a functional group with which polyether polyol P1 or a derivative of polyether polyol P1 may react, which functional group may be any one of the above-described functional groups, preferably an epoxide group or an isocyanate group, and (ii) which may contain a branched or straight chain hydrocarbon group containing from 1 to 8 carbon atoms, preferably 1 to 3 carbon atoms, or an aryl group containing from 6 to 12 carbon atoms which aryl group may be substituted by one or more, preferably one, branched or straight chain hydrocarbon groups containing from 1 to 8 carbon atoms, preferably 1 to 3 carbon atoms, wherein said functional group is attached to at least one of these substituents. For example, in the present invention, the macromer may be prepared by reacting polyether polyol P1 with a cyclic dicarboxylic acid anhydride not containing any ethylenic unsaturation, and subsequently reacting the adduct thus obtained (which is a derivative of polyether polyol P1) with an epoxide compound containing one ethylenic unsaturation. A suitable process for making a macromer using such two steps is disclosed in WO199940144. Said cyclic dicarboxylic acid anhydride preferably is phthalic anhydride. Further, said epoxide compound preferably is glycidyl methacrylate or glycidyl acrylate. Still further, the macromer may be prepared by reacting polyether polyol P1 directly with an epoxide compound containing one ethylenic unsaturation, such as said glycidyl methacrylate or glycidyl acrylate, without reacting polyether polyol P1 first with a cyclic dicarboxylic acid anhydride, such as phthalic anhydride. Still further, the macromer may be prepared by reacting polyether polyol P1 with maleic acid or maleic anhydride, which may be followed by isomerization of the maleate bond to the more reactive fumarate bond. In the present invention, it is preferred to prepare the macromer by reacting polyether polyol P1 or a derivative of polyether polyol P1 with an unsaturated isocyanate compound which contains at least one isocyanate group and which contains one ethylenic unsaturation. The above-mentioned unsaturated isocyanate compound may contain two or more isocyanate groups, but preferably it contains one isocyanate group. Suitable diisocyanates, containing two isocyanate groups, include various isomers of diphenylmethane diisocyanate and isomeric mixtures of diphenylmethane diisocyanate such as a mixture of 2,4'- diphenylmethane diisocyanate, 4,4'-diphenylmethane diisocyanate and/or 2,2'-diphenylmethane diisocyanate. A mixture of 2,4'-diphenylmethane diisocyanate and 4,4'- diphenylmethane diisocyanate is suitable. Other suitable diisocyanates include toluenediisocyanate, isophoronediisocyanate, hexamethylenediisocyanate and 4,4'- methylenebis(cyclohexyl isocyanate). Thus, preferably, the unsaturated isocyanate compound is a monoisocyanate containing one isocyanate group. Said monoisocyanate may be aromatic or non-aromatic. Most preferably, the unsaturated isocyanate compound comprises 1,1-dimethyl-meta-isopropenylbenzyl isocyanate (TMI) or 2- isocyanatoethyl methacrylate (IEM), which are both monoisocyanates. Preferably, the macromer is prepared by reacting polyether polyol P1 or a derivative of polyether polyol P1 with the unsaturated compound, in particular an unsaturated isocyanate compound, in the presence of a catalyst. Said catalyst preferably comprises tin or bismuth. Employed with particular preference as catalyst are dibutyltin dilaureate (DBTL) and bismuth neodecanoate. Further, said reaction may be performed at a temperature of 60 to 150 °C, preferably 80 to 130 °C, and a pressure of 0.5 to 2 bara (50 to 200 kPa), preferably 0.8 to 1.2 bara (80 to 120 kPa). In the present invention, the macromer may have a molecular weight of 5,000 to 25,000 g/mol, preferably 7,000 to 22,000 g/mol, more preferably 10,000 to 19,000 g/mol. In the present invention, polyether polyol P1 may have a hydroxyl (OH) functionality of from 2 to 8 or 3 to 8 or 4 to 8, preferably 5 to 7, more preferably 6, and may be prepared by (i.e. obtained by a process comprising) ring-opening polymerization of an alkylene oxide in the presence of an initiator having a plurality of active hydrogen atoms and a catalyst. Said catalyst may be a basic catalyst, such as potassium hydroxide (KOH), or a composite metal cyanide complex