Poly (vinyl chloride)

Poly (vinyl chloride) is used in industry on a very large scale in many applications, such as rigid plastics, plastisols, and surface coatings. The monomer, vinyl chloride, can be prepared from acetylene:

The reaction is exothermic and requires cooling to maintain the temperature between 100 and 108C. The monomer can also be prepared from ethylene:

The reaction of dehydrochlorination is carried out at elevated pressure of about 3 atm. Free-radical polymerization of vinyl chloride was studied extensively. For reactions that are carried out in bulk the following observations were made [292]:

1. The polymer is insoluble in the monomer and precipitates out during the polymerization.

2. The polymerization rate accelerates from the start of the reaction. Vinyl chloride is a relatively unreactive monomer. The main sites of initiation occur in the continuous monomer phase.

3. The molecular weight of the product does not depend upon conversion nor does it depend upon the concentration of the initiator.

 4. The molecular weight of the polymer increases as the temperature of the polymerization decreases. The maximum for this relationship, however, is at 30C.

There is autoacceleration in bulk polymerization rate of vinyl chloride [293]. It was suggested by Schindler and Breitenbach [294] that the acceleration is due to trapped radicals that are present in the precipitated polymer swollen by monomer molecules. This influences the rate of the termination that decreases progressively with the extent of the reaction, while the propagation rate remains constant. The autocatalytic effect in vinyl chloride bulk polymerizations, however, depends on the type of initiator used [295]. Thus, when 2, 20-azobisisobutyronitrile initiates the polymerization, the autocat alytic effect can be observed up to 80% of conversion. Yet, when benzoyl peroxide initiates the reaction, it only occurs up to 20–30% of conversion. Whenvinyl chloride is polymerized in solution, there is no autoacceleration. Also, a major feature of vinyl chloride free-radical polymerization is chain transferring to monomer [296]. This is supported by experimental evidence [297, 298]. In addition, the growing radical chains can terminate by chain transferring to “dead” polymer molecules. The propagations then proceed from the polymer backbone [297]. Such new growth radicals, however, are probably short lived as they are destroyed by transfer to monomer [299]. The 13C NMR spectroscopy of poly (vinyl chloride), which was reduced with tributyltin hydride, showed that the original polymer contained a number of short four-carbon branches [300]. This, however, may not be typical of all poly(vinyl chloride) polymers formed by free-radical polymerization. It conflicts with other evidence from 13C NMR spectroscopy that chloromethyl groups are the principal short chain branches in poly (vinyl chloride) [301, 302]. The pendant chloromethyl groups were found to occur with a frequency of 2–3/1,000 carbons. The formation of these branches, as seen by Bovey and coworkers, depends upon head-to-head additions of monomers during the polymer formation. Such additions are followed by 1,2 chlorine shifts with subsequent propagations [301, 302]. Evidence from still other studies also shows that some head-to-head placement occurs in the growth reaction [303]. It was suggested that this may be not only an essential step in formation of branches but also one leading to formation of unsaturation at the chain ends [303, 304]:

