Radical Solution Polymerization

For the copolymerization of ethene and vinyl acetate, solution polymerization, suspension polymerization, emulsion polymerization and bulk polymerization may be used, but solution polymerization is preferred (1). A method of either continuous type or batch type may be employed. Methanol is generally used as the solvent. 

Commonly used catalysts are 2,2 -azobisisobutyronitrile, or organic peroxides. 2,2 -Azobis(4-methoxy-2,4-dimethyl valeronitrile) is the most preferred catalyst (7). 

The polymerization temperature is in the range of 50-80°C. The ethene pressure is 2-8 M Pa. In the case of a continuous polymerization process, the average residence time should be in the range of 3-4 h (1). 

To stop the polymerization reaction, a polymerization inhibitor is added to reaction mixture. Unreacted ethylene gas is evaporated and removed from the solution. Further, unreacted vinyl acetate is extracted from the copolymer solution. Eventually, methanol is recovered by precipitation with a water containing separating and purifying solution. The vinyl acetate and methanol thus recovered may be reused in the copolymerization process. 

Polymeric binders based upon a vinyl acetate and ethylene backbone incorporating a self crosslinking monomer have been widely used in the nonwoven industry (3). 

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Anionic polymerization offers fast polymerization rates on account of the long life-time of polystyryl carbanions. Early studies have focused on this attribute, most of which were conducted at short reactor residence times (< 1 h), at relatively low temperatures (10—50°C), and in low chain-transfer solvents (typically benzene) to ensure that premature termination did not take place. Also, relatively low degrees of polymerization (DP) were typically studied. Continuous commercial free-radical solution polymerization processes to make PS, on the other hand, operate at relatively high temperatures (>100° C), at long residence times (>1.5 h), utilize a chain-transfer solvent (ethylbenzene), and produce polymer in the range of 1000—1500 DP. 

Polymers Polyacrylamide and hydrolyzed polyacrylamide were prepared by the American Cyanamid Company specifically for this project, starting with l C labelled monomer. The radioactivity level of the monomer was kept below 0.20 mC /g in order to avoid significant spontaneous polymerization, utilizing a copper inhibitor. The homopolymer was synthesized by free radical solution polymerization in water at 40°C, using monomer recrystallized from chloroform, an ammonium persulfate-sodium metabisulfite catalyst system, and isopropanol as a chain transfer agent. Sodium... 

Another class of chain scission positive resists is the poly(olefin-sulfones). These materials are alternating copolymers of an olefin and sulfur dioxide, prepared by free radical solution polymerization. The relatively weak C-S bond, 60 kcal/mole compared with 80 kcal/mole for a carbon-carbon bond, is readily cleaved upon irradiation (Gs values for these polymers are typically 10), and several sensitive resists have been developed based on this chemistry (53). One material that has been made commercially available is poly (butene-1-sulfone) (54). 

Polyols. Typical polyols used in automotive topcoats Include acrylic copolymers and polyesters which have varied number of hydroxyl groups. Acrylic copolymers ranging in number average molecular weight from 1,000 to 10,000 and containing 15-40% by weight of a hydroxy functional comonomer such as hydroxyethyl acrylate have been studied. The acrylic copolymers were prepared by conventional free radical solution polymerization. 

Materials. GMC and PCLS were synthesized by free radical solution polymerization initiated by benzoyl peroxide as described previously (5,6). Nearly mono and polydisperse polystyrenes were obtained from Pressure Chemical Co. and the National Bureau of Standards respectively. Molecular weight and polydispersity were determined by gel permeation chromatography (GPC) using a Water Model 244 GPC, equipped with a set (102-106 A) of —Styragel columns using THF as the elution solvent. The molecular parameters of the above three polymers are listed in Table I. The copolymer, poly(GMA-co-3-CLS), contained 53.5 mole % 3-CLS and 46.5 mole % GMA, as determined by chlorine elemental analysis. The structure of the copolymer is shown in Figure 1. 

Axiva in Frankfurt, Germany (now Siemens-Axiva), performed the radical solution polymerization of acrylate resins using micromixer-tube reactors [42]. 

Materials. Trimethylsilylmethyl methacrylate (Sl) and chloro-methylstyrene (CMS) (mixed m,p isomers) were obtained from Petrarch Systems Inc. and Dow Chemical Co. Inc., respectively. Both monomers were purified by distillation at reduced pressure. Copolymers were prepared by free-radical solution polymerization at 85 C in toluene. Reactions were initiated using benzoyl peroxide. 

Sadeghi, G.M.M. Morshedian, J. Barikani, M. The effect of initiator-to-monomer ratio on the properties of polybutadiene-ol synthesized by free radical solution polymerization of 1,3-butadiene. Polym. Int. 2003, 52, 1083. 