catalyst, which latter catalyst is frequently also referred to as double metal cyanide (DMC) catalyst. Further, said initiator having a plurality of active hydrogen atoms may have a hydroxyl (OH) functionality of from 2 to 8 or 3 to 8 or 4 to 8, preferably 5 to 7, more preferably 6. Such initiator may suitably comprise one or more of glycerol, trimethylolpropane, pentaerythritol, sorbitol and mannitol, preferably one or more of pentaerythritol, sorbitol and mannitol, more preferably sorbitol. Polyether polyol P1 may have a molecular weight of 5,000 to 25,000 g/mol, preferably 7,000 to 20,000 g/mol, more preferably 8,000 to 15,000 g/mol. Preferably, polyether polyol P1 comprises polyether chains containing propylene and/or butylene oxide content, more preferably propylene oxide (PO) content, and optionally ethylene oxide (EO) content. Preferably, polyether polyol P1 comprises polyether chains containing of from 0 wt.% to 25 wt.% of EO content. The EO content of polyether polyol P1 may be at most 25 wt.% or at most 20 wt.% or at most 15 wt.% or at most 12 wt.%. Further, the EO content of polyether polyol P1 may be 0 wt.% or at least 3 wt.% or at least 5 wt.% or at least 6 wt.% or at least 10 wt.% or at least 12 wt.% or at least 15 wt.%. Preferably, the remainder of the alkylene oxide content in the polyether chains of polyether polyol P1 is derived from propylene and/or butylene oxide. More preferably, the remainder of the alkylene oxide content in the polyether chains of polyether polyol P1 is derived from propylene oxide. Therefore, the polyether chains of polyether polyol P1 preferably comprise at least 75 wt.%, more preferably at least 80 wt.%, most preferably at least 85 wt.% of propylene oxide (PO) content. Further, the polyether chains of polyether polyol P1 may comprise 100 wt.% of PO content and preferably comprise at most 97 wt.%, more preferably at most 95 wt.%, most preferably at most 94 wt.% of PO content. In the present invention, the polyether chains of polyether polyol P1 may comprise no ethylene oxide content but may comprise only propylene and/or butylene oxide content, suitably only propylene oxide content. Further, polyether polyol P1 used in the process of the present invention preferably has a hydroxyl value of at least 15, more preferably at least 20, most preferably at least 25. Further, polyether polyol P1 preferably has a hydroxyl value of at most 50, more preferably at most 40, most preferably at most 35. In the present invention, the diluent with which the macromer is mixed may comprise one or more diluents. It is preferred that said diluent comprises a polyether polyol. Said diluent may comprise a diluent which is different from (i.e. not identical to) polyether polyol P1. Further, preferably, said diluent comprises a polyether polyol. Further, preferably, said polyether polyol in the diluent and polyether polyol P1 both comprise an EO only block or a PO/EO block wherein PO and EO are randomly distributed, wherein “EO” stands for ethylene oxide and “PO” stands for propylene oxide. The above blocks concern parts of the polyether polyol which are derived from ethylene oxide and/or propylene oxide. Alternatively, said diluent may comprise a diluent which is identical to polyether polyol P1. In the latter case, the above description of polyether polyol P1 equally applies to such diluent. Still further, it is preferred that said diluent comprises a diluent which is identical to a base polyol used in the below-described polymer polyol preparation process. In the latter case, the below description of the base polyol equally applies to such diluent. It is also envisaged in the present invention, that said diluent comprises a diluent which is identical to a chain transfer agent or an ethylenically unsaturated monomer used in the below-described polymer polyol preparation process. A suitable example of a chain transfer agent that may be used as such diluent, is isopropanol. Further, suitable examples of ethylenically unsaturated monomers that may be used as such diluent, are styrene and acrylonitrile. Furthermore, a solution of the polymerization initiator, for example in a base polyol, used in the below-described polymer polyol preparation process, may also be used as such diluent. Apart from achieving a relatively low viscosity and a relatively high filterability