Homogeneous bulk polymerization

Polymer dissolved in monomer None Homogeneous bulk polymerization Poly(methyl methacrylate) in methyl methacrylate monomer... 

By application of the simple rate expression for homogeneous bulk polymerization... 

Because of the above heat transfer problems, bulk polymerization of vir l monomers is restricted to those with relatively low reactivities and enthalpies of polymerization. This is exemplified by the homogeneous bulk polymerization of methyl methacrylate and styrene (Table 10.1). Some polyurethanes and polyesters are examples of step-reaction polymers that can be produced by homogeneous bulk polymerizations. The products of these reactions might be a solid, as in the case with acryhc polymers a melt, as produced by some continuous polymerization of styrene or a solution of polymer in monomer, as with certain aUcyd-type polyesters. 

Different processes are nsed in industry for the manufacture of polymers by fi-ee-radical chain polymerization. Among them homogeneous bulk polymerization is economically the most attractive and yields products of higher purity and clarity. But it has problems associated with the heat of polymerization, increases in viscosity, and removal of unreacted monomer. This method is nevertheless used for the manufacture of PVC, polystyrene, and poly(methyl methacrylate). More common processes are homogeneous solution polymerization and heterogeneous suspension polymerization. 

In bulk polymerization, the only components of the formulation are monomers and the catalyst or initiator. When the polymer is soluble in the monomer, the reaction mixture remains homogeneous for the whole process. Examples of homogeneous bulk polymerization are the production of low-density polyethylene (LDPE), general purpose polystyrene and poly(methyl methacrylate) produced by free-radical polymerization, and the manufacture of many polymers produced by step-growth polymerization including poly(ethylene terephthalate), polycarbonate and nylons. In some cases (e.g., in the production of HIPS and acrylonitrile-butadiene-styrene (ABS) resins), the reaction mixture contains a preformed... 

Radical polymerization is initiated by a free radical, which subsequently adds to a vinyl or diene monomer to produce a propagating radical. To obtain information about the structure and concentrations of initiating and propagating radicals in radical polymerizations, use of ESR spectroscopy has called the interest of physical or polymer chemists. However, ESR measurements on these radicals in solution poly merizati on were found to be difficult, except for the case where polymers precipitated, because otherwise the concentrations of the radicals were too low. Thus, these measurements had to be limited to polymerization systems in highly viscous solutions or in the solid state, where the disappearance of free radicals by bimolecular reactions is suppressed. Bresler et al. -i7) succeeded for the first time in obtaining ESR spectra of free radicals which were produced in homogeneous bulk polymerization of methyl methacrylate (MMA), methyl acrylate (MA) and vinyl acetate (VAc) at conversions of 50-60% (in the gel state). 

Currently, there has been only one report in the literature concerning ATRP of 2-(dimethylamino)ethyl acrylate (DMAEA) which involved homogeneous (bulk) polymerization (instead of polymer brush) formation [54]. To the authors knowledge, the only LRP technique applied in the s)mthe-sis of PDMAEA polymer brushes is nitroxide-mediated radical polymerization [55-57]. The first synthesis of PDMAEA brushes from Si substrates by SC-ATRP used a CuCl/CuCl2/Me6TREN/2-bromopropionyl type initiator catalyst system and reaction temperature of 70°C. PDMAEA dry layer thicknesses reached 34 5.9 nm after 72 h. 

The rate of termination reaction is slower than that observed in the homogenous bulk or solution polymerization since the limited number of free radicals exists in the polymerization loci having a reasonably small volume (i.e., monomer swollen forming latex particle). Higher degree of polymerizations can be achieved in an emulsion system relative to the homogenous polymerization due to the existence of this limitation. 

Suspension polymerizations are often regarded as "mini-bulk" polymerizations since ideally all reaction occurs w ithin individual monomer droplets. Initiators with high monomer and low water solubility are generally used in this application. The general chemistry, initiator efficiencies, and importance of side reactions are similar to that seen in homogeneous media. 

There are two different ways for carrying out polyreactions in a solvent. When both the monomer and the resulting polymer are soluble in the solvent, one speaks of a homogeneous solution polymerization on the other hand, if the polymer precipitates during the course of the reaction, it is called precipitation polymerization. By addition of a solvent, different effects are obtained Basically, the viscosity of the reaction mixture is decreased in comparison to a bulk polyreaction this facilitates heat transfer, mass transport, and handling. 

Shear yielding in the form of a quasi-homogeneous,bulk process can contribute substantially to the crack resistance of a polymeric solid. On the other hand, however, localized shear yielding in the form of shear micro-bands is befleved to be a precursor of brittle fracture in many semicrystalline and glassy thermoplas-... 

