Vacuum, polymerization

Usually CO is used in excess with respect to the concentration of the AC, and adsorption of carbon monoxide on the AC leads to complete quenching of polymerization. If carbon monoxide is removed from the polymerization system, for example by vacuum, polymerization begins again However, polymer... 

Conventional polymerization does not occur in gas phase, particularly in vacuum, because of the limitation set by the ceiling temperature of polymerization, and there are only a few cases in which the deposition of polymeric material occurs in vacuum. Those exceptional cases are plasma polymerization and Parylene polymerization, which is also a vacuum polymerization coating process using a gaseous monomer. The common denominators for these two processes are 1) the polymerizations yield solid-state polymer (in the form of film in most cases) from a gas phase monomer in vacuum and 2) the polymer formed by the processes... 

PVC can be made by either bulk polymerization, suspension polymerization, or emulsion polymerization of the VCM. Most of the resin (over 90%) is now made by free-radical-initiated suspension polymerization of VCM. The reactor used for emulsion polymerization of the monomer is also about the same as that for suspension polymerization except that stripping is usually carried out under vacuum. Polymerization of VCM is an exothermic reaction (+410 cal/g) and removal of heat from the system is an important consideration in large-scale manufacture. Controlling the temperature of the reaction is important as it dictates the average molecular weight and the polydispersity of the resin product. This is relatively easier to achieve in suspension polymerization compared to bulk polymerization processes because the former is carried out in a water medium. 

The authors concluded that the side reactions normally observed in amine-initiated NCA polymerizations are simply a consequence of impurities. Since the main side reactions in NCA polymerizations do not involve reaction with adventitious impurities such as water, but instead reactions with monomer, solvent, or polymer (i.e., termination by reaction of the amine end with an ester side chain, attack of DMF by the amine end, or chain transfer to monomer), it appears removal of water suppresses these side reactions. A possible explanation for the polymerization control observed under high vacuum is that impurities (e.g., water) act to catalyze side reactions of growing chains with monomer, polymer, or solvent. In this scenario, it is reasonable to speculate that polar species such as water can bind to monomers or the propagating chain end and thus influence their reactivity. Recently, in polymerizations of O-benzyl-L-tyrosine NCA (Bn-Tyr NCA) in DMF, it was determined that although most side reactions are insignificant in the high-vacuum polymerization, some termination of chains by reaction with DMF solvent does occur. ... 

To address this issue, Hadjichristidis developed the synthesis of 3- and 4-arm star copolypeptides by high-vacuum polymerization using the core-first method. They prepared 3-arm stars containing poly(Z-Lys) and poly(Bn-Glu) block copolymers that were simultaneously grown off of a 2(aminomethyl)-2-methyl-1,3-propanediamine initiator core (Scheme 2). This method produced well-defined star copolymers with narrow molecular weight distributions. Attempts to prepare 4-arm star copolymers... 

The methanol is vacuum distilled from the flask (this occurs between the temperature of 50-60°C), then allowed to cool. Sometimes upon cooling the concentrated mixture will polymerize into a big, white gummy mass. This will go away in the next step. To the concentrate is added 2000mL 3N HCI solution which will dis-... 

To a solution of 0.25 mol of the trimethylsilyl ether in 120 ml of dry diethyl ether was added in 20 min at -35°C 0.50 mol of ethyllithium in about 400 ml of diethyl ether (see Chapter II, Exp. 1). After an additional 30 min at -30°C the reaction mixture was poured into a solution of 40 g of ammonium chloride in 300 ml of water. After shaking, the upper layer was separated off and dried over magnesium sulfate and the aqueous layer was extracted twice with diethyl ether. The ethereal solution of the cumulenic ether was concentrated in a water-pump vacuum and the residue carefully distilled through a 30-cm Vigreux column at 1 mmHg. The product passed over at about 55°C, had 1.5118, and was obtained in a yield of 874. Distillation at water-pump pressure (b.p. 72°C/I5 mmHg) gave some losses due to polymerization. 

Concentration in a water-pump vacuum gave the chloroallene, n 1.5980, in more than 90% yield. The NMR spectrum showed that no starting compound was present and the purity was satisfactory. Attempts to distil the allene led to extensive polymerization. 

Polythiophene can be synthesized by electrochemical polymerization or chemical oxidation of the monomer. A large number of substituted polythiophenes have been prepared, with the properties of the polymer depending on the nature of the substituent group. Oligomers of polythiophene such as (a-sexithienyl thiophene) can be prepared by oxidative linking of smaller thiophene units (33). These oligomers can be sublimed in vacuum to create polymer thin films for use in organic-based transistors. 

Polymerizations are typically quenched with water, alcohol, or base. The resulting polymerizates are then distilled and steam and/or vacuum stripped to yield hard resin. Hydrocarbon resins may also be precipitated by the addition of the quenched reaction mixture to an excess of an appropriate poor solvent. As an example, aUphatic C-5 resins are readily precipitated in acetone, while a more polar solvent such as methanol is better suited for aromatic C-9 resins. 

Three generations of latices as characterized by the type of surfactant used in manufacture have been defined (53). The first generation includes latices made with conventional (/) anionic surfactants like fatty acid soaps, alkyl carboxylates, alkyl sulfates, and alkyl sulfonates (54) (2) nonionic surfactants like poly(ethylene oxide) or poly(vinyl alcohol) used to improve freeze—thaw and shear stabiUty and (J) cationic surfactants like amines, nitriles, and other nitrogen bases, rarely used because of incompatibiUty problems. Portiand cement latex modifiers are one example where cationic surfactants are used. Anionic surfactants yield smaller particles than nonionic surfactants (55). Often a combination of anionic surfactants or anionic and nonionic surfactants are used to provide improved stabiUty. The stabilizing abiUty of anionic fatty acid soaps diminishes at lower pH as the soaps revert to their acids. First-generation latices also suffer from the presence of soap on the polymer particles at the end of the polymerization. Steam and vacuum stripping methods are often used to remove the soap and unreacted monomer from the final product (56). 

R = alkyl, R = H) can be condensed, using acid, dehydrating agents (eg, phosphoms pentoxide), or vacuum to form polymeric peroxides (3). Most such polymeric peroxides decompose explosively. 

Resoles. Like the novolak processes, a typical resole process consists of reaction, dehydration, and finishing. Phenol and formaldehyde solution are added all at once to the reactor at a molar ratio of formaldehyde to phenol of 1.2—3.0 1. Catalyst is added and the pH is checked and adjusted if necessary. The catalyst concentration can range from 1—5% for NaOH, 3—6% for Ba(OH)2, and 6—12% for hexa. A reaction temperature of 80—95°C is used with vacuum-reflux control. The high concentration of water and lower enthalpy compared to novolaks allows better exotherm control. In the reaction phase, the temperature is held at 80—90°C and vacuum-refluxing lasts from 1—3 h as determined in the development phase. SoHd resins and certain hquid resins are dehydrated as quickly as possible to prevent overreacting or gelation. The end point is found by manual determination of a specific hot-plate gel time, which decreases as the polymerization advances. Automation includes on-line viscosity measurement, gc, and gpc. 

Depending on the final polymerization conditions, an equilibrium concentration of monomers (ca 8%) and short-chain oligomers (ca 2%) remains (72). Prior to fiber spinning, most of the residual monomer is removed. In the conventional process, the molten polymer is extmded as a strand, solidified, cut into chip, washed to remove residual monomer, and dried. In some newer continuous processes, the excess monomer is removed from the molten polymer by vacuum stripping. 

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