POLYMERSPOLYMERS

http://www.cem.msu.edu/~reusch/OrgPage/VirtualText/polymers.htm#polmr2b

plastic videos: http://www.tvo.org/iqm/plastic/video.html#

What is a polymer?

IntroductionWhile we take them for granted, prior to the early 1920's, chemists doubted the existence of molecules with molecular weights greater than a few hundred. This limiting view was challenged by Hermann Staudinger, a German chemist with experience in studying natural compounds such as rubber and cellulose.

In contrast to the prevailing rationalization of these substances as aggregates of small molecules, he proposed that they were macromolecules of 10,000 or more atoms. He formulated a polymeric structure for rubber, based on a repeating isoprene unit (referred to as a monomer). For his contributions to chemistry, Staudinger received the 1953 Nobel Prize.

Recognition that polymeric macromolecules make up many important natural materials was followed by the creation of synthetic analogs having a variety of properties.

The terms polymer and monomer were derived from the Greek roots poly (many), mono (one) and meros (part).

Applications of these materials as fibers, flexible films, adhesives, paints and tough but light solids have transformed modern society.

Writing Formulas for Polymeric MacromoleculesThe repeating structural unit of simple polymers reflects the monomer(s) structure. However, most polymers are not composed of identical molecules.

For linear high-density polyethylene (HDPE), ethylene is the monomer. The polymer is called polyethylene rather than polymethylene, (-CH2-)n, because ethylene was the actual monomer. This polymer is composed of macromolecules in which n (degree of polymerization, DP) ranges from 10,000 to 100,000 (molecular weight 2*105 to 3*106 ).  

How about in DNA?

Polyethylene molecules are all long carbon chains, but their lengths (DPs) may differ by thousands of monomer units. Because of this, polymer molecular weights are usually given as averages. Two experimentally determined values (by gel permeation chromatography, a process similar to column chromatography) are:

• Mn, the number average molecular weight, calculated from the mole fraction distribution of different sized molecules.• Mw, the weight average molecular weight, calculated from the weight fraction distribution of different sized molecules. Since larger molecules weigh more than smaller molecules, Mw is skewed to higher values, and Mw > Mn. The Mw / Mn ratio is called polydispersity. If all polymer molecules have identical weights (monodisperse sample), the ratio Mw / Mn = 1 (DNA). Typically Mw/Mn > 2.

Polymerizations occurs via either chain growth or step growth mechanism.

In chain growth (addition) an active chain propagates via continuous monomer addition (like a zipper).

http://www.youtube.com/watch?v=i3QTlshVSXo&feature=related

Radical Polymerization of Vinyl Chloride

Polymerization occurs via either chain growth or step growth mechanism.

In step growth (polycondensation) oligomers of various sizes grow simultaneously. They all have the same reactivity and propagate via elimination of a small molecule (H2O, HCl, etc)

Synthesis of Addition (chain growth) PolymersMost addition polymers are derived from substituted alkenes. The most common and thermodynamically favored chemical transformations of alkenes are addition reactions and proceed via reactive intermediates. Since a -bond in the monomer is converted to a -bond in the polymer, the polymerization reaction is usually exothermic by 8 to 20 kcal/mol. Explosively, uncontrolled polymerizations can happen. http://www.youtube.com/watch?v=w9uFymAPIu4

There are four chain growth polymerization mechanisms: •  Radical. The initiator is a radical, and the propagating site is a carbon radical. •  Cationic. The initiator is an acid, and the propagating site is a carbocation. •  Anionic. The initiator is a nucleophile, and the propagating site is a carbanion. • Coordination. The initiator is a transition metal complex, and the propagating site of is also attached to a metal complex (remember Grignard?).

Radical Chain-Growth PolymerizationMost vinyl monomers are subject to radical polymerization. Since this can be initiated by traces of oxygen or other minor impurities, monomers are often stabilized by small amounts of radical inhibitors to avoid unwanted reaction. A radical polymerization is started by using a radical initiator, such as a peroxide or certain azo compounds.

