POLYMER CHEMISTRY
 
A THUMBNAIL TUTORIAL FOR HIGH SCHOOL OR EARLY COLLEGE STUDENTS)
By Skip Thacker

What is a “Polymer”?----- Repeating units, as LONG CHAINS, (straight chained or branched chained) of one or more types of MONOMER molecules.
Poly (many) mers (units),<--------Mono (one) mer (unit)
The carbon atom, tetrahedronal with four bonding sites, is unique in its ability to form polymeric structures. Single-bonded carbons are free to rotate about their single bond, which provides flexibility and elasticity to organic (carbon-based) polymers. Double bonds, or “cross-linking” ladder-type structures, whether carbon-carbon bonded or carbon-oxygen- bonded, are rigid non-elastic structures. The former are called THERMOPLASTIC, and the latter are called THERMOSETS.

BASIC TYPES OF POLYMERS
:
I- Natural
Cellulose, Proteins, Polysaccharides, Starches
(examples: wood, egg whites, sugars, gelatins)

II- Synthetic (Plastics)
Classes A)- THERMOPLASTICS (can be remelted and processed again):
Examples:
Polyolefins: LDPE, HDPE, PP,
Vinyls: PVC, CPVC, PVA, EVA
Styrenics: PS, HIPS, ABS
Nylons: Polyamides
Acrylics: PMMA(Lucite), MBS
Thermoplastic Elastomers: TPR, TPE,TPU, etc.
High-Temp. Engineering Polymers: PC, PBT, PEEK, Polysulfone
Thermoplastic Polyesters: PET, Dacron, Mylar.

B)- THERMOSETS (crossed-linked, cannot be remelted and used again)
Examples:
Rubber
Phenolics
Unsaturated Polyesters (with Fiberglass reinforcement)
Epoxies
Urethanes
 
 
 
TYPES OF POLYMERS BY METHOD OF POLYMERIZATION (hooking monomers together):
A)- Addition Polymerization: Typically opening up double bonds (-C=C-) by free radical (peroxide) catalysis.
Examples:
Ethylene--- CH2=CH2 ------->~~CH2-CH2-CH2-CH2~~ Polyethylene (PE)
Propylene--- CH2=CH --------> ~~CH2-CH-CH2-CH-~~ Polypropylene* (PP)
                            /                           /           /
                          CH3                      CH3      CH3
*- Stereospecific polymerization, isotactic, syndiotactic, or atactic can occur depending on type of catalysis. This relates to the “conformation” of methyl groups along the polymer main chain--- same side, alternating, or random alignments.
Vinyl Chloride --- CH2=CH------->~~CH2-CH-CH2-CH~~ Polyvinylchloride(PVC)
                                  /                        /           /
                                Cl                      Cl         Cl
Styrene--- CH2=CH------------>~~CH2-CH-CH2-CH-~~ Polystyrene (PS)
                       /                             /            /
                Benzene                Benzene    Benzene
                  Ring                       Ring          Ring
                                                                                  /OOCH3
Methyl Methacrylate--- CH2=C-C-OCH3--------> ~~(CH2-C-)x~~ Acrylic (PMMA)
                                          /  //                                 /
                                     CH3 O                                CH3
B)- Condensation Polymerization: Typically esterification, with water elimination.
Examples:
Dibasic acid               +   Dihydric alcohol ------------> Polyester                                        + H2O
HO-C-[benzene]-C-OH + HOCH2CH2OH ---> ~~(OCH2CH2-OC-[benzene]-C-OCH2CH2)~~
      II                  II                                                               II                      II
      O                 O                                                              O                     O
terephthalic acid     +   ethylene glycol   ----------->  polyethylene terethphalate (MYLAR DACRON,PET)
 
di-isocyanates + glycols ---------> polyurethanes + H2O
Copolymers/Terpolymers-------- 2-3 co-monomers reacted together.
Examples: PVC/Vinyl acetate, EVA, ABS
Blends/ Alloys---------Compatible polymer blends to modify properties such as impact, tensile, heat distortion, elongation, etc.
Examples: Impact modified rigid PVC(with acrylic, MBS, ABS, etc.)
Plasticization--------- Addition of liquid or solid materials to promote flexibility, softness, elongation.
Examples: Plasticized PVC, PVC/EVA blends
 
Most polymers are COMPOUNDED with some combination of additives (fillers, plasticizers, heat & UV stabilizers, lubricants, process aids, colorants, foaming agents, etc.) depending on processing and end-product needs. These compounds are supplied as either pellets or (PVC mainly) powder blends.
COMPOUNDED polymers are processed to finished shapes by extrusion, injection molding, blow molding,calendering, coating-molding-casting of liquid systems-----by heating/melting & cooling of thermoplastics, or by exothermic reaction (cross-linking) of thermosets.

Commercial Importance of the Polymer Industry:
Estimated World Capacities (1997)- Large Volume Thermoplastic Polymers
LDPE------------------64 billion lbs.}________PE’s--------------- 98 billion lbs.
HDPE-----------------34 billion lbs. }
PP-----------------------------------------------------------------------------56 billion lbs.
PVC --------------------------------------------------------------------------65 billion lbs.
PS-----------------------------------------------------------------------------23 billion lbs.
PET-----------------------------------------------------------------------------8 billion lbs.
                                                                                250 billion lbs.
Most polymers (or plastics) are derived from petroleum feedstocks.
Probably 90+% of all petroleum is burned as fuel-- gasoline, heating oil, etc., and some is used for lubrication (oils, greases). About 5% is used in petrochemicals (plastics, pharmaceuticals, solvents,etc.), and of this 5%, 60%(or only 3% of total petroleum) is used to produce plastics!