the polymer component ( a ) used in the composition of the invention can be prepared in a variety of ways . one way to prepare such polymers is to prepare an acrylic monomer having a carbamate functionality in the ester portion of the monomer . such monomers are well - known in the art and are described , for example in u . s . pat . nos . 3 , 479 , 328 , 3 , 674 , 838 , 4 , 126 , 747 , 4 , 279 , 833 , and 4 , 340 , 497 , the disclosures of which are incorporated herein by reference . one method of synthesis involves reaction of a hydroxy ester with urea to form the carbamyloxy carboxylate ( i . e ., carbamate - modified acrylic ). another method of synthesis reacts an α , β - unsaturated acid ester with a hydroxy carbamate ester to form the carbamyloxy carboxylate . yet another technique involves formation of a hydroxyalkyl carbamate by reacting a primary or secondary amine or diamine with a cyclic carbonate such as ethylene carbonate . the hydroxyl group on the hydroxyalkyl carbamate is then esterified by reaction with acrylic or methacrylic acid to form the monomer . other methods of preparing carbamate - modified acrylic monomers are described in the art , and can be utilized as well . the acrylic monomer can then be polymerized along with other ethylenically - unsaturated monomers , if desired , by techniques well - known in the art . an alternative route for preparing the polymer ( a ) used in the composition of the invention is to react an already - formed polymer such as an acrylic polymer with another component to form a carbamate - functional group appended to the polymer backbone , as described in u . s . pat . no . 4 , 758 , 632 , the disclosure of which is incorporated herein by reference . one technique for preparing polymers useful as component ( a ) involves thermally decomposing urea ( to give off ammonio and hnco ) in the presence of a hydroxy - functional acrylic polymer to form a carbamate - functional acrylic polymer . another technique involves reacting the hydroxyl group of a hydroxyalkyl carbamate with the isocyanate group of an isocyanate - functional acrylic or vinyl monomer to form the carbamate - functional acrylic . isocyanate - functional acrylics are known in the art and are described , for example in u . s . pat . no . 4 , 301 , 257 , the disclosure of which is incorporated herein by reference . isocyanate vinyl monomers are well - known in the art and include unsaturated m - tetramethyl xylene isocyanate ( sold by american cyanamid as tmi ®). yet another technique is to react the cyclic carbonate group on a cyclic carbonate - functional acrylic with ammonia in order to form the carbamate - functional acrylic . cyclic carbonate - functional acrylic polymers are known in the art and are described , for example , in u . s . pat . no . 2 , 979 , 514 , the disclosure of which is incorporated herein by reference . a more difficult , but feasible way of preparing the polymer would be to trans - esterify an acrylate polymer with a hydroxyalkyl carbamate . the polymer ( a ) will generally have a molecular weight of 2000 - 20 , 000 , and preferably from 4000 - 6000 . molecular weight can be determined by the gpc method using a polystyrene standard . the carbamate content of the polymer , on a molecular weight per equivalent of carbamate functionality , will generally be between 200 and 1500 , and preferably between 300 and 350 . the glass transition temperature , t g , of components ( a ) and ( b ) can be adjusted to achieve a cured coating having the t g for the particular application involved . the average t g of unreacted components ( a ) and ( b ) should be between 10 ° c . and 80 ° c ., with the individual t g &# 39 ; s being adjusted to achieve optimum performance . the polymer component ( a ) can be represented by the randomly repeating units according to the following formula : ## str1 ## in the above formula , r 1 represents h or ch 3 . r2 represents h , alkyl , preferably of 1 to 6 carbon atoms , or cycloalkyl , preferably up to 6 ring carbon atoms . it is to be understood that the terms alkyl and cycloalkyl are to include substituted alkyl and cycloalkyl , such as halogen - substituted alkyl or cycloalkyl . substituents that will have an adverse impact on the properties of the cured material , however , are to be avoided . for example , ether linkages are thought to be susceptible to hydrolysis , and should be avoided in locations that would place the ether linkage in the crosslink matrix . the values x and y represent weight percentages , with x being 10 to 90 % and preferably 40 to 60 %, and y being 90 to 10 % and preferably 60 to 40 %. in the formula , a represents repeat units derived from one or more ethylenically unsaturated monomers . such monomers for copolymerization with acrylic monomers are known in the art . they include alkyl esters of acrylic or methacrylic acid , e . g ., ethyl acrylate , butyl acrylate , 2 - ethylhexyl