by using one of the above as diluent for the macromer, by using one or more of these as such diluent, advantageously, no new chemicals are introduced into the polymer preparation process. In the present invention, it is preferred that the diluent with which the macromer is mixed, cannot react with the macromer or is mixed with the macromer under conditions under which substantially no reaction between the diluent and macromer can take place. The present invention also relates to a process for preparing a polymer polyol, comprising mixing a base polyol, one or more ethylenically unsaturated monomers, a polymerization initiator, the mixture comprising the macromer obtained by the above-described process, and optionally a chain transfer agent, and polymerizing the mixture thus obtained at a temperature of 50 to 200 °C. Said mixture comprising the macromer also comprises the diluent with which the macromer is mixed. In addition to adding said mixture comprising macromer and diluent, the macromer may also be added as such, not as part of said mixture. It is preferred that of from 10% to 100%, more preferably of from 30% to 100%, more preferably of from 50% to 100%, more preferably of from 70% to 100%, more preferably of from 80% to 100%, more preferably of from 90% to 100%, most preferably 100%, of the total amount of macromer added in the above-mentioned polymer preparation process is added as part of the mixture comprising the macromer obtained by the above-described process, said mixture comprising macromer and diluent. The temperature at which polymerization is carried out, is comprised in the range of from 50 to 200 °C, preferably 70 to 150 °C, more preferably 80 to 130 °C. Further, preferably, the polymerization temperature is less than 120 °C, more preferably less than 115 °C, most preferably 110 °C or less. Further, it is preferred that during the entire polymerization process, the temperature is maintained constant at a certain value, with the proviso that a deviation of plus or minus 10 °C from said value is still allowable. Further, it is preferred that during the entire polymerization process, the temperature is maintained at a value comprised in the range of from 70 to 120 °C, preferably 80 to 115 °C. The pressure at which polymerization may be carried out, is suitably comprised in the range of from 0.01 to 5 bara (1 to 500 kPa), more suitably 0.05 to 4 bara (5 to 400 kPa). The base polyol used in the present polymer polyol preparation process preferably is a polyether polyol. Such polyether polyols may be obtained in the same way as described above for polyether polyol P1, by ring-opening polymerization of an alkylene oxide in the presence of an initiator having a plurality of active hydrogen atoms and a catalyst. Said catalyst may be a basic catalyst, such as potassium hydroxide (KOH), or a composite metal cyanide complex catalyst, which latter catalyst is frequently also referred to as double metal cyanide (DMC) catalyst. Further, in preparing the base polyol, said initiator having a plurality of active hydrogen atoms may have a hydroxyl (OH) functionality of from 2 to 8, preferably 2 to 4, more preferably 2 to 3. Such initiator may suitably comprise one or more of monopropylene glycol, glycerol, trimethylolpropane, pentaerythritol, sorbitol and mannitol, preferably one or more of monopropylene glycol, glycerol and trimethylolpropane, more preferably monopropylene glycol or glycerol, most preferably glycerol. The base polyol may have a hydroxyl (OH) functionality which is at least 2. The latter functionality may be of from 2 to 8, preferably 2 to 4, more preferably 2 to 3. The base polyol may have a molecular weight of 350 to 15,000 g/mol, preferably 1,000 to 10,000 g/mol, more preferably 2,000 to 6,000 g/mol. Preferably, the base polyol comprises polyether chains containing propylene and/or butylene oxide content, more preferably propylene oxide (PO) content, and optionally ethylene oxide (EO) content. The base polyol may comprise polyether chains containing of from 0 wt.% to at most 25 wt.%, more preferably at most 20 wt.%, most preferably at most 12 wt.% of EO content. Further, preferably, the base polyol comprises polyether chains containing at least 3 wt.%, more preferably at least 5 wt.%, most preferably at least 6 wt.