For example, the radical polymerization of styrene to polystyrene (lab): The first two equations illustrate the initiation, and the last two, the chain propagation. Each monomer unit adds to the growing chain in a manner that generates the most stable radical. Since carbon radicals are stabilized by most substituents, there is a preference for head-to-tail regioselectivity.

In principle, a radical polymerization could produce just a few very long chains. In practice, a large number of medium size chains are formed, because of chain-transfer and termination reactions. The most common termination processes are recombination and disproportionation. In both, two reactive radical sites are removed simultaneously. Since the concentration of radicals is small, the rate of termination is also very small. Therefore, most growing chains achieve moderate length before termination. The relative importance of the termination depends on the monomer. For acrylonitrile and styrene recombination is dominant while methyl methacrylate and vinyl acetate are terminated by disproportionation.

Another side reaction is the chain transfer of a radical from one location to another by an intermolecular or intramolecular hydrogen atom transfer.

Chain transfer reactions are especially prevalent in the high pressure radical polymerization of ethylene, the method used to make LDPE (low density polyethylene). The 1°-radical at the end of a growing chain is converted to a more stable 2°-radical by H atom transfer. Further polymerization at the new radical site generates a side-chain radical, and this may in turn lead to creation of other side chains by chain transfer reactions. As a result, the morphology of LDPE is an amorphous network of highly branched macromolecules.

Ring opening polymerization is another example of chain-growth

what is the mechanism?

Name(s) Formula Monomer Properties Uses Polyethylene-branchedlow density (LDPE)

- (CH2-CH2)n-ethyleneCH2=CH2

soft,waxy solid film wrap, plastic bagscandles

Polyethylene-linearhigh density (HDPE)

- (CH2-CH2)n-ethyleneCH2=CH2

rigid, translucent solid electrical insulationbottles, toys

Polypropylene(PP) different grades

-[C-H2-CH(CH3)]n-

propyleneCH2=CHCH3

atactic: soft,elastic solidisotactic: hard, strong solidhttp://www.youtube.com/watch?v=m3X3BRes__8&feature=related

similar to LDPEcarpet, upholstery

Poly(vinyl chloride)(PVC)

- (CH2-CHCl)n-vinyl chlorideCH2=CHCl strong rigid solid pipes, siding, flooring

Poly(vinylidene chloride)(Saran A)

- (CH2-CCl2)n-vinylidene chlorideCH2=CCl2

dense, high-melting solid seat covers, films

Polystyrene(PS)

- [CH2-CH(C6H5)]n-styreneCH2=CHC6H5

hard, rigid, clear solidsoluble in organic solvents

toys, cabinetspackaging (foamed)

Polyacrylonitrile(PAN, Orlon, Acrilan)

- (CH2-CHCN)n-acrylonitrileCH2=CHCN

high-melting solidsoluble in organic solvents

rugs, blanketsclothing

Polytetrafluoroethylene(PTFE, Teflon)

- (CF2-CF2)n-tetrafluoroethyleneCF2=CF2

resistant, smooth solid non-stick surfaceselectrical insulation

Poly(methyl methacrylate)(PMMA, Lucite, Plexiglas)

- [CH2-C(CH3)CO2CH3]n-methyl methacrylateCH2=C(CH3)CO2CH3

hard, transparent solid lighting covers, signsskylights

Poly(vinyl acetate)(PVAc)

- (CH2-CHOCOCH3)n-vinyl acetateCH2=CHOCOCH3

soft,sticky solid latex paints, adhesives

cis-Polyisoprenenatural rubber

- [CH2-CH=C(CH3)-CH2]n-isopreneCH2=CH-C(CH3)=CH2

soft,sticky solid requires vulcanizationfor practical use

Polychloroprene (cis/trans)(Neoprene)

- [CH2-CH=CCl-CH2]n-chloropreneCH2=CH-CCl=CH2

tough, rubbery solid synthetic rubberoil resistant

Many polymeric materials with chain-like structures similar to polyethylene are known. Polymers formed by linking together monomer units, with no loss of material, are called addition or chain-growth polymers.