acrylate , butyl methacrylate , isodecyl methacrylate , hydroxyethyl methacrylate , hydroxypropyl acrylate , and the like ; and vinyl monomers such as unsaturated m - tetramethyl xylene isocyanate ( sold by american cyanamid as tmi ®), styrene , vinyl toluene and the like . l represents a divalent linking group , preferably an aliphatic of 1 to 8 carbon atoms , cycloaliphatic , or aromatic linking group of 6 to 10 carbon atoms . examples of l include ## str2 ## --( ch 2 )--, --( ch 2 ) 2 --, --( ch 2 ) 4 --, and the like . in one preferred embodiment , -- l -- is represented by -- coo -- l &# 39 ;-- where l &# 39 ; is a divalent linking group . thus , in a preferred embodiment of the invention , the polymer component ( a ) is represented by randomly repeating units according to the following formula : ## str3 ## in this formula , r 1 , r 2 , a , x , and y are as defined above . l &# 39 ; may be a divalent aliphatic linking group , preferably of 1 to 8 carbon atoms , e . g ., --( ch 2 )--, --( ch 2 ) 2 --, --( ch 2 ) 4 --, and the like , or a divalent cycloaliphatic linking group , preferably up to 8 carbon atoms , e . g ., cyclohexyl , and the like . however , other divalent linking groups can be used , depending on the technique used to prepare the polymer . for example , if a hydroxyalkyl carbamate is adducted onto an isocyanate - functional acrylic polymer , the linking group l &# 39 ; would include an -- nhcoo -- urethane linkage as a residue of the isocyanate group . the composition of the invention is cured by a reaction of the carbamate - functional polymer component ( a ) with a component ( b ) that is a compound having a plurality of functional groups that are reactive with the carbamate groups on component ( a ). such reactive groups include active methylol or methylalkoxy groups on aminoplast crosslinking agents or on other compounds such as phenol / formaldehyde adducts , isocyanate groups , siloxane groups , cyclic carbonate groups , and anhydride groups . examples of ( b ) compounds include melamine formaldehyde resin ( including monomeric or polymeric melamine resin and partially or fully alkylated melamine resin ), urea resins ( e . g ., methylol ureas such as urea formaldehyde resin , alkoxy ureas such as butylated urea formaldehyde resin ), polyanhydrides ( e . g ., polysuccinic anhydride ), and polysiloxanes ( e . g ., trimethoxy siloxane ). aminoplast resin such as melamine formaldehyde resin or urea formaldehyde resin are especially preferred . even more preferred are aminoplast resins where one or more of the amino nitrogens is substituted with a carbamate group for use in a process with a curing temperature below 150 ° c ., as described in the concurrently - filed u . s . patent application entitled &# 34 ; carbamate - defunctionalized aminoplast curing for polymer compositions &# 34 ; in the names of john w . rehfuss and donald l . st . aubin . a solvent may optionally be utilized in the clearcoat composition used in the practice of the present invention . although the composition used according to the present invention may be utilized , for example , in the form of substantially solid powder , or a dispersion , it is often desirable that the composition is in a substantially liquid state , which can be accomplished with the use of a solvent . this solvent should act as a solvent with respect to both the carbamate - functional polymer ( a ) as well as the component ( b ). in general , depending on the solubility characteristics of components ( a ) and ( b ), the solvent can be any organic solvent and / or water . in one preferred embodiment , the solvent is a polar organic solvent . more preferably , the solvent is a polar aliphatic solvents or polar aromatic solvents . still more preferably , the solvent is a ketone , ester , acetate , aprotic amide , aprotic sulfoxide , or aprotic amine . examples of useful solvents include methyl ethyl ketone , methyl isobutyl ketone , m - amyl acetate , ethylene glycol butyl ether - acetate , propylene glycol monomethyl ether acetate , xylene , n - methylpyrrolidone , or blends of aromatic hydrocarbons . in another preferred embodiment , the solvent is water or a mixture of water with small amounts of aqueous co - solvents . the clearcoat composition used in the practice of the invention may include a catalyst to enhance the cure reaction . for example , when aminoplast compounds , especially monomeric melamines , are used as component ( b ), a strong acid catalyst may be utilized to enhance the cure reaction . such catalysts are well - known in the art and include , for example , p - toluenesulfonic acid , dinonylnaphthalene disulfonic acid , dodecylbenzenesulfonic acid , phenyl acid phosphate , monobutyl maleate , butyl phosphate , and hydroxy phosphate ester . other catalysts that may be useful in the composition of the invention include lewis acids , zinc salts , and tin salts . in a preferred embodiment of the invention , the solvent is present in the clearcoat composition in an amount of from about 0 . 