Principles_of_Polymerization

2018

➢ Polymer classification Homopolymer: Co-polymer Formed of one type of monomer foined in regular sequence Blok co-polymer :styreneisopropylene Alternating co-polymer: styrenebutadiene Random co-polymer : linear branched crosslinking PE-PP PP Phenol--formaldehyde

 are macro (large) molecules made from smaller repeating units called monomers.  Monomers are the building blocks of polymers.  A polymer might be made from identical monomers or different types of monomers.  Homopolymers-from only one type of monomer.  E.g., polyethylene is synthesized by the polymerization of one type monomer, ethene (ethylene)  Copolymers-prepared by polymerizing more than one kind of monomer unit.  E.g. ethane (H2C=CH2) and propene (H 2 C=CH-CH 3) can be copolymerized to produce a polymer that has two kinds of repeating units:  classified as random or regular, based on the way the monomers are arranged along the polymer chain  Random polymers contain repeating units arranged in a random fashion.  Regular polymers contain a sequence of monomers in regular alternating repeating units. Where, A and B represent monomer units.  On the basis of their source, polymers are classified as synthetic and natural.  Natural polymers,-found in nature. E.g. proteins, carbohydrates, nucleic acids & natural rubber  Synthetic polymers are man-made polymers. Most synthetic polymers are organic compounds.  E.g. nylon, polyester (Dacron), Teflon, Bakelite, polyethylene, and polyvinyl chloride. POLYMERIZATION  The process by which monomer units combine together to form polymers.  can be carried out either through addition polymerization or condensation polymerization. Addition Polymerization  requires a large number of monomer molecules which can be polymerized by addition reactions  also known as chain growth polymerization.  E.g. addition polymerization of  ethylene molecules yields polyethylene  styrene molecules gives polystyrene