Heterogeneous Copolymerization. When copolymer is prepared in a homogeneous solution, kineiic expressions can be used to predict copolymer composition Bulk and dispersion polymerization are somewhat different since the reaction medium is heterogeneous and polymeri/aiion occurs simultaneously in separate loci. In bulk polymerization, for example, the monomer swollen polymer particles support polymerization within the particle core us well as on the particle surface, lit aqueous dispersion or emulsion polymeri/aiion the monomer is actually dispersed in two or three distinct phases a continuous aqueous phase, a monomer droplet phase, and a phase consisting of polymer particles swollen at Ihe surface with monomer. This affect the ultimate polymer composition because llie monomers are partitioned such that the monomer mixture in the aqueous phase is richer in the more water-soluble monomers than the two organic phases. 

This review deals with current ideas on the mechanisms operative in acrylonitrile polymerization. The topic is of importance in its own right and also because the study of acrylonitrile has cast light on heterogeneous polymerizations in general. It is an active field of research and the interpretations are still controversial. We shall look first at free-radical polymerization in homogeneous solution, where the monomer behaves in a more or less classical fashion. Next we shall consider the complications that arise where the monomer is at least partially soluble in the reaction medium but where the polymer precipitates. These conditions are encountered in bulk polymerization and in most aqueous or organic diluents. Finally we shall examine the less extensive literature on anionic polymerization and show important differences between the radical and the ionic processes. 

In emulsion polymerization a single polymer particle can be regarded as a locus of bulk polymerization with intermittent initiation. A decrease in termination rate constant, which is observed in bulk polymerization, should, therefore, also occur in a single polymer particle and must be taken into account in model simulation of emulsion polymerization. However, since the polymer particles from the very beginning of the reaction contain a certain percentage of polymer, typically 20-40%, then the termination rate constant is always smaller in the beginning of emulsion polymerization than in the similar homogeneous bulk... 

In bulk polymerizations, the initial reaction mixture consists essentially of monomer. If the process is a chain-growth reaction, the mixture will also contain initiator and chain transfer agent, if needed. If the polymer and monomer are miscible, the system remains homogeneous during the polymerization reaction. 

Heal removal and mixing are problems in bulk chain-growth polymerizations for the reasons outlined in the previous section. Thus, homogeneous bulk step-growth reactions are driven to high conversions to achieve high molecular weights... 

Poly (ethylene terephihalate) and nylon-6,6 manufacture are homogeneous bulk step-growth reactions. The molecular weight of the polymer produced is limited by the high viscosity of the reaction mixture at very high conversions. Post polymerization techniques such as that described in connection with reaction (5-39) can be used to increase the polymer molecular weight for some applications. 

Polystyrene and poly(methyl methacrylate) polymerizations are typical of homogeneous bulk chain-growth reactions. The molecular weight distributions of the products made in these reactions are broader than predicted from consideration of classical, homogeneous phase free-radical polymerization kinetics because of autoacceleration (Section 6.13.2) and temperature rises at higher conversions. 

The mechanism of the polymerization reaction is presumed to be essentially that of a homogeneous bulk or solution free-radical polymerization. The concern is exclusively with the polymerization by double-bond opening of carbon compounds that contain at least one caibon-carbon double bond. The reactive species that propagates to produce the polymer chain is a free radical formed by opening of the rc-bond of the carbon-carbon double bond. The basic steps of the polymerization reaction are initiation, propagation, termination (by various means), and various transra reactions. Tbe structure of the polymer produced is determined by the balance of the propagation, termination, and transfer reactions. 

T T omogeneous catalysis is of major importance in polymer chemistry - - and technology—e.g., in producing vinyl polymers, polydienes, and polycondensation polymers. It permits precise control of all reactants, usually eliminates the necessity of removing catalyst residues from the product, and allows the use of relatively simple apparatus. On the other hand, polymerizations in homogeneous media, especially bulk polymerizations of vinyl monomers, are sometimes so vigorous that proper heat removal may not be eflFected on a large scale. This drawback has limited some of the possible industrial applications (34). 

The reaction mixture is homogeneous throughout the entire polymerization—i.e., the catalyst, monomer, polymer, and solvent (if used) form a single phase. This is the subject discussed here. Bulk polymerization and solution polymerization are examples of reactions that are usually homogeneous. 

Styrene polymerization was investigated with some pyridinimine ligands. Homogeneous living polymerizations can be achieved with L-9 (R = /v-Oct) in xylene104 and with L-12 in the bulk,105 both with a bromide initiator and CuBr, where the former gave narrower MWDs than the latter (MJMn =1.2 and 1.5—1.8, respectively). L-9 (R = n-Oct) is also applicable for MA polymerization to give relatively narrow MWDs.81... 

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