PROCESS TO MAKE POLYCARBONATE FROM BISMETHYLSAL1CYLCARBONATE Background of the Invention: This invention relates lo a methpd of preparing polycarbonate. More particularly the method relates to a method whereby a solution comprising a solvent and an oligomenc polycarbonate is introduced into a devolatilizing extruder wherein the oligomenc polycarbonate is convened into high molecular weigh! polycarbonate while simultaneously removing the solvent. More particularly, the instant invention relates to the formation under mild conditions of polycarbonates having extremely low levels of Fries rearrangement products, a high level of endcapping and low levels of residual solvent. Polycarbonates, such as-bisphenol A polycarbonate, are typically prepared either by interfacial or melt polymerization methods. The reaction of a bisphenol such as bisphendl A (BPA) with phosgene in the presence of water, a solvent such as methylene chloride, an acid acceptor .such as sodium hydroxide and a phase transfer catalyst such as triethylaminc is typical Of the interfacial methodology. The reaction of bisphenol A with a source of carbonate units such as diphenyl carbonate at high temperature in the presence of a catalyst such as sodium hydroxide is typical of currently employed melt polymerization methods. Each method is practiced on a large scale commercially and each presents significant drawbacks. The interfacial method for making polycarbonate has several inherent disadvantages. First it is,a disadvantage to operate a process which requires phosgene as a reactant due to obvious safety concerns. Second it is a disadvantage to operate a process which requires using large amounts of an organic solvent because expensive precautions must be taken to guard against any adverse environmental impact. Third, the interfacial method requires a relatively large amount of equipment and capital investment. Fourth, the polycarbonate produced by the interfacial process is prone to having inconsistent color, higher levels of particulars, and higher chloride content, which can cause corrosion. The melt method, although obviating the need for phosgene or a solvent such as methylene chloride requires high temperatures and relatively long reaction times. As a result, by-products may be formed at high temperature, such as the products arising by Fries rearrangement ot carbonate units along the growing polymer chains. Fries rearrangement gives rise to lindesired and uncontrolled polymer branching which may negatively impact the polymer's flow properties and performance. The melt method further requires the use of complex processing equipment capable of operation at high temperature and low pressure, and capable of efficient agitation of the highly viscous polymer melt during the relatively long reaction times required to achieve high molecular weight. Some years ago, it was reported in U.S. Pat. No. 4,323,668 that polycarbonate could be formed under relatively mild conditions by reacting a bisphenol such as BPA with the diary I carbonate formed by reaction phosgene with methyl salicylate. The method used relatively high levels of transesterification catalysts such as lithium stearate in order to achieve high molecular weight polycarbonate. High catalyst loadings are particularly undesirable in melt polycarbonate reactions since the catalyst remains in the product polycarbonate following the reaction. The presence of a transesterificalion catalyst in the polycarbonate may shorten the useful life span of articles made therefrom by promoting increased water absorption, polymer degradation at high temperatures and discoloration. In US 6,420,5] 2, extrusion of a mixture of an ester-substituted diary) carbonate, such as bis-methyl salicyl carbonate, a dihydroxy aromatic compound, such as bisphenol A, and a transesterification catalyst, such as tetrabutylphosphonium acetate (TBPA), afforded high molecular weight polycarbonate. The extruder employed was equipped with one or more vacuum vents to remove by-product ester-substituted phenol. Similarly, a precursor polycarbonate having ester-subsiituted phenoxy endgroups, for example methyl salicyl endgroups, when subjected to extrusion afforded a polycarbonate having a significantly increased molecular