Plastics Classification - Chemical Classifications

Types:
 
    So, in the continuing discussion of how we classify polymers, we now need to talk about their chemical classification.  For the purposes of this blog and for the time being I will restrict the discussion of polymer chemistry to thermoplastics, and those most typically used in injection molding; although many of these are also made in specific grades meant for other processes such as Thermoforming, Extrusion, Rotational Molding, etc.
 
    Polymers, being organic compounds created via reactions of other organic monomers and reactants with 2 or more active sites on the molecule, are classified by the chemical groups that make up the repeating segments of the polymer chain.

    The following is a list of polymer chemical classes:
1) Polyolefins
2) Polystyrenes
3) Polyesters
4) Polyethers
5) Polyamides
6) Polyimides
7) Poly-Aldehydes (i.e. Polyoxymethylene - polymerized formaldehyde)
8) Poly-Acrylates


The next several blog posts will be devoted to discussing these and other various polymer chemical groups, the type of polymerization reaction involved in the making of those specific polymers, typical uses and properties for those polymers.


Common Polymerization Reactions:

Polymerization reactions include the following common processes as well as others: Linear Step, Non-Linear Step, Free Radical, Ionic, Cationic, Anionic Polymerization.

Step Polymerization - The combining of a monomer as a whole unit into chains, step-by-step.
 
    Linear step polymerization (i.e. condensation polymerization) is where a monomer will maintain it's structure but become chemically linked into chains, covalently bonded, of varying length.  Polyethylene is one example of this.  Ethane is catalyzed and the molecules link together to form longer hydrocarbon chains.  In this way, octane, heptane, decane, and other hydrocarbons might be looked at as being linked in family with polyethylene.  The term "linear" means that it will yield a thermoplastic material as there 2 and only 2 active sites on the monomer chain.

    Non-Linear Step Polymerization is essentially just like the linear except that there are 3 or more active sites on the monomer and thus a thermoset is formed.  The pint where the first side chain is formed is referenced as the "gel point."  Essentially, these lead to network polymers.

 Living  Polymerization -A Living polymer is one where there is no termination to the polymerization reaction.  These polymers can "self-heal" but the trade-off is that they're not safe for food or drug applications.

Free Radical Polymerization - The monomer has Pi-bonds (double and/or triple bonds) that are broken via the introduction of a source of free radicals.  This then opens the active sites and allows for the formation of the polymer chain and with its creation, the free radicals are recovered like a catalyst.  Typically, free radicals are introduced in the form of peroxides, and can be excited (activated) by heat or UV light.  Polystyrene is the most common example of free radical polymerization.


Ionic Polymerization - Chain polymerization that uses active centers with an ionic charge.  This is done with olyfinic monomers.  There are two types:

    Cationic Polymerization - The active centers are positively charged.  It has a defined termination process.

    Anionic Polymerization - The active centers are negatively charged.  Lacks a termination process. Instead, there is a chain transfer stage.  Polystyrene formed in liquid ammonia was one of the first applications for this process.

Ring Opening Polymerization - Polymers with a structure that looks like:  -[R-Z]n-  (the linking group is Z) can be polymerized by step polymerization.   In the case of a cyclic monomer, the ring can be broken in order to facilitate polymerization.  The primary mechanism that drives this is either the relief of bond angle strain or steric repulsions.
   
Solid State Polymerization - Polymerization of a monomer that exists in the solid and crystals are formed by adding heat or ultraviolet radiation.  These tend to yield highly oriented polymers.


Group Transfer Polymerization - Generally used for acrylic and methacrylic monomers (i.e. methacrylates - PMMA), this type of polymerization propagates by way of reacting a terminal silyl ketene acetal with some monomer by addition thus transferring the group to the monomer and creating a new terminal silyl ketene acetal group on the growing polymer.  It is initiated with monomers containing 2 or more silyl ketene acetal sites, and is catalysed by anions.



Homopolymers versus Copolymers (and Terpolymers):

    The term homopolymer refers to a polymer comprised of a single repeating monomer.  Copolymers have two different monomer links that are connected, perhaps alternating, along the polymer chain.  Examples of co-polymers include Styrene-Acrylonitrile, Butadiene-Styrene (aka High Impact Polystyrene or HIPS).

    Terpolymers are, as one might now imagine, polymers comprised of three monomer units that alternate along the polymer chain.  The most common example of a terpolymer is Acrylonitrile-Butadiene-Styrene (ABS - It's NOT the break system in your car).