% of EO content. Preferably, the remainder of the alkylene oxide content in the polyether chains of the base polyol is derived from propylene and/or butylene oxide. More preferably, the remainder of the alkylene oxide content in the polyether chains of the base polyol is derived from propylene oxide. Therefore, the polyether chains of the base polyol preferably comprise at least 75 wt.%, more preferably at least 80 wt.%, most preferably at least 85 wt.% of propylene oxide (PO) content. Further, the polyether chains of the base polyol may comprise 100 wt.% of PO content and preferably comprise at most 97 wt.%, more preferably at most 95 wt.%, most preferably at most 94 wt.% of PO content. In the present invention, the polyether chains of the base polyol may comprise no ethylene oxide content but may comprise only propylene and/or butylene oxide content, suitably only propylene oxide content. Further, the base polyol preferably has a hydroxyl value of at least 10, more preferably at least 20, most preferably at least 25. Further, the base polyol preferably has a hydroxyl value of at most 150, more preferably at most 75, most preferably at most 65. It is possible to have all base polyol fed initially, while it is also possible to add the majority of the base polyol after initiation of polymerization. The additional base polyol optionally added after initiation of polymerization can be the same or can be different from the base polyol as initially fed. Preferably, the base polyol remains the same. The polymer made in the above polymer polyol preparation process is a polymer resulting from the polymerization of the one or more ethylenically unsaturated monomers and the above- described macromer. Suitable ethylenically unsaturated monomers comprise vinyl aromatic hydrocarbons, like styrene, alpha-methyl styrene, methyl styrene and various other alkyl- substituted styrenes. Of these, styrene is preferred. Other suitable ethylenically unsaturated monomers comprise acrylonitrile, methacrylonitrile, vinylidene chloride, various acrylates and conjugated dienes like 1,3-butadiene and isoprene. Of these, acrylonitrile is preferred. Further, it is preferred to use both styrene and acrylonitrile as ethylenically unsaturated monomers, in addition to the macromer, thus resulting in a terpolymer. In the latter case, preferably, the weight ratio of styrene to acrylonitrile is 30:70 to less than 100:0, more preferably 50:50 to 75:25. In the present invention, the above-mentioned ethylenically unsaturated monomer may have a molecular weight of at most 500 g/mol, preferably at most 250 g/mol, more preferably at most 150 g/mol. Further, the molecular weight of the ethylenically unsaturated monomer may be at least 28 g/mol, preferably at least 50 g/mol. The polymerization initiator is usually applied in an amount of from 0.01 to 5% by weight based on total weight of ethylenically unsaturated monomer(s). Chain transfer agents may also be added to or be present in the polymerization reaction medium. Preferably, they are fed to the reactor in the initial phase of the process. The use of chain transfer agents and their nature is known in the art. Chain transfer agents enable a control of the molecular weight and/or the cross-linking occurring between the various polymer molecules and hence may affect stability of the polymer polyol. If used at all, a chain transfer agent is suitably used in an amount of from 0.1 to 20% by weight, more suitably 0.2 to 10% by weight, and most suitably 0.3 to 7% by weight, based on total weight of end product. Examples of suitable chain transfer agents are 1-butanol, 2-butanol, isopropanol, ethanol, methanol, cyclohexane and mercaptans, such as dodecanethiol, ethanethiol, 1-heptanethiol, 2- octanethiol and toluenethiol. Preferably, isopropanol is used as a chain transfer agent. Other compounds, such as compounds facilitating mixing of the various components, compounds which have a viscosity- lowering effect and/or compounds which enable one or more of the components used to better dissolve in the reaction medium may also be applied. An example of a compound having a viscosity-lowering effect, thus enabling a better mixing of the components, is toluene. Auxiliaries like toluene can be present in the feed and/or in the reactor. It is preferred in the above polymer polyol preparation process that at least part, which