01 weight percent to about 99 weight percent , preferably from about 10 weight percent to about 60 weight percent , and more preferably from about 30 weight percent to about 50 weight percent . coating compositions can be coated on the article by any of a number of techniques well - known in the art . these include , for example , spray coating , dip coating , roll coating , curtain coating , and the like . for automotive body panels , spray coating is preferred . pigmented basecoat compositions for such composite coatings are well - known in the art , and do not require explanation in detail herein . polymers known in the art to be useful in basecoat compositions include acrylics , vinyls , polyurethanes , polycarbonates , polyesters , alkyds , and polysiloxanes . preferred polymers include acrylics and polyurethanes . in one preferred embodiment of the invention , the basecoat composition also utilizes a carbamate - functional acrylic polymer . basecoat polymers are preferably crosslinkable , and thus comprise one or more type of cross - linkable functional groups . such groups include , for example , hydroxy , isocyanate , amine , epoxy , acrylate , vinyl , silane , and acetoacetate groups . these groups may be masked or blocked in such a way so that they are unblocked and available for the cross - linking reaction under the desired curing conditions , generally elevated temperatures . useful cross - linkable functional groups include hydroxy , epoxy , acid , anhydride , silane , and acetoacetate groups . preferred cross - linkable functional groups include hydroxy functional groups and amino functional groups . basecoat polymers may be self - cross - linkable , or may require a separate cross - linking agent that is reactive with the functional groups of the polymer . when the polymer comprises hydroxy functional groups , for example , the cross - linking agent may be an aminoplast resin , isocyanate and blocked isocyanates ( including isocyanurates ), and acid or anhydride functional cross - linking agents . after an article is coated with the above - described layers , the composition is subjected to conditions so as to cure the coating layers . although various methods of curing may be used , heat - curing is preferred . generally , heat curing is effected by exposing the coated article to elevated temperatures provided primarily by radiative heat sources . curing temperatures will vary depending on the particular blocking groups used in the cross - linking agents , however they generally range between 93 ° c . and 177 ° c ., and are preferably between 121 ° c . and 141 ° c . the curing time will vary depending on the particular components used , and physical parameters such as the thickness of the layers , however , typical curing times range from 15 to 60 minutes . a three - necked 5 - 1 round bottom flask was fitted with an agitator at the center neck and a thermal couple at one of the side necks to monitor the reaction temperature . a nitrogen purge line was also fed through this neck . the second side neck was fitted with a claissen adaptor and water cooled condenser . 198 g urethane - grade mixed aromatics solvent ( solvesso ® 100 ) and 225 g urethane - grade toluene were charged to the flask . the mixture was agitated and heated to reflux with a nitrogen purge . as the mixture reached reflux temperature , 127 ° c ., the nitrogen purge was discontinued . 923 g tmi ® ( unsaturated m - tetramethyl xylene isocyanate , american cyanamid ), 692 g ethyl hexyl acrylate and 269 g of a 50 % solution of t - butyl peracetate in odorless mineral spirits were charged to a separate container . this mixture was pumped to the refluxing solvents over a period of 3 . 5 hour . at the conclusion of this first feed , a second addition of 27 g of the t - butyl peracetate solution and 27 g urethane grade mixed aromatics were charged over 30 minutes . 8 . 2 g urethane - grade mixed aromatics was flushed through the pump and into the reaction mixture after the second initiator feed . the reaction mixture was then held at reflux , 135 ° c . for one hour . after this hold period , the batch was cooled to 70 ° c . 1 . 1 g dibutyltin dilaurate was charged and mixed into the batch for five minutes . at this point , 565 g hydroxypropyl carbamate was charged to the reaction mixture over 30 minutes . the batch was then slowly heated to 100 ° c . and held at this temperature until isocyanate functionality had disappeared as determined by infrared spectroscopy or titration . upon the disappearance of the isocyanate , 852 g monobutyl ether of ethylene glycol was charged to the vessel and allowed to homogenize . the heat to the reaction was turned off and the carbamate functional acrylic was removed from the vessel . a three - necked 5 - 1 round - bottomed flask was fitted with a vacuum sealed agitator at the center neck and a thermocouple at a side neck to monitor the reaction temperature . the second side neck as temporarily fitted with a water cooled condensor . vacuum was applied through a collecting vessel and supercooled condensor via this side neck of the reaction flask . 