Molecular Weight Distributions in Ideal Polymerization Reactors

View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by CONICET Digital Latin American Applied Research 41: 389-401(2011) MOLECULAR WEIGHT DISTRIBUTIONS IN IDEAL POLYMERIZATION REACTORS. AN INTRODUCTORY REVIEW G.R. MEIRA† and H.M. OLIVA‡ † INTEC (Univ. Nac. del Litoral and CONICET), Santa Fe (3000), Argentina. [email protected] ‡ Escuela de Ingeniería Química, Univ. del Zulia, Maracaibo, Venezuela. [email protected] Abstract The ultimate aim of polymerization million tons by 2015. In volume, the annual production reaction engineering is the production of polymers of polymers exceeds that of the 2 most important met- with tailor-made properties. An introductory review als: iron and aluminum. The raw materials for the pro- into this field is presented, with emphasis on the ef- duction of around 95% of all synthetic polymers are fects on the molar mass distribution (MMD), of the non-renewable sources (fossil oil, gas, and coal). sought combination of polymerization mechanism, Polymerization Reaction Engineering deals with reactor type, and reactor control. Three ideal poly- problems involving the measurement, mathematical merization mechanisms are analyzed: free-radical, modeling, optimization, and control of industrial poly- “living” anionic, and step-growth. “Living” anionic merization processes. It aims at improving both the and step-growth polymerizations are similar in that productivity of the polymerization process and the qual- their growing chains remain reactive while inside the ity of the produced polymer. Some general references reactor; and for these systems the narrowest MMDs on this area are: Ray (1972), Reichert and Moritz are produced in reactors with narrow residence time (1989), Hamielec and Tobita (1992), Kiparissides distributions (RDT); i.e.: batch or continuous tubu- (1996), Ray et al. (2004), Yoon et al. (2004), Meyer and lar reactors. In contrast, in conventional free-radical Keurentjes (2005), Villa (2007) and Asua (2007). polymerizations, the polymer molecules grow in a Polymers are high molar mass substances characte- fraction of a second and thereafter remain inactive rized by the repetition (neglecting ends, branch junc- while inside the reactor. In this case, the RTD does tions, and other minor irregularities) of one or more not affect the MMD, and the homogeneous conti- types of monomeric repeating units. While homopoly- nuous stirred-tank reactors provide the narrowest mers contain a single type of chemical repeating unit, MMDs. Representative mathematical models of po- copolymers contain 2 or more. Polymers may be syn- lymerization reactors are useful for: a) quantifying thetic or natural (such as proteins, carbohydrates, etc.). the interrelationships between their numerous inputs In spite of their highly sophisticated structures, natural and outputs; and b) developing open- and closed- macromolecules are synthesized at ambient temperature loop strategies for increasing reactor productivity and in mild aqueous media, with the aid of specialized and product quality. catalysts or enzymes (themselves also polymers). In Keywords Molecular Weight Distribution, contrast, synthetic polymers are considerably simpler in Polymerization, Reactor. their chemical and structural characteristics, are mostly I. INTRODUCTION soluble in organic solvents, and their syntheses typically Synthetic polymers are important materials that find in- require stringent conditions of pressures, temperatures, numerate applications as plastics, composites, rubbers, and solvents. fibers, adhesives, and coatings. Unlike low molar mass The total number of repeating units in a chain is the substances where quality is mainly determined by puri- chain length or degree of polymerization. Most synthet- ty, synthetic polymers are mixtures of a large variety of ic polymers are linear molecules made up of repeating molecular species and morphologies, and therefore are units of functionality 2. Due to their high molar masses difficult to characterize. The physical properties of po- and chain entanglements, polymers are solids at room lymers (both in the solid and in the melt) depend on temperature, but may become viscous liquids between complex interrelationships with: a) the molecular struc- 100 and 300 ºC. Their average molar masses are typical- ture (described by the distributions of molecular ly between 20,000 and 300,000 g/mol. These values are weights, of isomers, of the chemical composition in co- a compromise between mechanical properties (such as polymers, of the molecular topology in long-branched elastic modulus, and tensile strength that all increase polymers, etc.); and b) the supramolecular morphology with the molar mass), and ease of processability in the (degree of crystallinity, particle