may be at least 50% or at least 60% or at least 70% or at least 80% or at least 90%, most preferably all (100%), of the total amount of diluted macromer to be added, is mixed with one or more ethylenically unsaturated monomers, optionally base polyol and optionally a chain transfer agent before mixing with polymerization initiator, wherein any remainder of the total amount of diluted macromer is added after and/or during said mixing with the polymerization initiator. Further, it is preferred in the above polymer polyol preparation process that at least part, which may be at least 50% or at least 60% or at least 70% or at least 80% or at least 90%, most preferably all (100%), of the total amount of diluted macromer to be added, is mixed with at most 50 wt.% of the total amount of the one or more ethylenically unsaturated monomers to be added, optionally base polyol, optionally a chain transfer agent and optionally polymerization initiator before mixing with the remainder of the total amount of the one or more ethylenically unsaturated monomers, wherein any remainder of the total amount of diluted macromer is added and the remainder of the total amount of the one or more ethylenically unsaturated monomers are added after said pre-mixing. The period of time for such pre-mixing step may be of from 1 to 120 minutes, suitably of from 5 to 90 minutes. Said amount of the one or more ethylenically unsaturated monomers to be added in the above- described optional pre-mixing step is greater than 0 wt.% and may be at least 1 wt.%, and is at most 50 wt.%, preferably at most 30 wt.%, more preferably at most 20 wt.%, more preferably at most 10 wt.%, most preferably at most 5 wt.%, of the total amount of the one or more ethylenically unsaturated monomers to be added in the above polymer polyol preparation process. The dispersed polymer in the polymer polyol resulting from the above-described process is suitably present in an amount of from 10 to 60 wt.%, suitably 15 to 55 wt.%, more suitably 30 to 45 wt.%, based on total weight of the polyol and polymer. The present invention also relates to a polymer polyol obtainable by the above-described process. The present invention also relates to a process for preparing a polyurethane foam using the above-described polymer polyol, which process comprises reacting the polymer polyol and a polyisocyanate in the presence of a blowing agent. In the present polyurethane foam preparation process, the polyisocyanate may be an aromatic polyisocyanate or an aliphatic polyisocyanate, preferably an aromatic polyisocyanate. The aromatic polyisocyanate may for example comprise tolylene diisocyanate (TDI) or polymeric TDI, xylylene diisocyanate, tetramethylxylylene diisocyanate, methylene diphenyl diisocyanate (MDI) or polymeric MDI (i.e. polymethylene polyphenyl isocyanate), or a modified product thereof. Preferably, the aromatic polyisocyanate comprises tolylene diisocyanate (TDI), i.e. non-polymeric TDI. The TDI may be a mixture of 80 wt.% of 2,4-TDI and 20 wt.% of 2,6- TDI, which mixture is sold as “TDI-80”. Further, the aliphatic polyisocyanate may for example comprise hexamethylene diisocyanate, dicyclohexylmethane diisocyanate, lysine diisocyanate or isophorone diisocyanate, or a modified product thereof. Further, the polyisocyanate may be any mixture of two or more of the polyisocyanates mentioned above. For example, the polyisocyanate may be a mixture of TDI and MDI, in particular a mixture wherein the weight ratio of TDI:MDI varies from 10:90 to 90:10. In the present invention, the blowing agent may be a chemical blowing agent or a physical (non-chemical) blowing agent. Within the present specification, by “chemical blowing agent” reference is made to a blowing agent that may only provide a blowing effect after it has chemically reacted with another compound. Preferably, in the present invention, the blowing agent is a chemical blowing agent. Further, preferably, the chemical blowing agent comprises water. Water reacts with isocyanate groups of the polyisocyanate, thereby releasing carbon dioxide which causes the blowing to occur. Further, preferably, substantially no physical (non-chemical) blowing agent is added in the present process. However, other suitable blowing agents, such as for example, acetone, gaseous or liquid carbon dioxide, halogenated