1708 g hexamethoxylated monomeric melamine and 1044 g butyl carbamate were charged to the flask . the mixture was homogenized with agitation while heating slowly to 60 ° c . as the mixture reached 60 ° c ., 1 . 2 g dodecylbenzyl sulfonic acid was charged to the vessel . the condensor was removed and the flask fitted to the vacuum set - up . the mixture was heated to 100 ° c . at a rate of 1 ° c ./ min . when the mixture reached 70 ° c ., 15 - 20 &# 34 ; vacuum was applied . the methanol was collected as it condensed in the supercooled condensor . a stoichiometric amount of methanol , 279 g , was removed in 2 . 5 hours at 25 &# 34 ; vacuum and 100 ° c . after this amount was removed , the heat and vacuum were discontinued . the vessel was charge with 433 g xylene , homogenized , and carbamate - modified melamine separated from the mixture . a clear coating composition was prepared by combining the following materials : the coating composition was sprayed over steel panels that had been previously sprayed with an acrylic pigmented base coat and flashed . viscosity was adjusted to seconds with butyl acetate . the panels were baked 10 minutes at 82 ° c . and 20 minutes at 132 ° c . the panel of example i was subjected to 16 weeks of severe weathering conditions in jacksonville , fla ., and exhibited significantly reduced environmental etch versus comparison panels coated having clearcoats of hydroxyl - functional acrylic polymer cross - linked with melamine . a clear coating composition was prepared by combining the following materials : the coating composition was sprayed over steel panels that had been previously sprayed with an acrylic pigmented basecoat and flashed . viscosity was adjusted to 20 seconds with butyl acetate . the panels were baked 10 minutes at 82 ° c . and 20 minutes at 132 ° c . the panel of example 2 was subjected to 16 weeks of severe weathering conditions in jacksonville , fla ., and exhibited significantly reduced environmental etch versus comparison panels coated having clearcoats of hydroxyl - functional acrylic polymer cross - linked with melamine . a three - necked 5 - 1 round bottom flask was fitted with an agitator at the center neck and a thermal couple to monitor the reaction temperature at one of the side necks . a nitrogen purge / sparge line was also fed through this neck . the second side neck was fitted with a claissen adaptor and water - cooled condenser . 235 g xylene and 356 g amyl acetate were charged to the flask . the mixture was agitated and heated to reflux with a nitrogen purge . as the mixture reached reflux , 143 ° c ., the nitrogen purge was discontinued . 301 g styrene , 196 g ethylhexyl acrylate , 337 g ethylhexyl methacrylate 445 g hydroxyethyl methacrylate , 226 g cyclohexyl methacrylate , 123 g of a 50 % solution of t - butyl peracetate in odorless mineral spirits , and 116 g xylene were charged to a separate container . this mixture was pumped to the refluxing solvent over a period of four hours . at the conclusion of this feed , 35 g xylene was added through the pump and into the reaction mixture . the reaction mixture was held at reflux , 140 ° c ., for one hour . the mixture was cooled to 120 ° c . and charged with 205 g urea . the temperature dropped as the urea dissolved . the reaction mixture was slowly heated to 150 ° c . and held for the remainder of the synthesis . the vessel was then charged with 2 g of king industry catalyst nacure ® xp - 348 ( metal carbalate ). at this point , the reaction was sparged with nitrogen to facilitate the evacuation of ammonia formed from the thermal decomposition of the urea . incremental additions of the catalyst ( 0 . 5 g ) were added once an hour . the reaction was monitored for the disappearance of hydroxyl by titration . when no hydroxyl was detected by titration , the nitrogen sparge and heat were cut , and 560 g methyl isobutyl ketone was added to the mixture . the mixture was homogenized , followed by separation of the polymer . a coating composition was formed by blending 50 g of the carbamate - functional acrylic from preparation 3 , 7 . 7 g hexamethoxylated monomeric melamine , and 0 . 6 g oxizolidine - blocked dodecylbenzyl sulfonic acid . the composition was coated onto a glass plate , followed by vacuum drawdown to form an 200 μm - thick layer . the cured coating was baked at 132 ° c . for 30 minutes . the coating passed a test of 200 mek rubs . the invention has been described in detail with reference to preferred embodiments thereof . it should be understood , however , that variations and modifications can be made within the spirit and scope of the invention .