weight relative to the precursor polycarbonate. The reaction to form a higher molecular weight polycarbonate may be catalyzed by residual transesterification catalyst present in the precursor polycarbonate, or by a combination of any residua] catalyst and an additional catalyst such as TBPA introduced in the extrusion step. Fries rearrangement products were not observed in the product polycarbonates, Although the methods described in US 6,420,512 represent significant enhancements in the preparation of polycarbonate relative (o older methods, additional improvements are needed. For example, it would be highly desirable to increase the throughput rate of starting materials through the extruder in order to achieve greater efficiency without observing the entrainment of the feed and polycarbonate at the vent ports of the extruder. In addition, it would be highly desirable to avoid having to isolate a precursor polycarbonate having ester-substituted phenoxy endgroups prior to its extrusion to afford a higher molecular weight polycarbonate. Summary of the Invention: Applicants have discovered a superior process for the production of polycarbonate using an ester-substituted carbonate: In accordance with an embodiment of the present invention polycarbonate is prepared by the method comprises the steps of: (j) introducing to an extruder through a feed port a plurality of reaction components comprising a polycarbonate oligomer, an activated carbonate residue, and a transesterification catalyst, wherein the extruder comprises the feed port, a first back vent port, and a polycarbonate exit port, wherein the feed port is located between the first back vent port and the polycarbonate exit port, and wherein the resistance to flow of the reaction components from the feed port to the first back vent port is less than or equal to the resistance to flow of the reaction components from the feed port to the polycarbonate exit port, and (ii) extruding the reaction components at one or more temperatures in a range between 100 °C and 400 °C wherein during the extrusion of the reaction components, activated carbonate residue is removed through the first back vent port, thereby preparing a polycarbonate. The present invention is an improvement over the prior art. Jt has been found thai the process of the present invention allows for superior polycarbonate production using a back vented extruder whi le minimizing residence time and the number of barrels required in typical extrusion methods. Further it has been found that a back vent port on an extruder may be operated under conditions to remove activated carbonate residue without the entramment of oligomer at the vent port even in the circumstance that the resistance to flow of the reaction components from the feed port to the back vent port is less than or equal to the resistance to flow of the reaction components from the feed port to the polycarbonate exit port. Brief Description of the Drawings Fig. !:• shows an apparatus for practicing the invention Detailed Description of the Invention The present invention may be understood more readily by reference to the following detailed description of preferred embodiments of the invention and the examples included therein. In the specification and the claims which follow, reference will be made to a number of terms ,which shall be defined to have the following meanings: The singular forms "a", "an" and "the" include plural referents unless the context clearly dictates otherwise "Optional" or "optionally" means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where the event occurs and instances where it does not. As used herein, the terms "polycarbonate oligomer" and "oligomeric polycarbonate" are synonymous and refer to a polycarbonate oligomer having a number average molecular weight of less than 7000 Dahons and includes oligomeric polycarbonates comprising polycarbonate repeat units derived from one or more dihydroxy aromatic compounds. As used herein, when describing an oligomeric polycarbonate, the expression "polycarbonate repeat units derived from ai least one dihydroxy compound" means a repeal unit incorporated into an oligomeric polycarbonate by reaction of a dihydroxy compound with a source of carbonyl units, for