hydrocarbons, aliphatic alkanes and alicyclic alkanes may be employed additionally or alternatively. Due to the ozone depleting effect of fully chlorinated, fluorinated alkanes (CFC’s) the use of this type of blowing agent is generally not preferred, although it is possible to use them within the scope of the present invention. Halogenated alkanes, wherein at least one hydrogen atom has not been substituted by a halogen atom (the so-called HCFC’s) have no or hardly any ozone depleting effect and therefore are the preferred halogenated hydrocarbons to be used in physically blown foams. One suitable HCFC type blowing agent is 1-chloro-l,1-difluoroethane. In the present invention, in a case where the blowing agent comprises water, water may be used in an amount of from 0.1 to 10 parts per hundred parts by weight of polyol (pphp), more preferably of from 0.5 to 8 pphp, more preferably of from 1 to 6 pphp, more preferably of from 1.5 to 4 pphp. In case of halogenated hydrocarbons, aliphatic alkanes and alicyclic alkanes, the amount of the blowing agent may be of from 1 to 50 parts per hundred parts by weight of polyol (pphp), suitably of from 1 to 30 pphp, more suitably of from 1 to 20 pphp. The above blowing agents may be used singly or in mixtures of two or more. In the present process, the isocyanate index (or NCO index) may be at most 150, more suitably at most 140, more suitably at most 130, more suitably at most 125, most suitably at most 120. The isocyanate index is preferably higher than 90, more preferably higher than 100, most preferably higher than 105. Within the present specification, “isocyanate index” is calculated as 100 times the mole ratio of —NCO groups (isocyanate groups) to NCO—reactive groups in the reaction mixture. In other words, the “isocyanate index is defined as: [(actual amount of isocyanate)/(theoretical amount of isocyanate)]*100, wherein the “theoretical amount of isocyanate” equals 1 equivalent isocyanate (NCO) group per 1 equivalent isocyanate-reactive group. Such “isocyanate-reactive groups” as referred to above include for example OH groups from the polyether polyols and from any water that may be used as a blowing agent. Isocyanate groups also react with water. Additionally, other components may also be present during the polyurethane foam preparation process of the present invention, such as one or more polyurethane catalysts, surfactants and/or cross-linking agents. Polyurethane catalysts are known in the art and include many different compounds. For the purpose of the present invention, suitable catalysts include tin-, lead- or titanium-based catalysts, preferably tin-based catalysts, such as tin salts and dialkyl tin salts of carboxylic acids. Specific examples are stannous octoate, stannous oleate, dibutyltin dilaureate, dibutyltin acetate and dibutyltin diacetate. Other suitable catalysts are tertiary amines, such as, for instance, bis(2,2'-dimethylamino)ethyl ether, trimethylamine, triethylamine, triethylenediamine and dimethylethanolamine (DMEA). Examples of commercially available tertiary amine catalysts are those sold under the tradenames Niax, Tegoamin and Dabco (all trademarks). The catalyst is typically used in an amount of from 0.01 to 2.0 parts by weight per hundred parts by weight of polyether polyol (pphp). Preferred amounts of catalyst are from 0.05 to 1.0 pphp. The use of cross-linking agents in the production of polyurethane foams is also well known. Polyfunctional glycol amines are known to be useful for this purpose. The polyfunctional glycol amine which is most frequently used and is also useful in the preparation of polyurethane foams, especially flexible polyurethane foams, is diethanol amine, often abbreviated as DEOA. If used at all, the cross-linking agent is applied in amounts up to 4 parts by weight per hundred parts by weight of polyol (pphp), but amounts in the range of from 0.01 to 2 pphp are more suitably applied, most suitably 0.01 to 0.5 pphp. In addition, other well known auxiliaries, such as colorants, flame retardants and fillers, may also be used during the polyurethane foam preparation process of the present invention. In specific, an auxiliary which promotes cell opening may also be used during the polyurethane foam preparation process of the present invention. The present polyurethane foam preparation process