Prepolymerization

Afterthe polybutadiene is dissolved,0.3 g fert-dodecylmercaptane (MW-regulator) are added and the reaction mixture heated under stirring (100 rpm) to 80 °C. Now, 0.6 g bisbenzoylperoxide (BPO), dissolved in 1 ml of styrene, are added and the temperature kept at 80 °C. After 2 hours another 0.6 g BPO in 1 ml of styrene are added and the temperature further held at 80 °C until a conversion of 35-40% is reached.This is the case after 2-3 h. In order to control the conversion, samples are taken every 30 min.They are weighed and then heated for 10 min at 220 °C in vacuo in order to remove the styrene monomer. After a solid content of 35-40% is reached, the prepolymerization phase is finished. 

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Remember from Sec. 1.3 that graft copolymers have polymeric side chains which differ in the nature of the repeat unit from the backbone. These can be prepared by introducing a prepolymerized sample of the backbone polymer into a reactive mixture—i.e., one containing a source of free radicals—of the side-chain monomer. As an example, consider introducing polybutadiene into a reactive mixture of styrene ... 

Stabilizers, pigments, and other additives are milled in spinning solvent, normally along with small amounts of the urethane polymer to improve dispersion stabiUty this dispersion is then blended to the desired concentration with polymer solution after chain extension. Most producers combine prepolymerization, chain extension, and additive addition and blending into a single integrated continuous production line. 

In the first step of the polymerization process, a prepolymer is prepared as a slurry in water. Excess diamine is added to control the degree of polymerization, eg, degree of polymerization = 6-14 (158). This prepolymerization step is conducted at approximately 200°C under autogenous pressure for less than 90 min. 

Many proprietary methods have been developed for casting and shaping DADC, especially for lenses. In one method DADC containing 3.5% diisopropyl percarbonate is prepolymerized by warming to a symp of viscosity 40—60 mm /s (=cSt) (15). Polymerization is continued in a lens for 18 h at 90°C followed by annealing at 120°C. 

Scratch resistance of polymer from DADC is improved by novel mixtures of peroxide initiators such as 5% isopropyl percarbonate with 3.5% benzoyl peroxide (16). In order to force completion of polymerization and attain the best scratch resistance in lenses, uv radiation is appHed (17). Eyeglass lenses can be made by prepolymerization in molds followed by removal for final thermal cross-linking (18). 

The reaction product of 4,4 -bismaleimidodiphenylmethane and 4,4 -diaminophenylmethane, known as Kerimide 601 [9063-71-2] is prepolymerized to such an extent that the resulting prepolymer is soluble in aprotic solvents such as /V-methy1pyrro1idinone [872-50-4] dimethylformamide [68-12-2] and the like, and therefore can be processed via solution techniques to prepreg. Kerim ide 601 is mainly used in glass fabric laminates for electrical appHcations and became the industry standard for polyimide-based printed circuit boards (32). 

Fig. 6-5. Model of the binding site for L-PA based on spectroscopic and chromatographic characterization of the prepolymerization monomer-template assemblies.

PVC can be prepolymerized in bulk to approximately 7-8% conversion. It is then transferred to an autoclave where the particles are polymerized to a solid powder. Most vinyl chloride, however, is polymerized... 

PA-6,6 is still often synthesized in a one-pot batch process with a prepolymerization stage under pressure. Continuous processes have long been described, and for prepolymerization, in particular, they seem to work well. However, with this method, there does seem to be a problem with quality control. 

PA-6,6 is made from the relatively expensive materials hexamethylene diamine and adipic acid. An alternative synthesis of PA-6,6 from adiponitrile and hexamethylene diamine utilizing water is under investigation.16 PA-6 can be synthesized in a continuous process at atmospheric pressure, but reaction times are very long as the ring-opening initiation step is particularly slow. The reaction time can be shortened considerably by carrying out prepolymerization in the presence of excess water at pressure however, this makes the continuous polymerization process more complex. Copolymers with amide units of uniform length (diamides) are relatively new the diamide units are able to crystallize easily and have a thermally stable crystalline structure. 

The main method of PA synthesis is by melt polymerization. The polymerization of PA-6,6 occurs in two stages, a prepolymerization of the PA salt at elevated pressures followed by a melt polymerization at atmospheric pressure. The prepolymerization stage requires an autoclave, preferably with a glass insert. The glass insert allows easy extraction of the polymer. PA-6 polymerization is simple it can be carried out at atmospheric pressure, and the evaporating water stirs the reaction medium. 

The AA-BB polyamides are nearly always prepared using bulk polymerization with a prepolymerization step at higher pressure, although not every laboratory has die facility to carry out a polymerization in an autoclave. Nielinger50 has reported bulk polymerization of PA-6,1 from its salt solution at atmospheric pressure (Example 11). This mediod may also be usable for other laboratory polymerizations like PA-6,6. 

Continuous polymerization processes for PA-6,6 have been reported for over 30 years.5,6,28 Prepolymerization in tubular (Fig. 3.21) or baffled reactors is particularly well suited to continuous polymerization. The polymerization of prepolymers to high-molecular-weight materials in a continuous process is more difficult to control as small differences is molecular weights result in large differences in viscosities. Viscosity differences result in different hold-up times in die reactor and thus nonhomogeneous products. 