example the reaction of bisphenol A with bis(methyl salicyl) if carbonate. As used herein, the term "high molecular weight polycarbonate" means polycarbonate having a number average molecular weight, Mn, of 8000 Daltons or more. As used herein, the term "solvent" can refer 10 a single solvent or a mixture of solvents. As used herein, the term "solution comprising a solvent and an oligomeric polycarbonate" refers to a liquid oligomeric polycarbonate comprising at least 5 percent by weight solvent. As used herein, the terrh "rrielt polycarbonate" refers to a polycarbonate made by the transesterification of a diaryl carbonate with a dihydroxy compound. As used in the examples section of the specification the term "TLTM" refers to values that were too low to measure. "BPA" is herein defined as bisphenol A or 2,2-bis(4-hydroxyphenyl)propane. As used herein the term "Fnes product" is defined as a structural unit of the product polycarbonate which upon hydrolysis of the product polycarbonate affords a carboxy-substituted dihydroxy aromatic compound bearing a carboxy group adjacent to one or both of the hydroxy groups of said carboxy-substituted dihydroxy aromatic compound. For example, in bisphenol A polycarbonate prepared by a melt reaction method in which Fries reaction occurs, the Fries product includes those structural features of the polycarbonate which afford.2-carboxy bisphenol A upon complete hydrolysis of the product polycarbonate. The terms "Fries product" and "Fries group" are used interchangeably herein. The terms "Fries reaction" and "Fries rearrangement" are used interchangeably herein. The terms "double screw extruder" and "twin screw extruder" are used interchangeably herein. As used herein the term "monofunctional phenol" means a phenol comprising a single reactive hydroxy group The terms "vent port" and "vent" are used interchangeably herein. "Polycarbonate" refers to polycarbonates incorporating repeat units derived from at least one dihydroxy aromatic compound and includes copolyesiercarbonaies, for example a polycarbonate comprising repeal uniis derived from resorcinol, bisphenol A, and dodecandioic acid. Nothing in the description and claims of this application should be taken as limiting the polycarbonate to only one dihydroxy residue unless the context is expressly limiting. Thus, the application encompasses copoJycarbonates with residues of 2, 3, 4, or more types of dihydroxy compounds. Numerical values in the specification and claims of this application reflect average values. Furthermore, the numerical values should be understood to include numerical values which are the same when reduced to the same number of significant figures and numerical values which differ from the stated value by less than the experimental error of the measurement technique used in the present application to determine the value. US patent application 10/389,009 (pub. No. US 2003/0236384), herein incorporated by reference, shows a back vented extruder in figure 3. However, US 2003/0236384 discloses the use of an active element, (i.e. a kneading element), that is placed between the back vent and the feed port of the extruder to increase the resistance of flow of the reaction components to the back vent to reduce or eliminate the entrainment of oligomer/polymer in the back vent. It has herein been found that no such active element is required to prevent the entrainment of the oligomer/polymer in the back vent. Thus, the extruder of the present invention has a back vent wherein the resistance to flow of the reaction components from the feed pon to the back vent pon is less than or equal to the resistance to flow of the reaction components from the feed port to the polycarbonate exit pon. The method of the invention effects both the conversion of the oligdmeric polycarbonate to a product polycarbonate having higher molecular weight, and a separation of the solvent initially present in the solution of the oligomeric polycarbonate from the product polycarbonate. Additionally, the method provides for the removal of other volatile materials which may be present in the initial solution of oligomeric polycarbonate, or formed as by-products as the oligomeric polycarbonate is transformed in the extruder to the product