SUMMARY / RELATED TOPICS

Tacticity

Tacticity is the relative stereochemistry of adjacent chiral centers within a macromolecule. The practical significance of tacticity rests on the effects on the physical properties of the polymer; the regularity of the macromolecular structure influences the degree to which it has rigid, crystalline long range order or flexible, amorphous long range disorder. Precise knowledge of tacticity of a polymer helps understanding at what temperature a polymer melts, how soluble it is in a solvent and its mechanical properties. A tactic macromolecule in the IUPAC definition is a macromolecule in which all the configurational units are identical. Tacticity is significant in vinyl polymers of the type -H2C-CH- where each repeating unit with a substituent R on one side of the polymer backbone is followed by the next repeating unit with the substituent on the same side as the previous one, the other side as the previous one or positioned randomly with respect to the previous one. In a hydrocarbon macromolecule with all carbon atoms making up the backbone in a tetrahedral molecular geometry, the zigzag backbone is in the paper plane with the substituents either sticking out of the paper or retreating into the paper.

This projection is called the Natta projection after Giulio Natta. Monotactic macromolecules have one stereoisomeric atom per repeat unit, ditactic to n-tactic macromolecules have more than one stereoisomeric atom per unit. Two adjacent structural units in a polymer molecule constitute a diad. If the diad consists of two identically oriented units, the diad is called a meso diad reflecting similar features as a meso compound. If the diad consists of units oriented in opposition, the diad is called a racemo diad as in a racemic compound. In the case of vinyl polymer molecules, a meso diad is one in which the book carbon chains are oriented on the same side of the polymer backbone; the stereochemistry of macromolecules can be defined more with the introduction of triads. An isotactic triad is made up of two adjacent meso diads, a syndiotactic triad consists of two adjacent racemo diads and a heterotactic triad is composed of a meso diad adjacent to a racemo diad; the mass fraction of isotactic triads is a common quantitative measure of tacticity.

When the stereochemistry of a macromolecule is considered to be a Bernoulli process, triad composition can be calculated from the probability of finding meso diads. When this probability is 0.25 the probability of finding: an isotactic triad is Pm2 or 0.0625 an heterotactic triad is 2Pm or 0.375 a syndiotactic triad is 2 or 0.5625with a total probability of 1. Similar relationships with diads exist for tetrads; the definition of tetrads and pentads introduce further sophistication and precision to defining tacticity when information on long-range ordering is desirable. Tacticity measurements obtained by Carbon-13 NMR are expressed in terms of the relative abundance of various pentads within the polymer molecule, e.g. mmmm, mrrm. The primary convention for expressing tacticity is in terms of the relative weight fraction of triad or higher-order components, as described above. An alternative expression for tacticity is the average length of meso and racemo sequences within the polymer molecule.

The average meso sequence length may be approximated from the relative abundance of pentads as follows: M S L = m m m m + 3 2 m r r r + 2 r m m r + 1 2 r m r m + 1 2 r m r r 1 2 m m m r + r m m r + 1 2 r m r m + 1 2 r m r r Isotactic polymers are composed of isotactic macromolecules. In isotactic macromolecules all the substituents are located on the same side of the macromolecular backbone. An isotactic macromolecule consists of 100% meso diads. Polypropylene formed by Ziegler–Natta catalysis is an isotactic polymer. Isotactic polymers are semicrystalline and form a helix configuration. In syndiotactic or syntactic macromolecules the substituents have alternate positions along the chain; the macromolecule consists 100% of racemo diads. Syndiotactic polystyrene, made by metallocene catalysis polymerization, is crystalline with a melting point of 161 °C. Gutta percha is an example for Syndiotactic polymer. In atactic macromolecules the substituents are placed randomly along the chain; the percentage of meso diads is between 1 and 99%.

With the aid of spectroscopic techniques such as NMR it is possible to pinpoint the composition of a polymer in terms of the percentages for each triad. Polymers that are formed by free-radical mechanisms such as polyvinyl chloride are atactic. Due to their random nature atactic polymers are amorphous. In he

Heartland Collegiate Athletic Conference

The Heartland Collegiate Athletic Conference is an intercollegiate athletic conference affiliated with the NCAA's Division III. Member institutions are located in Indiana and Ohio. Founded as the Indiana Collegiate Athletic Conference in 1987, it reincorporated under its current name in 1998 with the addition of several schools from Ohio. Original members of the HCAC included Anderson, Franklin, Manchester, Mount St. Joseph and Wilmington. Of the ten current members, six were founding members of the former ICAC. Former members include DePauw, Taylor and Wilmington. Rose–Hulman Institute of Technology re-joined as of July 1, 2006; the Indiana Collegiate Athletic Conference was formed in June 1987, with 1990–91 being the first full season of competition. Charter members in 1987 included Anderson University, DePauw University, Franklin College, Hanover College, Manchester College, Wabash College. Rose-Hulman Institute of Technology and Taylor University joined in 1988; the addition of three Ohio schools and the departure of two Indiana schools during the 1998–99 season prompted a change in name to Heartland Collegiate Athletic Conference.