2011

Polymer is a collection of many macromolecules which are not identical chemical compounds; they can differ in molecular weight and chemical structure. How big has to be a molecule to call it macromolecule or polymer? The line between oligomer and polymer is not sharp and is set by the material behavior. We can find in the literature such a description as low polymers and high polymers. The former are characterized by molecular weight (M) of 10 000-20 000 Da while for the latter M is higher and sometimes reaches even several million. This distinction is however arbitrary. The process of polymer synthesis is called polymerization and from the mechanism point of view one can distinguish chain-reaction and step-reaction polymerization. Linear polymers consist of a long chain of skeletal atoms to which are attached the substituent groups. They are linear only in the topological sense, their real shape can be very complicated. The representative of this group of polymers are polyethylene, poly(methyl methacrylate), or poly(vinyl chloride). There are also special types of linear polymercyclolinear, polymerswhich are formed by linking together ring systems. Introducing the ring in the chain makes polymer more rigid and thermally stable. The poly(p-xylen) is an example of such polymers: CH 2 CH 2 8 Some polymers have branched chains which are often the result of side reactions during polymerization. A few types of branched polymers are known: with random branches comblike polymer star polymer dendrimer Ladder polymers, as the name suggests, resemble the ladder in which two skeletal strands are linked together in a regular sequence. In practice the aromatic rings may constitute the linking, as illustrated below: Cyclomatrix polymer it is a system of connected rings that form a tridimensional matrix. Such polymers are rigid, insoluble and thermally stable. The special example of such polymer is graphite. Crosslinked or network polymers are ones in which chemical linkages exist between the chains. 9 O CH 3 CH 3 CH 2 CH 2 CH 2 CH 2 CH CH 2 CH 2 CH CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH 2 CH CH 2 or (C2H5)2AlH CH 2 CHCH 2 CH 2 Al(C 2 H 5) 2 TiCl3 CH 2 =CHR CH 2 CHCH 2 CH 2 CH 2 CHCH 2 CH R R • Changing the functional groups present in a polymer into the initiating centra. Some examples are given below. Chloromethylated polystyrene in the presence of AlBr 3 (cationic mechanism) reacts with 2-methylpropene, resulting in polystyrene grafted with poly(2-methylpropene): Poly(vinyl chloride) grafted with styrene also according to the cationic mechanism: Acrylonitrile grafted anionically on poly(p-chlorostyrene) (THF, in the presence of sodium naftalene): Unsaturated polymers can be grafted with Ziegler-Natta catalysts: CH 2 CH CH 2 CH CH 2 Cl AlBr 3 CH 2 CH CH 2 CH CH 2 + AlBr 3 Cl-CH 2 =C(CH 3) 2 CH 2 CH CH 2 CH CH 3 CH 2 CH 3 C CH 3 CH 3 CH 2 C CH 2 AlCl 3 CH 2 =CHC 6 H 5 CH 2 CH Cl CH 2 CH Cl CH 2 CH CH 2 CH Cl + AlCl 4 _ CH 2 CH Cl CH 2 CH CH CH 2 C 6 H 5 CH CH 2 C 6 H 5 CH 2 CH CH 2 CH Cl CH 2 CH Na Na naftalen THF CH 2 CHCN CH 2 CH CN CH 2 CH 3 CH 2 C CH 3 CH 3 CH 2 CH CH 2 CH CH 3 CH 3 Poly(butene-1) T g ~-24 o C Mechanical properties are like those of LDPE, but it has higher flexibility, resistance to creep, environmental stress cracking, chemicals, and abrasion. It is used for hot water containers, pipes, cable insulation, wrapping films, sealing tapes, shoe soles, floor covering and polyolefins modification. In a blend with PE it is used in medical packaging. Polyisobutylene (PIB)-butyl rubber T g ~-73 o C It is properly a copolymer but the other monomer-isoprene-is used in a very low content (about 1%). The latter is used only to make possible the vulcanization of material. The polymer is amorphous and at room temperature exhibits elastomeric behavior. Its main property is its impermeability to gases, and resistance to most chemicals and water. It has found an application in production of tubeless tires, sealing compounds, joints, various coatings and as an additive to gasoline increasing the motors power. It is used also as lubricating oil additive, hot-melt adhesives and chewing gum base. From biomedical applications the most important is using as implant material for soft tissue replacement and reconstruction and in medical plasters. Poly(4-methyl-1 pentene) (PMP) This polymer has the lowest density (0.83) from all olefins, it is transparent, flexible and chemicals resistant. It is used for production of household articles, medical tools (sterilized), hoses and food wrapping. Double C=C bond in the main chain The aliphatic carbon-carbon double bonds are expected to generate chain stiffness. However such an effect is observed only for chains with alternating single and double bonds 92 CH 2 CH CH CH 2 CH 2 C CH CH 2 CH 3 CH 2 CH C 6 H 5 (polyacetylene). In other polymers the torsional mobility of the adjacent single bonds is enhanced. Double bonds in a hydrocarbon skeleton are sites that are sensitive to chemical attack, especially by ozone or by oxygen. Polybutadiene (BR-butadiene rubber) T g ~-110 o C Polyisoprene (IR-isoprene rubber) T g ~-72 o C Both polymers have similar properties. Double bonds in the chain make polymer vulcanization possible. Cis-isomers are used in tire industry along with natural rubber, also shoes soles, adhesives. Trans-isomer has a limited and specific application. Huge amounts of polybutadiene go for production of high-impact polystyrene. Various rubber products find an application in medical industry-the examples are gloves, catheters, diaphragms, tubing, tube caps and stoppers, wheels and casters for hospital beds and trolleys, and many others. Aryl side groups Phenyl rings are hydrophobic, rigid, and relatively bulky, so they impose stiffness and steric hindrance on to any chain to which they are attached. Aromatic rings adsorb UV energy thus undergo photolytic reactions that can lead to discoloration and decomposition of a polymer. Polystyrene (PS) (poly(1-phenylethylene) T g ~ 100 o C Commercial polymer is the amorphous, thermoplastic material, transparent, hard and brittle. Used for production of many small articles (toys, boxes, trays, disposable dishes and cases), optical components and others. In the form of rigid foams PS is utilized in the packaging industry and in the insulation buildings. umerous polystyrene products as diagnostic instruments and disposable labware, petri dishes, tissue culture components, flasks, pipettes, trays, syringes, they find an application in biomedical laboratories. Polystyrene derivative-sodium sulfonate-is used to treat a high level of potassium in blood and as an immune-modulation drug for the complement inhibition. 93 CH CHCl CH 2 CCl 2 CH 2 C Cl CH CH 2 Chlorine as a side group unit Chlorine connected directly to a carbon skeleton, increases chain stiffness and usually (except of benzyl chloride units) enhances its chemical resistance and reduces the solubility in hydrocarbon solvents or fuels. Carbon-chlorine bonds are prone to photochemical reactions in sunlight and the addition of stabilizers is necessary. The chlorine-containing polymers are not suitable for use as biomaterials but many products are used in medical industry. The most important in this polymer group is Poly(vinyl chloride) (PVC) T g ~ 80 o C As thermoplastic polymer is utilized in two forms: rigid and plasticized. Pure polymer is very hard, stiff, used for example for production of pipes, house siding, doors and window frames, small electrical equipment. PVC fibers are used to manufacture fireproof furnishing fabrics. Plasticized PVC is elastic well extruded into film or tubes and used as a cable insulator, artificial leather, floor covering, shower curtains, gaskets. PVC is used to manufacture medical tubing for dialysis, blood transfusion and feeding, endotracheal, feeding and pressure monitoring, heart and lung bypass sets, hard valves in medical devices catheters, drip chambers and caps, suction tubing connectors, blood containers, urine continence and ostomy products, oxygen tents, inhalation masks, surgical and examination gloves, blister pack for pills and tablets and others. Poly(vinylidene chloride) (PVDC) T g ~-17 o C Its application is determined by two physicochemical characteristics-very low permeability to gases and liquids (films in food packaging, containers, coatings) and flame resistance (textile fibers for furnishing fabrics). Polychloroprene (CR-chloroprene rubber, neoprene) It is the oldest synthetic rubber, cured with metal oxides. It finds applications due to its durability, oil, fuel, and ozone resistance, good adhesive properties, and fireproof character. It is widely used for the manufacture of flexible tubes, sealing joints, adhesives, and paints.