F Teymour. The Dynamic Behavior of Free-Radical Solution Polymerization in Continuous Stirred Tank Reactors. PhD thesis, University of Wisconsin, Madison, 1989. 

The methyl methacrylate-itaconic acid copolymer, P(MMA-co-ItaA), was prepared by slow free-radical solution polymerization in methanol under nitrogen using 2,2 -azobis-(2,4-dimethyl valeronitrile)(du Pont Vazo 52) as initiator. The molar ratio of monomer to initiator was in the range of 5xl03 to 10xl03. Reaction at 50°C for 30 to 40 hrs gave conversions of 10 to 30%. The reaction mixture was added to cold, deionized water and the precipitated polymer obtained was rinsed with 2-propanol. 

Photopolymerization. The free radical solution polymerization of NVK in THE at temperatures in the range of —20°C, to 20°C with photoinitiation of ADMVN as the radical initiator showed an overall rate proportional to the square root of the initiator concentration. At low temperatures and small concentrations of the initiator, weight average molecular weights of 510,000 Dalton were obtained. The same is true, when 1,1,2,2-tetra-chloroethane is used as a solvent. ... 

Methacrylate-based copolymers are used often for anticorrosive applications. Copolymers from N-vinylcarbazole and glycidyl methacrylate have been synthesized using a free radical solution polymerization technique [229]. 

This phenomenon can be observed in free radical polymerization (nonisothermal) due to the exothermic nature of the polymerization reaction. However, due to the gel effect, it is also observed in some isothermal free radical polymerizations in a CSTR [11-14]. Figure 5.2 shows the rate of polymerization plotted versus monomer conversion for the free radical solution polymerization of methly methacrylate. Unlike a more common reaction in which the rate of reaction falls monotonically with conversion, the rate of reaction rises with conversion due to the onset of the gel effect. Thus the system can be thought of as autocatalytic. At high conversions the polymerization... 

Figure 5.2 Rate of polymerization versus monomer conversion for the free radical solution polymerization of methyl methacrylate (from [14]).

CSTR polymerization reactors can also be subject to oscillatory behavior [12]. A nonisothermal CSTR free radical solution polymerization can exhibit damped oscillatory approach to a steady state, unstable (growing) oscillations upon disturbance, and stable (limit cycle) oscillations in which the system never reaches steady state, and never goes unstable, but continues to oscillate with a fixed period and amplitude. However, these phenomena are more commonly observed in emulsion polymerization. 

Primarily, a radical solution polymerization with high transfer constant, leading to products of relatively low molecular weight (telomers, with MW 10,000) containing built-in fragments of the solvent. 

Surfactant-acrylamide Polymers. Copolymers of acrylamide and alkylarylpoly(ethoxy)-acrylate were prepared using standard free radical solution polymerization techniques. The alkylarylpoly(ethoxy)-acrylate monomers referred to as Surf" monomers were water dispersible and thus additional surfactants were not needed to effect random copolymerization. These so called "PAM-SURF" polymers were prepared with a variety of Surf monomers containing different amounts of ethylene oxide and different alkylaryl functionality. 

The hydrosols may be either linear, graft or branched polymers. The linear acrylic hydrosol polymers can be conveniently prepared by a conventional free radical solution polymerization process. Useful examples of initiators include dibenzoyl peroxide, hydrogen peroxide and other peroxy compounds such as tert-butyl peroxyp-ivalate, ferf-butyl peracetate, ferf-butyl peroctoate, and azo compounds such as 2,2 -azobisisobutyronitrile. The solvent used for the polymerization should be miscible with water so that the polymer can be conveniently inverted. 

Copolymers synthesized by a sodium persulfate-initiated free-radical solution polymerization of acrylamide and AMPS (at different pH values) as a new type of super-absorbent polymer have been studied in terms of swelling or viscosity behavior in water (168). 

Concerning the growing radicals in polymerization reactions, they can be studied directly by ESR spectroscopy as in the case of triphenylmethyl methacrylate and MMA [95]. In the latter case it was concluded that there are two stable conformations of the propagating radicals. The steric effect of the a-methyl group of MMA is not only responsible for the comparatively low heat of the polymerization reaction, but also for a certain control of the propagation steps. Therefore, in radical solution polymerization the polymethacrylates exhibit in most cases a favored syndiotacticity. 

FIGURE 6.10 Proposed scheme for control of monomer conversion and Mw in free-radical solution polymerizations. 

FIGURE 17.3 Rate of polymerization versus monomer conversion for free radical solution polymerization of methyl methacrylate. With permission from Reference 19, Kwalik KM. Bifurcation characteristics in closed-loop polymerization reactors [PhD thesis]. Atlanta School of Chemical Engineering, Georgia Institute of Technology 1988. 

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