Poly(vinyl chloride) prepared with boron alkyl catalysts at low temperatures possesses higher amounts of syndiotactic placement and is essentially free from branches [305–307]. Many attempts were made to polymerize vinyl chloride by ionic mechanisms using different organometallic compounds, some in combinations with metal salts [308–312]. Attempts were also made to polymerize vinyl chloride with Ziegler–Natta catalysts complexed with Lewis bases. To date, however, it has not been established unequivocally that vinyl chloride does polymerize by ionic mechanism. Use of the above catalysts did yield polymers with higher crystallinity. These reactions, however, were carried out at low temperatures where greater amount of syndiotactic placement occurs by the free-radical mechanism [313]. Vinyl chloride was also polymerized by AlCl(C₂H₅)O₂H₅ + VO(C₃H₇O₂) without Lewis bases [312]. Here too, however, the evidence indicates a free-radical mechanism. On the other hand, butyllithium–aluminum alkyl-initiated polymerizations of vinyl chloride are unaffected by free-radical inhibitors [313]. Also the molecular weights of the resultant polymers are unaffected by additions of CCl₄ that acts as a chain transferring agent in free-radical polymerizations. This suggests an ionic mechanism of chain growth. Furthermore, the reactivity ratios in copolymeri zation reactions by this catalytic system differ from those in typical free-radical polymerizations [313]. An anionic mechanism was also postulated for polymerization of vinyl chloride with t-butylmagnesium in tetrahydrofuran [314]. Commercially, by far the biggest amount of poly(vinyl chloride) homopolymer is produced by suspension polymerization and to a lesser extent by emulsion and bulk polymerization. Very little polymer is formed by solution polymerization. One process for bulk polymerization of vinyl chloride was developed in France where the initiator and monomer are heated at 60C for approximately 12 h inside a rotating drum containing stainless steel balls. Typical initiators for this reaction are benzoyl peroxide or azobisisobutyr onitrile. The speed of rotation of the drum controls the particle size of the final product. The process is also carried out in a two-reactor arrangement. In the first one approximately 10% of the monomer is converted. The material is then transferred to the second reactor where the polymerization is continued until it reaches 75–80% conversion. Special ribbon blenders are present in the second reactor. Control of the operation in the second reactor is quite critical [315]. Industrial suspension polymerizations of vinyl chloride are often carried out in large batch reactors or stirred jacketed autoclaves. Continuous reactors, however, have been introduced in several manufacturing facilities [315]. Typical recipes call for 100 parts of vinyl chloride for 180 parts of water, a suspending agent, like maleic acid–vinyl acetate copolymer, a chain transferring agent, and a monomer soluble initiator. The reaction may be carried out at 100 lb/in.2 pressure and 50C for approximately 15 h. As the monomer is consumed the pressure drops. The reaction is stopped at an internal pressure of about 10 lb/in.2 and remaining monomer (about 10%) is drawn off and recycled. The product is discharged. Emulsion polymerizations of vinyl chloride are usually conducted with redox initiation. Such reactions are rapid and can be carried out at 20C in 1–2 h with a high degree of conversion. Commercial poly(vinyl chloride)s range in molecular weights from 40,000 to 80,000. The polymers are mostly amorphous with small amounts of crystallinity, about 5%. The crystalline areas are syndiotactic [317, 318]. Poly(vinyl chloride) is soluble at room temperature in oxygen-containing solvents, such as ketones, esters, ethers, and others. It is also soluble in chlorinated solvents. The polymer, however, is not soluble in aliphatic and aromatic hydrocarbons. It is unaffected by acid and alkali solutions but has poor heat and light stability. Poly(vinyl chloride) degrades at temperatures of 70C or higher or when exposed to sun light, unless it is stabilized. Heating changes the material from colorless to yellow, orange, brown, and finally black. Many compounds tend to stabilize poly(vinyl chloride). The more important ones include lead compounds, like dibasic lead phthalate and lead carbonate. Also effective are metal salts, like barium, calcium, and zinc octoates, stearates, and laurates. Organotin compounds, like dibutyl tin maleate or laurate, also belong to that list. Epoxidized drying oils are effective heat stabilizers, particularly in coatings based on poly(vinyl chloride). Some coating materials may also include aminoplast resins, like benzoguanamine–formaldehyde condensate. The process of degradation is complex. It involves loss of hydrochloric acid. The reactions are free radical in nature, though some ionic reactions appear to take place as well. The process of dehydro chlorination results in formations of long sequences of conjugated double bonds. It is commonly believed that formation of conjugated polyenes, which are chromophores, is responsible for the darkening of poly (vinyl chloride). In addition, the polymer degrades faster in open air than it does in an inert atmosphere. This shows that oxidation contributes to the degradation process. All effective stabilizers are hydrochloric acid scavengers. This feature alone, however, can probably not account for the stabilization process. There must be some interaction between the stabilizers and the polymers. Such interaction might vary, depending upon a particular stabilizer.

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Poly tetra fluoro ethylene

Polyacrylamide, Poly(acrylic acid), and Poly(methacrylic acid)

Acrylonitrile and Methacrylonitrile Polymers

Thermoplastic and Thermoset Acrylic Resins

Acrylic Elastomers

Polymerizations of Acrylic and Methacrylic Esters

Polymers of Acrylic and Methacrylic Esters

ABS Resins

High-Impact Polystyrene