Synthetic Polymers

Synthetic specialty polymers are obtained by the polymerization of monomers synthesized from petroleum or natural gas precursors. Linear or branched macromolecules may be formed from one or many monomers. Distribution of monomers, along the backbone or side chain, can be controlled in a number of ways, including: controlling monomer reactivity, concentration, addition order, and reaction conditions.

Major commercial synthetic specialty polymers are made by chain-growth polymerization of functionalized vinyl monomers, carbonyl monomers, or strained ring compounds. Depending on monomer structure, the polymerization may be initiated free radically, anionically, or cationically. Copolymers or terpolymers with random, alternating, block, or graft sequences can be prepared under appropriate reaction conditions. There are numerous methods used to prepare specialty polymers in the research laboratory. However, only a few are of commercial interest. Of particular commercial interest is synthesis of specialty polymers in solutions, dispersions, suspensions, or emulsions.

Commercial Advantages of Synthetic Polymers

 Synthetic, semisynthetic, and natural polymers frequently can perform similar functions. However, synthetic polymers have a number of inherent advantages and are preferred in many applications for a number of reasons, including:

· Greater Flexibility - Synthetic polymers can be designed at the molecular level and are frequently used as model compounds to develop structure-property relationships for the more complex natural polymers.

· Greater Versatility - Synthetic polymers can be tailored to provide a specific property or properties for a given application. The naturals and semisynthetics are limited in the types of chemical modification because of the fragile polysaccharide backbone.

· Greater Efficacy - Since synthetic polymers can be tailored for a specific property, significantly less synthetic polymers are needed to facilitate the same performance as natural polymers.

· Lower Biological Oxygen Demand (BOD) - Since synthetic polymers have lower BOD, the effluents containing synthetic polymers are easier to treat than the other types.

· Greater Product Consistency - Since the raw materials and the reaction conditions can be well defined and controlled, synthetic polymers can be manufactured with more consistent quality than natural or semisynthetic polymers.

· Greater Price Control - Synthetics are less subject to variations in price than natural polymers due to availability.

Functionality of Specialty Polymers

Specialty polymers are best known for their built in functionality and water-solubility. This functionality is present in natural and synthetic polymers and can be broken down into two broad categories nonionic and ionic polymers. These two categories will be discussed in greater detail.

Nonionic Polymers

Nonionic specialty polymers contain diverse functional groups that do not bear a charge, such as, vinyl esters, acrylates, acrylamides, imines, and ethers. Most of these commercially important polymers are soluble in water. Their water solubility is the result of a high number of polar or hydrogen-bonding functional groups per repeat unit. Nonionic polymers are typically formed from free-radical polymerization of vinyl monomers or ring opening polymerization of strained ring compounds. An example of each and their commercial applications are given below.

Polyacrylamide is formed from the free-radical polymerization of acrylamide. Acrylamide is unique among vinyl monomers because it can be polymerized to ultrahigh molecular weight . Although polyacrylamide dissolves slowly, it is soluble in water in all proportions. Since polyacrylamide can be polymerized to very high molecular weight, it is a highly efficient viscosifier. Applications of polyacrylamides include flocculants, rheology control agents, and adhesives. High molecular weight copolymers of acrylamide are the most widely used polymer for water treatment. Approximately several hundred million pounds of polyacrylamide is used annually in water treatment, with a market value of approximately one billion dollars .

Poly(ethylene oxide) is prepared by ring-opening polymerization of ethylene oxide. It is a white free-flowing powder with commercial grades ranging from 100,000 to 5,000,000 molecular weight (10). Polyethylene oxide)s are completely soluble in water at room temperature, but show a lower critical solution temperature (LCST) near the boiling point of water. The solution properties of poly(ethylene oxide)s have been extensively used in commercial applications for rheology control. The unique phase transition properties are being explored for control-release in thermally-responsive applications.

Ionic Polymers

Polymers possessing ionic groups pendent to the backbone are perhaps the most important class of macromolecules, ranging from biopolymers such as polynucleotides and proteins to technologically important rheology control agents and polysoaps.

These ion-containing polymers may be divided into two groups, polyelectrolytes and polyampholytes. Polyelectrolytes have either anionic or cationic groups along the chain while the polyampholytes have both anionic and cationic groups present. Both have high charge densities and typically are water-soluble.

Water-soluble ionic polymers share a number of common properties with water-soluble nonionics, e.g., they both can act as viscosifiers. However, differences arise from the presence of charge on the macromolecular backbone and from the electrostatic interactions of mobile counterions. These differences have a significant impact on the structure of ionic polymers in solution and will be discussed further.