polycarbonate. One embodiment of the present invention provides a method for preparing polycarbonate comprising the steps of: (i) introducing to an extruder through a feed port a plurality of reaction components composing a polycarbonate oligomer, an activated carbonate residue, and a transesterification catalyst, wherein the extruder comprises the feed port, a first back vent port, and a polycarbonate exit port, wherein the feed port is located between the first back vent port and the polycarbonate exit port, and wherein the resistance to flow of the reaction components from the feed port to the first back vent port is less than or equal to the resistance to flow of the reaction components from the feed port to the polycarbonate exit port, and (ii) extruding the reaction components at one or more temperatures in a range between ]00 °C and 400 °C, wherein during the extrusion of the reaction components, activated carbonate residue is removed through the first back vent port. The Activated Diaiyl Carbonate: The carbonate is preferably derived from an activated dicarbonale or a mixture of an activated carbonate with diphenyl carbonate. A preferred activated carbonate of the present invention is an activated diarylcarbonate such as bismethylsalicylcarbonate (BMSC). However, as used herein the term "activated carbonate" is defined as a diarylcarbonate which is more reactive than diphenylcarbonate toward transesterification reactions. Such activated carbonates are of the general formula: (Figure Remove)herein Ar is a substituted aromatic radical having 6 to 30 carbon atoms. The preferred activated carbonates have the more specific general formula: wherein Q and Q' are each independently activating groups. A and A' are each independently aromatic rings which can be the same or different depending on the number and location of their substituent groups, and n or n' are whole numbers of zero up to a maximum equivalent to the number of replaceable hydrogen groups substituted on the aromatic rings A and A', wherein n -)- n' is greater than or equal to 1 R and R' are each independently substituent groups such as alky), substituted alkyl, cycloaikyl, alkoxy, aryl, alkylaryl, cyano, nitro, halogen, and carboalkoxy. The number of R groups is a whole number and can be 0 up to a maximum equivalent to the number of replaceable hydrogen groups on the aromatic rings A minus the number n. The number of R' groups is a whole number and can be 0 up to a maximum equivalent to the number of replaceable hydrogen groups on the aromatic rings A minus the number n'. The number, type and location of the R and R' substituents on the aromatic ring are not limited unless they deactivate the carbonate and lead to a carbonate which is less reactive than diphenylcarbonate. Non-limiting examples of activating groups Q and Q' are: alkoxycarbonyl groups, halogens, nitro groups, amide groups, sulfone groups, sulfoxide groups, or imihe groups with structures indicated below: (Figure Remove)Specific and non-limiting examples of activated carbonates include bis(o-methoxycarbonylphenyi)carbOTiatc, bis(o-cnJorophenyl)carbonate, bis(o-nitrophenyl)carbonaie, bis(o-acetylphenyl)carbonate, bis(o-phenylketonephenyl)carbonate, bis(o-formylphenyl)carbonate. Unsymmetrical combinations of these structures, where the substitution number and type on A and A' are different, are also possible to employ in the current invention. A prefeTed structure for an activated carbonate is an ester-substituted diarylcarbonate having the structure: and b is independently at each occurrence an integer 0-4. At least one of the substituents CO;R' is preferably attached in an ortho position relative,to-the carbonate group. Examples of preferred ester-substituted diarylcarbonates include but are not limited to bis(methylsalicyl)carbonate (CAS Registry No. 82091-12-1), bis(ethyl salicyl)carbonaie, bis(propyl salicyl) carbonate, bis(butylsalicyl) carbonate, bis(benzyl salicyl)carbonate, bis(methyl 4-chlorosalicyl)carbonaie and the like. Typically BMSC is preferred for use in melt polycarbonate synthesis due to its lower molecular weight and higher vapor pressure. One method for determining whether a cenain diarylcarbonate is activated or is not activated is to carry out