• Step-growth (condensation) polymerizations proceed by functional group transformations. They often occur with loss of a small byproduct, such as water, and frequently combine two different components in an alternating structure. • In contrast to chain-growth polymers, most of which grow by C-C bond formation, step-growth polymers generally grow by C-heteroatom bond formation (C-O & C-N in Dacron & Nylon below).

Examples of naturally occurring condensation polymers include cellulose, polypeptide (proteins), and DNA.

protein

• Condensation polymers form slower than addition polymers, often requiring heat, and are generally of lower Mn. The 2 terminal functional groups on a chain remain active, and groups of shorter chains combine into longer chains in the late stages of polymerization.

• Polar functional groups on the chains often enhance chain-chain attractions, and thus crystallinity and tensile strength.

For commercial synthesis, the carboxylic acid components may be employed in the form of derivatives such as esters.

For polyurethanes, the polymerization does not eliminate a small molecule.

Nylon 6 is also made by ring opening (chain growth).

Formula Type ComponentsTg ° C

Tm ° C

~[CO(CH2)4CO-OCH2CH2O]n~

Polyester HO2C-(CH2)4-CO2HHO-CH2CH2-OH 

< 0 50

                                           

PolyesterDacronMylar

para HO2C-C6H4-CO2HHO-CH2CH2-OH 

70 265

                                            

Polyester meta HO2C-C6H4-CO2HHO-CH2CH2-OH 

50 240

                                             

PolycarbonateLexan (CDs)

(HO-C6H4-)2C(CH3)2

  (Bisphenol A)X2C=O  (X = OCH3 or Cl)

150 267

~[CO(CH2)4CO-NH(CH2)6NH]n~

PolyamideNylon 66

HO2C-(CH2)4-CO2HH2N-(CH2)6-NH2

 45 265

~[CO(CH2)5NH]n~ PolyamideNylon 6Perlon

                53 223

                                            

PolyamideKevlar

para HO2C-C6H4-CO2Hpara H2N-C6H4-NH2

 

--- 500

                                       PolyamideNomex

meta HO2C-C6H4-CO2Hmeta H2N-C6H4-NH2

 

273 390

                                             

PolyurethaneSpandex

HOCH2CH2OH                       

52 ---

Note that the same polymer can be synthesized by both chain (ring opening) and step polymerizations.

The Nylon Rope Trick

http://www.youtube.com/watch?v=7nCfbZwG

http://www.youtube.com/watch?feature=endscreen&v=yFEHKRdXb9Y&NR=1 WK8

Thermosetting vs. Thermoplastic PolymersMost linear polymers are thermo-plastic. Above their glass/softening transition (Tg) (which is a type of melting) they may be shaped or pressed into molds, spun or cast from melts or dissolved in solvents for later fashioning.

Other polymers which are crosslinked resist deformation and are insoluble. They are usually prepared directly in molds that yield the desired object. Since they cannot be reshaped by heating, they are called thermo-sets. Bakelite is one of the first completely synthetic plastics to see commercial use (~1910).      

http://www.youtube.com/watch?v=NpO2DAvMWi4&feature=related

In the lab: radical polymerization of styrene with benzoyl peroxide book, page 753, experiment 69.4.4 4th edition67.4 5th ed., p 770-772 6th ed.

To ReadChapter 69 Pages 757-779 6th; 813-836 5th 739-761 (4th)

Homework: Questions 2 and 4

Final

If anybody is interested in doing research in polymers, please come and see me! ;-)

• FINAL Thursday 05/01/14 5-5:50pm

• Will concentrate primarily on the second semester, but will assume you already know the techniques discussed in the first semester