PA-4,2 is an even-even PA with a very high melting temperature (390°C). It is prepared from an oxalic ester and 1,4-tetramethylenediamine in a two-step procedure a prepolymerization in solution and a polymerization of the prepolymers in the solid state. 

At higher water concentrations (at higher pressure) (Fig. 3.24), the reaction is considerably more rapid. The prepolymer obtained is further polymerized in another reactor, which has a working pressure of 1 bar or less. The total reaction time of the prepolymerization can be considerably shortened by processing it in an autoclave.812 28 In a laboratory, PA-6 can be synthesized in several ways from m-aminocaprolic acid, from lactam and from lactam and water, and anionically. 

Figure 3.24 Influence of PA-6 polymerization process on relative viscosity as function of reaction time (i) atmospheric in VK column (ii) prepolymerization atmospheric followed by water removal (iii) prepolymerization at elevated pressure followed by water removal.31...

The partially aromatic PAs are exclusively made of die diamine-diacid type and not die amine-acid type. The aromatic diamines, similar to phenylene diamines, color easily and dieir polymers are conjugated, having a golden brown color. The aromatic diacids used in the formation of partially aromatic PAs are mainly terephthalic and isophthalic acids. Starting with the diacids, the PA salt is made first and with this the salt prepolymers are prepared. The prepolymerization is usually carried out in an autoclave to prevent die sublimation of the reactants. In a laboratory synthesis it would be preferable to avoid this autoclave step as one is not always available. It is possible to start with the more reactive esters, such as diphenyl isophtiialate, or with the acid chlorides starting with the reactive isocyanates is, in principle, also possible. The terephthalic and isophthalic acids are also used to modify PA-6,6 and PA-4,6 to more dimensionally stable copolymers.6,18... 

In the diacid mediod, die PA salt is made first. A solution of this PA salt in water can be used for the polymerization. In the temperature range where the reaction rates are high, the diamines are volatile, and thus, it is preferable to carry out the prepolymerization under pressure. The prepolymerization can be carried out either at 220-250°C for 1 h or at 280-320°C in a matter of seconds. In the latter case, die reaction is carried out in a small-diameter tubular reactor.64 Although a prepolymerization under pressure is preferred, Nielinger28 has described a polymerization at atmospheric pressure at 210°C, whereby the loss in diamine is compensated for. 

Polylactides, 18 Poly lactones, 18, 43 Poly(L-lactic acid) (PLLA), 22, 41, 42 preparation of, 99-100 Polymer age, 1 Polymer architecture, 6-9 Polymer chains, nonmesogenic units in, 52 Polymer Chemistry (Stevens), 5 Polymeric chiral catalysts, 473-474 Polymeric materials, history of, 1-2 Polymeric MDI (PMDI), 201, 210, 238 Polymerizations. See also Copolymerization Depolymerization Polyesterification Polymers Prepolymerization Repolymerization Ring-opening polymerization Solid-state polymerization Solution polymerization Solvent-free polymerization Step-grown polymerization processes Vapor-phase deposition polymerization acid chloride, 155-157 ADMET, 4, 10, 431-461 anionic, 149, 174, 177-178 batch, 167 bulk, 166, 331 chain-growth, 4 continuous, 167, 548 coupling, 467 Friedel-Crafts, 332-334 Hoechst, 548 hydrolytic, 150-153 influence of water content on, 151-152, 154... 

Process flow for a typical batch-mass polystyrene process(1) is shown in Figure 1. Styrene monomer is charged to the low conversion prepolymerization reactor with catalyst and other additives, and the temperature is increased stepwise until the desired conversion is reached. It is then transferred into the press. Polycycles are 6 to 14 hours in the low conversion reactor, and 16 to 24 hours in the press. At completion, the cakes are then cooled with water and removed from the press to be ground and then (usually) extruded into pellets. 

The prepolymerization reactor for HIPS is similar ( ). A solution of rubber and styrene monomer is charged to the reactor along with catalysts, antioxidants, and other additives, and the temperature program is carried out until the desired conversion is reached. This is usually close to the point where increasing viscosity seriously limits mixing and temperature control. 

Suspension Polymerization. Suspension polymerization probably remains the most widely practiced method of producing PS. It can also be used to produce HIPS. To improve quality of the latter, however, a batch-mass prepolymerization of the rubber syrup is normally carried out first the syrup is then suspension polymerized to completion. 

The principal difference is that a prepolymerizer CSTR has been placed ahead of the first reactor. An example cited on a production scale similar to the example discussed in Section 2.3.5, reveals the prepol erizer to be a CSTR with a 3-blade agitator operating at 110°C and 7.5% solids. The first reactor is a roughly similar CSTR operating at 125°C and 35% solids. 

In the prepolymerizer, the polymer particles are formed and polymerized to 7-8 wt. % conversion before being transferred to the autoclave where the particles are polymerized to a solid powder at about 88% conversion. 

The final polymer particles have a narrow particle size distribution. Figure 2 (] ), and the mean particle size is a strong function of the agitation in the prepolymerizer. 

Figure 3. Effect of prepolymerizer agitator speed on the mean particle size of...

Figure 4. Effects of prepolymerizer agitator size and speed on the mean particle...

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