molecular weight is different for polymers than it is for small molecules.

Consistency Let's think about a small molecule, say, hexane. –Hexane has a molecular weight of 86. Every hexane molecule has a molecular weight of 86. –Now if we add another carbon to our chain, and the appropriate amount of hydrogen atoms, we've increased our molecular weight to 100.

That's fine, but the molecule is no longer hexane. –It's heptane! If we have a mixture of some molecules of hexane and some of heptane, –the mixture won't act like pure heptane, nor will it act like pure hexane. –The properties of the mixture, say its boiling point, vapor pressure, etc., will be neither those of pure hexane nor pure heptane.

But polymers are different. Imagine polyethylene. If we have a sample of polyethylene, –and some of the chains have fifty thousand carbon atoms in them, and others have fifty thousand and two carbon atoms in them, –this little difference isn't going to amount to anything. If you really want to know the truth, one almost never finds a sample of a synthetic polymer in which all the chains have the same molecular weight. –Instead, we usually have a bell curve, a distribution of molecular weights. Some of the polymer chains will be much larger than all the others, at the high end of the curve. Some will be much smaller, and at the low end of the curve. The largest number will usually be clumped around a central point, the highest point on the curve. So we have to talk about average molecular weights when we talk about polymers. And we're not going to stop there. –The average can be calculated in different ways, and – each way has its own value.

The Number Average Molecular Weight, M n –It is just the total weight of all the polymer molecules in a sample, divided by the total number of polymer molecules in a sample. The Weight Average Molecular Weight, M w –The weight average is a little more complicated. –It's based on the fact that a bigger molecule contains more of the total mass of the polymer sample than the smaller molecules do.

Demography

The Plot Thickens: –Viscosity Average Molecular Weight, Mv Molecular weight can also be calculated from the viscosity of a polymer solution. The principle is a simple one: –Bigger polymers molecules make a solution more viscous than small ones do. the molecular weight obtained by measuring the viscosity is a different from either the number average or the weight average molecular weight. –But it's closer to the weight average than the number average..

Distribution With all these different molecular weights out there, things can get a little confusing. No single one of them tells the whole story. –So it's usually best to try to know the molecular weight distribution. –The distribution is a plot, like the one in the picture. It plots molecular weight on the x- axis, and plots the amount of polymer at a given molecular weight on the y-axis.

Molecular weight (I) W i is weight fraction of chains of length i x i is number fraction of chains of length i å = i i n M x M å = i i w M w M

Renegade Distributions If we lived in a perfect world, where molecular distributions were always so nice and bell shaped, just knowing the averages might be enough. But they aren't always like that. Sometimes they are like this: This kind of distribution can result from something called a Tromsdorff effect, which we find in free radical vinyl polymerization. Sometimes the distribution is even nastier, like this:free radical vinyl polymerization

Here our number average molecular weight is a complete lie! There isn't a single molecule of that weight in the whole sample! Cases like these illustrate the need to know the complete distribution. The distribution can be given by a technique called size exclusion chromatography, and also by a new method called MALDI mass spectrometry. size exclusion chromatographyMALDI mass spectrometry

Molecular shape The angle between the singly bonded carbon atoms is~109 O – carbon atoms form a zigzag pattern in a polymer molecule.

Moreover, while maintaining the 109 O angle between bonds polymer chains can rotate around single C-C bonds (double and triple bonds are very rigid).

Molecular shape Molecular chains may thus bend, coil and kink Neighboring chains may intertwine and entangle Large elastic extensions of rubbers correspond to unraveling of these coiled chains Mechanical / thermal characteristics depend on the ability of chain segments to rotate

Molecular structure The physical characteristics of polymer material depend not only on molecular weight and shape, but also on molecular structure: Linear polymers: Van der Waals bonding between chains. Examples: polyethylene, nylon. Branched polymers: Chain packing efficiency is reduced compared to linear polymers - lower density

Molecular structure Cross-linked polymers: Chains are connected by covalent bonds. Often achieved by adding atoms ormolecules that form covalent links between chains. Many rubbers have this structure. Network polymers: 3D networks made from trifunctional mers. Examples: epoxies, phenolformaldehyde

Isomerism Isomerism: Hydrocarbon compounds with same composition may have different atomic arrangements. Physical properties may depend on isomeric state (e.g. boiling temperature of normal butane is -0.5 O C, of isobutane O C) Two types of isomerism are possible: stereoisomerism and geometrical isomerism Butane - C 4 H 10 - Isobutane

Geometrical isomerism Geometrical isomerism: consider two carbon atoms bonded by a double bond in a chain. H atom or radical R bonded to these two atoms can be on the same side of the chain (cis structure) or on opposite sides of the chain (trans structure). Cis-polyisopreneTrans-polyisoprene

I suppose that if we're going to have a page about something called "tacticity", it might be a good idea to let you the netsurfer know just what tacticity is. Tacticity is simply the way pendant groups are arranged along the backbone chain of a polymer. We talk about tacticity a lot when dealing with vinyl polymers. To illustrate tacticity, we're going to use one of those vinyl polymers, our good friend polystyrene. pendant groupsvinyl polymers polystyrene

Now polystyrene is a lot of times drawn as a flat picture like this: But polymers aren't really flat like that. The carbon atoms aren't really in a straight line, like that, nor are the hydrogens and phenyl groups all placed at perfect right angles. The carbon chain is more of a zigzag like this:

The pendant groups tend to be point away from the chain, like this: In that picture you see all the phenyl groups are located on the same side of the polymer chain. But they don't have to be this way. To illustrate let's look at a chain of polystyrene from above. You can see that the pendant phenyl groups can be either on the right or left side of the chain. If all of the phenyl groups are on the same side of the chain, we say the polymer is isotactic. If the phenyl groups come on alternating sides of the chain, the polymer is said to be syndiotactic. If the phenyl groups are on both sides and right and left follow in no particular order, in a random fashion, than we say the polymer is atactic.

What does this have to do with anything, you ask? A lot! You see, when polymers have a regular arrangement of their atoms, like we see in the isotactic and syndiotactic polystyrene, it is very easy for them to pack together into crystals and fibers. But if there is no order, as is the case with the atactic polystyrene, packing can't occur. This is because molecules pack best with other molecules of the same shape. Try packing a box full of identical objects, and different objects, and you'll get the idea.crystals fibers

Ziegler and Natta Come to the Rescue This can be a big problem in some polymers. –Tacticity makes a big difference in polystyrene. Free radical vinyl polymerization normally can only produce atactic polymers.Free radical vinyl polymerization –Atactic polystyrene is a hard plastic, and completely amorphous. –It can't crystallize at all. Then metallocene catalysis vinyl polymerization was invented and with it syndiotactic polystyrene became possible.metallocene catalysis vinyl polymerization It is not only crystalline, but it doesn't melt until 270 oC. Another vinyl polymer, polypropylene is a good example of the effects of tacticity. At first, there was only atactic polypropylene. It is kind of soft and sticky, not very strong, and not much good for anything.polypropylene Then along came two scientists named Robert L. Banks and J. Paul Hogan. –They invented a new type of vinyl polymerization which ended up being named Ziegler-Natta polymerization. Ziegler-Natta polymerization –This new process could make isotactic polypropylene. –This new polypropylene could crystallize, and could be used to make fibers, for things like indoor-outdoor carpeting.