a model transesteritication reaction between the cenain diarylcarbonalc with a phenol such as p-(l ,1,3,3-tetramethyl)butylphenol This phenol is preferred because it possesses only one reactive site, possesses a low volalility and possesses a similar reactivity to 10 bisphenol-A The model transesterification reaction was carried out at temperatures above the melting points of the certain diarylcarbonate and p-( 1,1,3,3-tetramethyl)butylphenol and in the presence of a transesterification catalyst, which is usually an aqueous solution of sodium hydroxide or sodium phenoxide. Preferred concentrations of the transesterification catalyst are 0.001 mole % based on the number of moles of the phenol or diarylcarbonate. And a preferred reaction temperature is 200 °C. But the choice of conditions and catalyst concentration can be adjusted depending on the reactivity of the reactants and melting points of the reac'ants to provide a convenient reaction rate. The only limitation to reaction temperature is that the temperature must be below the degradation temperature of the reactants. Sealed tubes can be used if the reaction temperatures cause the reactants 10 volatilize and affect the reactant molar balance. The determination of the equilibrium concentration of reactants is accomplished through reaction sampling during the course of the reaction and then analysis of the reaction mixture using a well-known detection method to those skilled in the art such as HPLC (high pressure liquid chromatography). Particular care needs to be taken so that reaction does hot. . .continue after the sample has been removed from the reaction vessel. This is accomplished by cooling down the sample in an ice bath and by employing a reaction quenching acid such as acetic acid in the water phase of the HPLC solvent system. It may also be desirable to introduce a reaction quenching acid directly into the reaction sample in addition to cooling the reaction mixture. A preferred concentration for the acetic acid in the water phase of the HPLC solvent system is 0.05 % (v/v). The equilibrium constant was determined from the concentration of the reactants and product when equilibrium is reached. Equilibrium is assumed to have been reached when the concentration of components in the reaction mixture reach a point of little or no change on sampling of the reaction mixture. The equilibrium constant can be determined from the concentration of the reactants and products at equilibrium by methods well known to those skilled in the art A diarylcarbonate which possesses a relative equilibrium constant (KKopc) of greater than i is considered to possess a more favorable equilibrium than diphenylcarbonate and is an activated carbonate, whereas a diarylcarbonate which possesses an equilibrium constant of 1 or less is considered to possess the same or a less favorable equilibrium than 11 diphenylcarbonate and is considered not to be activated It is generally preferred to employ an activated carbonate with very high reactivity compared to diphenylcarbonate when conducting 'transesterification reactions; Preferred are activated carbonates with an equilibrium constant greater than at least 10 times that of diphenylcarbonate. Some non-limiting examples of non-activating groups which, when present in an ortho position relative to the carbonate group, would not be expected to result in activated carbonates are alkyl, cycolalkyl or cyano groups. Some specific and non-limiting examples of non-activated carbonates are bis(o-methylphenyl)carbonate, bis(p-cumylphenyl)carbbnate, bis(p-(l,l,3,3-tetramethyl)butylphenyi)carbonateand bis(o-cyanophenyl)carbonate. Unsymmetrical combinations of these structures are also expected to result in non-activated carbonates. : Unsymmstrical diarylcarbbnates wherein one aryl group is activated and one aryl is unactivated or de-acti vated would also be useful in this invention if the activating group renders the diary! carbonate still more reactive than diphenyl carbonate. The carbonate may also be derived from dicarboxylic acids, dicarboxylic acid esters, or dicarboxyljc acid halides. Such constituent repeating units are typically polyester-polycarbonate units. Non-limiting examples of dicarboxylic acids include terephthalic