may involve combining the polymer polyol, the polyisocyanate, the blowing agent, a foam stabiliser, a catalyst and optionally crosslinker, flame retardant, colorant and/or filler, in any suitable manner to obtain the polyurethane foam. For example, the present process may comprise stirring the polymer polyol, the blowing agent, a foam stabiliser, a catalyst and any other optional component(s) except the polyisocyanate together for a period of at least 30 seconds; and adding the polyisocyanate under stirring. Further, the process of the invention may comprise forming the foam into a shaped article before it fully sets. Suitably, forming the foam may comprise pouring the liquid mixture containing all components into a mould before gelling is complete. The present invention also relates to a polyurethane foam obtainable by the above-described process, and to a shaped article comprising said polyurethane foam. The invention is further illustrated by the following Examples. Examples Materials (polyether polyols, polyisocyanate and other components) used in the experiments are described in Table 1 below. Table 1 DMC = double metal cyanide; KOH = potassium hydroxide; EO = ethylene oxide; PO = propylene oxide; EO and PO contents based on total polyol weight; MW = molecular weight; PHC = primary hydroxyl content 1. Preparation of macromer 882 g of Polyol A was charged to a double jacketed glass reactor while continuously stirring. The reactor contents were heated to 90 °C by external heating of the reactor wall using a heated oil bath. Once a temperature of 90 °C was attained, 18.2 g of 1,1-dimethyl-meta-isopropenylbenzyl isocyanate (TMI; 1.2 mol per mol of polyether polyol) was added to the reactor and the reactor contents were then mixed for 50 minutes. Then 0.1 g (100 ppmw) of a catalyst (bismuth neodecanoate) was added to the reactor. The reactor contents were then further stirred for 150 minutes. After this, the reactor was allowed to cool and the macromer was collected. No free TMI was present in the macromer. 2. Preparation of mixture of macromer and diluent The macromer thus obtained and a diluent were mixed in a weight ratio of macromer to diluent of 50:50 at room temperature (20 °C), resulting in a mixture comprising macromer and diluent, hereinafter also referred to as diluted macromer. The diluent was added to the macromer continuously for 10 minutes. The mixture was stirred at room temperature (20 °C) for 16 hours using a roller bank, before further use. The diluent was the same polyether polyol as applied as base polyol (i.e. Polyol B) in the below-described polymer polyol preparation process. 3. Preparation of polymer polyol A polymer polyol was prepared by applying the following batch procedure, comprising a pre-dose phase and a main dose phase. In the pre-dose phase, 495 g of base polyol (i.e. Polyol B), 47 g of diluted macromer prepared in the above way (invention) or undiluted macromer (comparison), 85 g of isopropanol (IPA), 15 g of styrene and 7 g of acrylonitrile were fed all-at-once to a double jacketed glass reactor while continuously stirring. The reactor contents were heated to 100 °C by external heating of the reactor wall using a heated oil bath. Said pre-dose phase took 30 minutes. In the main dose phase, once a temperature of 108 °C was attained, the polymerisation was started by continuously feeding 7.5 g of a 60 wt.% solution of a polymerization initiator (1,1-dimethyl propyl peroxy-2-ethylhexanoate) in base polyol, 528 g of base polyol, 570 g of styrene and 270 g of acrylonitrile to the reactor. The polymerization temperature within the reactor was maintained between 100 and 110 °C. Said main dose phase took 120 minutes. The resulting polymer polyols (dispersions) contained 45 wt.% of solids. The above-mentioned amount of 47 g of diluted or undiluted macromer, as fed in the pre-dose phase, corresponds with 2.4 wt.%, based on total amount of starting materials (as fed in pre-dose and main dose phases, said total amount excluding isopropanol which was removed at the end of the batch process). Further, another experiment using 4.8 wt.% (instead of said 2.4 wt.