Monomer Reactivity Ratios Acrylic Acid-Methylmethacrylate Copolymerization in Dimethylsulfoxide Kieran I. Ekpenyong University of Jos, Nigeria Many polymeric materials in common use (e.g., conirntction and huusehold plastic materials, resins, films, foams, etc.1 are copolymers, the polymerization product of two or more monomers. The current level of development in the manufacture of synthetic copolymers derives from the prior knowledge and understanding of the chemistry of natural copolymers, such as polysaccharides formed from pentose and hexose sugars, proteins as condensation products of different amino acids. and DNA and RNA formed bv the reaction of purine and pyrimidine bases with inorganic ihosphate. These natural copolymers play significant roles in the structure and functions of various plant and animal tissues. is nracticallv The number of known svnthetic conolvmers . . infinite. This is not to say, however, that any monomer can form a copolymer with every other monomer. On the contrary, compatibility with respect to chemical reactivity of a monomer with other monomers is essential for copolymer formation. Thus besides acrylic acid @A)-methylmethacrylate (MMA) copolymer, the subject of this report, others which may be cited include: AA-vinylidene, MMA-styrene, and styrenehutadiene. Also numerous copolymers are formed by grafting of compatible homopolymers ( I ) . The main goal in copolymer production is to obtain a polymer with improved physico-mechanical properties (e.g., processibility, elastic modulus, thermal and electrical conductivitv. etc.) (2). A stu& of the kinetics of copolymerization has, therefore, as one of its objectives, evaluation of the relative reactivities of the monomers with one another, with particular reference to the propagation step of the reaction. The reactivity of a monomer with other mouomers is generally defined by its relative reactivity ratio. Monomer reactivity ratios have been determined for a large numt~erof wpolymt.r systems as evident from the literarure ( I , 31. The hasic principles of copdymcrizntion and the n m cept of monomer reactivity ratios have been discussed in THIS JOURNAL recently ( 4 ) . The magnitudes of these ratios (as ratios of rate constants) depend on the copolymer system and on whether the copolymerization is carried out in bulk (monomers acting as their own solvent) or in a solvent (which is not a monomer). The copolymerization equation is
where m l and mz are molar concentrations of monomer "1" and "2", respectively, in copolymer, and M I and Mz are their corresponding concentrations in the monomer mixture prior to polymerization. The monomer reactivity ratios r1 and r z are defined as
and k22 kzt
1%= -
where h l l and kpp are rate constants in a propagation step in which a growing polymer chain radical with a particular
monomer unit a t the chain end adds another monomer unit of its kind. On the other hand. h , and ~ hm are rate constants for cross-initiated polymer chainradicai;. Equation (1)can be rearranged to give
Other transformations of eqn. (1)are possible ( 5 , 6 ) .In eqn. (2) one chooses arbitrary values for rl and with known values of M and m calculates rz. Thus from one copolymerization experiment one obtains several r z values from a set of arbitrarily chosen rl values. These r * values can then be olotted against r , to rive a linear plot. ~ G r e p e t i t i o nof this process wgh diffe;enicopolymer compositions, one obtains a family of straight lines that intersect a t a point from which the true experimental values of r l and rz for the copolymer system can be read. This method has been used in this study in the determination of reactivity ratios for the system acrylic acidlmethyl methacrylate in dimethylsulfoxide. The subscript "1" refers to acrylic acid and "2" to methylmethacrylate Materials and Methods Acrylic acid (AA),methylmethacrylate (MMA),dimethylsulfoxide (DMSO),dimethylformamide (DMF),tetrahydrafuran (THF) and methanol all were of analytical grade. AA and MMA were each shaken with 5%aqueous NaOH solution to removestabilizer and were dried over anhydrous NazSOn. Azabisisobutyronitrile (AIBN)was used as free radical initiator. (Largeamounts of AIBN should preferably be stored below r w m temperature, when not in use.) The polymerization was carried out dilatometrieallv with the eauioment . . described oreviously (7). Monomer mixturedof AA and MMA were such thai far each run their combined total concentration in DMSO stood at 2.5 M. Seven monomer mixtures were investigated, the AIBN eoncenM. The polymerization, tration kept constant in all cases at 5 X carried out at 50°C, was allowed to proceed to low conversions only (