Wabash and Wilmington departed in the 1998–99 and 1999–00 seasons respectively. Transylvania University joined in 2001. Rose-Hulman re-joined the HCAC for the 2006–07 season; the most recent expansion was when Earlham College of Richmond, Indiana was accepted as the 10th member of the conference in October 2009 to begin competition in the fall of 2010. All of the conference's members throughout its history have been private schools. All have a religious affiliation except former member Wabash. NoteRose–Hulman left the HCAC following the end of the 1997–98 season. NoteWabash left the HCAC for all-sports after the 1998–99 season except for football. Member teams compete in women's basketball, cross country, soccer, tennis and field and volleyball and men's baseball, cross country, golf, soccer and track and field. Official website

Cucurbita ficifolia

Cucurbita ficifolia is a species of squash, grown for its edible seeds and greens. It has many common names in English such as the fig-leaf gourd, Malabar gourd, black seed squash and cidra. Although it is related to other squashes in its genus, such as the pumpkin, it shows considerable biochemical difference from them and does not hybridize with them. Black-seed squash cidra fig-leaf gourd figleaf gourd fig-leaved gourd Malabar gourd pie melon sidra Thai marrow Abóbora-chila or abóbora gila Alcayota Black seeded pumpkin, written as kurodane kabocha, 黒種南瓜 and フィシフォリア Cabell d'Àngel Calabaza de cabello de ángel Cayote Chiberre Chiverre Chilacayote, Chilacayotl or Tzilacayote Courge à choucroute de cheveux d'ange Courge de Siam Lacayote Mboga ya kimasai shark fin melon, written as 鱼翅瓜 in Mandarin Calabaza Zambo Zucca del Siam or Zucca del Malabar Like most members of the genus Cucurbita, C. ficifolia is a climbing vine, an annual in temperate climates and a perennial in tropical zones. Unlike some other Cucurbita species, it does not have swollen storage roots.

The plant stem can grow five to fifteen meters and produces tendrils that help it climb adjacent plants and structures. It may root unlike most other curcubits; the vine can become semiwoody if left to grow perennially, although most commercial plants are annual. Its leaves resemble fig leaves, hence its most common name in English – fig-leaf gourd – and its Latin species name; the fruit is oblong, with wide black seeds. In stark contrast to other Cucurbita, its fruit is uniform in size and color; the plant is monoecious with imperfect flowers and are pollinated by insects bees. The color of the flowers is yellow to orange; the fruit is oblong with a diameter of eight inches or 20 centimeters, weighs eleven to 13 pounds, can produce up to 500 seeds. Its skin can vary from dark green to cream. One plant can produce over 50 fruit; the fruit can last without decomposing for several years. It is native to the Americas. Linguistic evidence suggests Mexico, because of the wide use of names based on the Nahuatl name "chilacayohtli" as far south as Argentina.

However, archaeological evidence suggests Peru. Biosystematics has been unable to confirm either hypothesis. Archeological records show that it was the most widespread variety of Cucurbita in the Americas, cultivated from northern Chile and Argentina to Mexico. Now it is grown as far north as southern California. In the sixteenth and seventeenth centuries, Europeans introduced it to the Mediterranean regions of Europe as well as India. From there it picked up more names; the fig-leaved gourd grows in temperate highlands at elevations up to 2,000 metres. It is used as a grafting rootstock for other less resistant cucurbits. C. ficifolia can be propagated by layering. Tendrils can grow into roots if anchored into the soil, can propagate new plants once cut, which can be moved to new sites; because it is not resistant to frost, it is planted after this risk has passed. Established plants, however can withstand short overnight frosts; the flowers and tender shoots are used in Mexico and other countries as greens.

The most nutritional part of Cucurbita ficifolia is its fat- and protein-rich seeds, which can vary in color from white to black. They are used in Mexico to make a sweet similar to peanut brittle; the fruit has several uses as food. The immature fruit is eaten cooked, while the mature fruit is sweet and used to make confectionery and beverages, sometimes alcoholic; the fruit is low in beta-carotene, as can be seen from its white flesh, is low in vitamins and minerals, moderately high in carbohydrates. In Europe: In Spain this squash is used to make a jam known as "cabello de ángel", "cabell d'àngel" in Catalan, used to fill pies and confectionery. In Portugal, where the fruit is known as "chila" or "gila", it is still used extensively in the production of traditional Portuguese sweets and confectionery. In Latin America: In Chile and Argentina, jam is made out of the fruit of "alcayota" or "cayote". In Costa Rica, it is traditional to make empanadas stuffed with sugared "chiverre" filling at Easter time.

In Asia, the pulp strands are used to make soup, quite similar to shark fin soup, hence the name "shark's fin melon". The cultivation and this usage feature in the film Grow Your Own. Across Asia, eating this melon is said to help people with diabetes. Several scientific studies have confirmed its hypoglycemic effect, it is used to treat diabetes due to its high D-Chiro-Inositol content. The vine and fruit are used for fodder; because of its ability to keep for a long time, the ripe fruit was taken on voyages on ships, used for food for livestock on board. Acosta-Patiño, J. L.. A.. C.. "Hypoglycemic action of Cucurbita ficifolia on Type 2 diabetic patients with moderately high blood glucose levels". Journa