Chem 109H Fall08

It was discovered that Group IV metals, especially titanium, were effective polymerization catalysts for ethylene. Following Ziegler’s successful preparation of linear polyethylene in 1953, Giulio Natta prepared and isolated isotactic (crystalline) polypropylene at the Milan Polytechnic Institute. This was immediately recognized for its practical importance. Ziegler and Natta shared the Nobel Prize in Chemistry in 1963.

A Ziegler-Natta catalyst is composed of at least two parts: a transition metal component and a main group metal alkyl compound. The transition metal component is usually either titanium or vanadium. The main group metal alkyl compound is usually an aluminum alkyl. In common practice, the titanium component is called "the catalyst’ and the aluminum alkyl is called "the co-catalyst".

In some instances, especially for catalyzing the polymerization of propylene, a third component is used. This component is used to control stereoregularity

Addition polymers are usually made from molecules that have the following general structure: Different W, X, Y, and Z groups distinguish one addition polymer from another.

In the first stage, a substance is split into two identical parts, each with an unpaired electron. (Peroxides, which contain an O-O bond, are often used in this role.) A molecule with an unpaired electron is called a free radical. The free radical then initiates the reaction sequence by forming a bond to one of the carbon atoms in the double bond of the monomer. One electron for this new bond comes from the free radical, and the second electron for the new bond comes from one of the two bonds between the carbon atoms. The remaining electron from the broken bond shifts to the carbon atom on the far side of the molecule, away from the newly formed bond, forming a new free radical. Each half-headed arrow indicates the shift of one electron.

The chain begins to grow--propagate, stage two--when the new free radical formed in the initiation stage reacts with another monomer to add two more carbon atoms. This process repeats over and over again

Pyrimidine Bases Cytosine and thymine are pyrimidines. The 6 stoms (4 carbon, 2 nitrogen) are numbered 1-6. Like purines, all pyrimidine ring atoms lie in the same plane. Structure of C and T

DNA is a polymer. The monomer units of DNA are nucleotides, and the polymer is known as a "polynucleotide." Each nucleotide consists of a 5-carbon sugar (deoxyribose), a nitrogen containing base attached to the sugar, and a phosphate group. There are four different types of nucleotides found in DNA, differing only in the nitrogenous base.

Adenine and guanine are purines. Purines are the larger of the two types of bases found in DNA. Structures are shown below: Structure of A and G

Chemistry : the molecular science

8.8 Formal Charge

Note that the edges of these cells all connect lattice points in the structure, points with the same environments.

Atomic Radius

Electronegativity

Ionic Radius vs. Atomic Radius

Slater's Rules

LIGHT: ITS SECRETS REVEALED

Waves can take many forms, but there are two fundamental types of waves: “longitudinal” and “transverse” (see Figures 1 and 2).

Nylon was developed as a synthetic substitute for silk.

Silk is a protein. Like all proteins, it is a polypeptide, and it has a structure something like this:

It will produce a loud sound when a resonant frequency is reached.

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Starting with a structure indicating only atom connections

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