%) of diluted macromer was performed, thereby using the same amount of macromer as in the comparison experiment but diluted to 50%. Still further, two other comparison experiments were performed, wherein the macromer was prepared using 0.6 or 0.4 mol of TMI per mol of macromer. In Table 2, the differences in these 5 experiments are shown (3 comparison experiments and 4 experiments in accordance with the invention). The viscosity and filtration behaviour of the polymer polyols thus obtained were measured. The measured viscosity was the kinematic viscosity (at 25 °C; mPa∙s). Further, the filterability was measured by first mixing 200 g of the polymer polyol with 400 g of isopropanol for 30 minutes on a roller bank, followed by filtering the polymer polyol containing mixture (same amount for all experiments), at 25 °C and atmospheric pressure, through filters having pore sizes of 30 µm and 100 µm and measuring the time at which polymer polyol was no longer passing through the filter. The shorter the latter time period, the better the filterability is because of a higher filtration rate which advantageously makes processing and use of the polymer polyol easier. The measurement results are shown in the last 2 columns of Table 2 below. Table 2 (*) = not according to invention; n.a. = not applicable; (1): for diluted macromer the mixing ratio is shown, which is the weight ratio of macromer to diluent; (2): wt.% based on total amount of starting materials (as fed in pre-dose and main dose phases) Table 2 - continued As can be seen from Table 2 above, when using 0.6 or less mol of TMI per mol of macromer when making the macromer (in Comparison Experiments 1 and 2), a relatively low filterability of the polymer polyol was obtained as compared to a case wherein more TMI is used (as in Comparison Experiment 3 wherein 1.2 mol of TMI per mol of macromer was used). However, in the latter Comparison Experiment 3, the viscosity of the polymer polyol was disadvantageously relatively high as compared to Comparison Experiments 1 and 2. However, as can also be seen from Table 2 above, surprisingly and advantageously, it was found that when both (i) the macromer was prepared using greater than 0.6 mol of TMI per mol of macromer (in Experiments 4 and 5: 1.2 mol) and (ii) the resulting macromer was diluted with a diluent prior to making a polymer polyol from that macromer (in Experiments 4 and 5: diluted in a weight ratio of 50:50), both the polymer polyol viscosity was reduced and the filterability of the polymer polyol was improved, which advantageously makes processing and use of the polymer polyols of the invention easier. Furthermore, in addition to achieving a reduced viscosity and an improved filterability as described above, it is also advantageous that the polymer polyol as obtained in Experiment 4 was made using 50% less macromer (i.e. 1.2 wt.%), because prior to polymer polyol preparation the macromer was diluted to 50% whereas the amount of diluted macromer used during polymer polyol preparation remained the same as the amount of undiluted macromer in the comparison experiments (see e.g. Comparison Experiment 3). This reduced macromer usage advantageously results in improved production efficiency. Still further, as can also be seen from Table 2 above, when less diluent is used in diluting the macromer, as in Experiments 6 and 7 as compared to Experiment 4, the above- described reduction in viscosity and improvement in filterability as described above, were advantageously also observed. Further experiments were done, wherein instead of the above-described base polyol Polyol B, another diluent was used to dilute the macromer before making the polymer polyol, as described in Table 3 below. These other diluents comprised other base polyols and isopropanol. Advantageously, by using these other diluents a relatively low viscosity and a relatively high filterability were also achieved. In addition, as with base polyol Polyol B, no new chemicals were introduced into the polymer preparation process by using these other diluents, because in case of Polyol C, D or E as diluent that was also used as a base polyol in the subsequent polymer preparation process. Further, in case of isopropanol as diluent, that same isopropanol was also used as a chain transfer agent in the subsequent polymer preparation process.

Table 3 (1): the mixing ratio is the weight ratio of macromer to diluent; (2): wt.% based on total amount of starting materials (as fed in pre-dose and main dose phases) Table 3 - continued