IJPSM, 2019

Polymers are a large class of materials consisting of many small molecules. Polymers such as cotton, wool, rubber, Teflon(tm), and all plastics are used in nearly every industry. Evidence for the existence of cellulose comes from the close match of an infrared absorption spectrum of celestial origin with that of cellulose. The surface film is a pellicle of cellulose. In most elastic materials, such as metals used in springs, the elastic behavior is caused by bond distortions. In its relaxed state, rubber consists of long, coiled-up polymer chains that are interlinked at a few points. Polymeric delivery systems are mainly intended to achieve controlled or sustained drug delivery. Targeting of drugs to the colon following oral administration has also been accomplished by using polysaccharides. Polymeric materials have a vast potential for exciting new applications in the foreseeable future. Polymer uses are being developed in such diverse areas.

2019

The mechanical properties of polymers are characterized by the way in which these materials respond to applied mechanical stresses, the latter being of the stress or strain type. The nature of this response depends on the chemical structure, temperature, time, and morphology defined during polymer processing. The molecular structure of the polymers provides a viscous behavior, such as liquids, superimposed with an elastic behavior, such as Hookean solids. This phenomenon is called viscoelasticity and occurs for both plastics and fibers. The elastomers have a unique behavior known as rubber elasticity. This type of elasticity is very particular because it imposes great deformations in the rubbery chains that are amorphous, cross-linked, and very flexible. Another parameter to consider is the timescale in which the polymer is stressed. The mechanical testing can be carried out quickly, called a short duration, or slowly, called a long duration. Testing under impact is classified as a very short duration, and the polymer is requested only for a few milliseconds. The creep and stress relaxation testing, in turn, characterize the mechanical behavior of the polymer over a much longer timescale, reaching many years. The importance of the duration of the applied mechanical demand is related to the time the polymer needs to relax during this period of time. The evaluation of the mechanical properties can be performed in a static or dynamic way. In addition, the characterization of the mechanical behavior can be done by reaching or not reaching the breaking of the material. For example: elastic moduli, yield strain and stress, maximum stress, etc., are parameters to be characterized without reaching the rupture of the polymer. On the other hand, tensile and deformation at rupture, impact strength, number of life cycles under fatigue, etc., are mechanical properties determined upon the rupture of the polymer. The mobility of a polymer chain determines the physical characteristics of the product whether it is a hard and brittle material, rubbery and tough plastic, or a 9 238