acid, isophthalic acid, sebacic acid, decanedioic acid, dodecanedioic acid, etc. Non-limiting examples of dicarboxylic acid esters include diphenyl sebacate, diphenyl terephthalate, diphenyl isophthalate, diphenyl decanedioate, diphenyl dodecanedioate, etc. Non-limiting examples of dicarboxylic acid halides include terephthaloyl chloride, isophthaloyl chloride, sebacoy) chloride, decanedioyl chloride, dodecanedioyl chloride, etc. Such polyester-polycarbonate units may be present in proportions of up to 50 mole %, preferably not more than 30 mole %, in copolymerized polycarbonates in accordance with the present invention. The theoretical stoichiomelry of the reaction within the equilibration vessel requires a molar ratio of dihydroxy compound to diaryl carbonate composition of 1.: 1. However, in practicing the present invention the molar ratio in the equilibration vessel is typically between 0.25:1 to 3:1, more preferably 1:0.95 to 1:1.05 and more preferably 1:0.98 to 1:1.02, The Dihydroxy: The dihydroxy compound is not limited to aromatic dihydroxy compounds. However; such dihydroxy aromatic compounds are frequently preferred for use in these types of applications. It is contemplated that the dihydroxy compound comprises aliphatic diols and/or acids. The following is a non limiting list of such compounds: Aliphatic DiolsIsosorbide: 1,4:3,6-d'anhydro-D-sorbitoI, Tricyclodecane-dimethanol (TCDDM), 4,8-Bis(hydroxymethyl)tricyclodecane, Tetramethylcyclobutanediol (TMCBE)), 2,2,4,4,-tetrame,thylcyclobutane-l,3-diol, mixed isomers, cis/trans-l,4-Cyclohexanedimethanol (CHDM), cis/trans-1,4-Bis(hydroxymetnyl)cyclohexane, cyclohex-1,4-ylenedimethaiiol. trans-l,4-Cyclohexanedimethanol (tCHDM), trans-l,4-Bis(hydroxymethyl)cyclohexane, cis-1,4-Cyclohexanedimethanol (cCHDM), cis-1,4-Bis(hydroxymethyl)cyclohexane, cis-l,2,-cyclohexanedimethanol, l.r-bKcyclohexylM.'-dioI.dicylcohexyM'-diol, 4,4'-dihydroxybicyclohexyl, and Poly(ethylene glycol). Acids: 1,10-Dodecanedioic acid (DDDA), Adipic acid, Hexanedioic acid, Isophthalic acid, 1,3-Benzenedicarboxylic acid, Teraphthalic acid, 1,4-Benzenedicarboxylic acid, 2,6-Naphthalenedicarboxylic acid, 3-hydroxybenzoic acid (mHBA), and 4-hydroxybenzoic acid (pHBA). It is further contemplated that the dihydroxy composition comprises a dihydroxy aromatic compound. A preferred dihydroxy aromatic composition of the present invention is bispheno! A (BPA). However, other dihydroxy aromatic compounds of the present invention can be used and are selected from the group consisting of bisphenols having structure, (Figure Remove)Suitable bisphenols are illustrated by 2,2-bis(4-hydroxyphenyl)propane (bisphenol A); 2,2-bis(3-chloro-4-hydroxyphenyl)propane; 2,2-bis(3-bromo-4-hydroxyphenyl)propane; 2,2-bis(4-hydroxy-3-methylphenyl)propane; 2,2-bis(4-.hydroxy-3-isopropylphenyl)propane; 2,2-bis(3-t-buiyl-4-hydroxyphenyI)propane; 2,2-bis(3-phenyl-4-hydroxyphenyl)propane; 2,2-bis(3,5-dichloro-4-hydroxyphenyl)-propane; 2,2-bis(3,5-dibromo-4-hydroxyphenyl)propane; 2,2-bis(3,5-dimethyl-4-hydroxyphenyI)propane; 2,2-bis(3-chloro-4-hydroxy-5-methylphenyl)propane; 2,2-bis(3-bromo-4 hydroxy-5-methylphenyl)propane; 2,2-bis(3-chloro-4 hydroxy-5-isopropylphenyl)propane; 2,2-bis(3-bromo-4-hydroxy-5-isopropylphenyl)propane; 2,2-bis(3~t-butyl-5-chloro-4-hydroxyphenyl)propane; 2,2-bis(3-bromo-5-l-bulyl-4-hydroxyphenyl)propane; 2,2-bis(3-chloro 5-phenyl-4-hydroxyphenyl)propane; 2,2-pis(3-bromo-5-phenyl-4-hydroxyphenyl)propane; 2,2-bis(3,5-disopropyl-4-hydroxypheny])propane; 2,2-bis(3,5-di-t-butyl-4-hydroxyphenyJ)propane; 2,2-bis(3,5-diphenyl-4-hydroxyphenyl)propane; 2,2-bis(4-hydrox y-2,3,5,6-tetrach Icirophenyl )propane; 2,2-bis(4-hydrox y-2,3,5,6-tetrabrornophenyl )propane; 2,2-bJs(4-hydroxy-2,3,5,6-tetrarnethylphe]iyl)propane; 2,2rbis(2,6-dichloro-3,5-dimcihyl-4-hydroxyphenyI)propane; 2,2-bis(2,6-dibromo-3,5-dimethyl-4-hydroxypheny))propane; l,J-bis(4-hydroxyphenyl)cyclohexane; l.l-bisfS-chloro-hydroxyphenylyclohexane; l,l-bis(3-bromo-4-hydroxyphenyl)cyclohexanc; l,l-bis(4-hydroxy-3-methylphenyl)cyclohexane; l,l-bis(4-hydroxy 3-isopropylphenyl)cyc]ohexane; l,lbis(3-t-butyl-4-hydroxyphenyl)cyclohexane; i ,l-bis(3-phcnyl4-hydroxyphenyl)cyclohexane; l,l-bis(3,5-dichJoro-4-hydroxyphenyl)cycJohexane; l,l-bis(3,5-