size in heterogeneous molten state, favored by a low melt viscosity (or a low solids, etc.). “Commodity” thermoplastics and fibers molar mass). In contrast, if the polymer is crosslinked or such as polyethylene, polypropilene, PVC, and polysty- cured, then the material is essentially a single molecule rene are synthesized in large continuous processes that that unless degraded, it will not flow upon increasing were mostly developed in the mid-20th Century. In the temperature. “Reactors” for producing crosslinked 2007, the World production of synthetic polymers was articles are not stirred, and their shapes provide the around 260 million tons, and it is expected to reach 350 shape of the final article (e.g.: a mould for producing a 389 Latin American Applied Research 41: 389-401(2011) rubber tire by vulcanization of a styrene-butadiene pre- mers (free-radicals are unaffected by water). In contrast, polymer). aqueous media are incompatible with anionic polymeri- The two main polymerization mechanisms are: chain zations. Anionic coordination catalysts are mostly sup- (or monomer-growth) and step (or polymer-growth). In ported on magnesium chloride or silica particles, but chain polymerizations, the linear molecule grows by may also be used in liquid form. Anionic coordination reaction between a monomer and a reactive site on the polymerizations are carried out in dispersions or in ho- growing chain end, with the reactive site being regene- mogeneous organic media. In coordination polymeriza- rated after each propagation step. According to the na- tions, the monomer in contact with the catalyst can be ture of the reactive site, chain polymerizations are clas- either a gas (in the fluidized-bed gas phase process), a sified into free-radical, anionic, and cationic. Many im- pure liquid (in the liquid solution process), or dissolved portant polymers such as low-density polyethylene, in a diluent (in the slurry process). Other heterogeneous poly(vinyl chloride) and polystyrenes are produced via polymerizations are the interfacial processes for the free-radical polymerizations. However, from the point production of nylons and polycarbonates. of total production, the most important synthetic poly- For the random nature of polymerization reactions, mers are obtained via catalyzed chain coordination po- all synthetic polymers exhibit a molecular weight distri- lymerizations. Examples of these are high-density po- bution, or better: a molar mass distribution (MMD). lyethylene (HDPE), linear low-density polyethylene MMDs totally characterize the molecular macrostruc- (LLDPE), isotactic polypropylene, isotactic poly(1- ture of linear homopolymers. In long-branched homopo- butene), various ethylene–propylene co- and terpoly- lymers and in linear copolymers, the macrostructures re- mers, cis-1,4-polybutadiene, and cis-1,4-polyisoprene. spectively include distributions of the number of In step polymerizations, all the molecules exhibit branches per molecule and of the chemical composition. reactive chain ends, and the chains grow by reaction be- Apart from the macrostructure, the microstructure of a tween any two molecules (including the monomers). homopolymer examines the orientation of the different Most step polymerizations are also polycondensations, geometrical or optical isomers along the chain (e.g.: the when a low-molar-mass by-product is generated at each trans content in a dienic monomer or the tacticity of a propagation step. Two important industrial polyconden- mono-substituted vinyl monomer). A homopolymer is sations are the syntheses of Nylon 6,6 (by reaction be- atactic if there is no systematic or regular configuration tween hexamethylenediamine and adipic acid), and of the repeating units along the chain. Atactic polymers poly(ethylene terephtalate) or PET (by reaction between are amorphous materials that soften around the glass terephtalic acid and ethylene glycol). transition temperature (Tg). In contrast, polymers with Most polymerizations are highly exothermic, to chain regularity such as iso- or syndiotactic polypropy- compensate for the reduction in entropy produced when lene are semi-crystalline when cooled slowly from the transforming many disordered monomer molecules into melt. Semi-crystalline materials exhibit a (lower) sof- a reduced number of ordered polymer structures. Thus, tening temperature at Tg, and a (higher) softening tem- most polymerization reactors require an efficient heat perature at the melting point (Tm) associated with the extraction system. The heat extraction capacity limit the crystals fusion. Compared with amorphous polymers, rate of polymerization of many industrial stirred-tank semi-crystalline polymers generally present improved reactors, due to the reduction

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