聚合物
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聚合物是通稱一些由非常長的分子组成的化合物，分子之间由結構單位和重覆單位經共價化學鍵連接一起。聚合物與其它化合物不同之處在於他們是由許多相同、相似或互補的亞單位重複所組成。這些亞單位或稱單體是一些低至中等分子質量的分子，他們經聚合作用的化學反應便可組成聚合物。
相似的單體由於有不同的取代基，所以不會是完全相同的。由不同的單體組成的聚合物會有不同的特性例如：溶解度、彈性和強度。譬如在蛋白質裡、這些差異使聚合物能形成其獨特的生物活躍構象(參見自我組裝)。當中相同單體如有非活潑的支鏈會導致聚合物鏈形成一無規線團——即根據數學模型所描述的理想鏈。雖然多數聚合物是有機化合物，並由碳基之單體所組成，但是也有無機聚合物例如，矽橡膠(又稱硅樹脂)是由交替的硅和氧原子所組成。
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[编辑] 聚合物命名法
Polymers are typically classified according to four main groups:
The term polymer covers a large, diverse group of molecules, including substances from proteins to stiff, high-strength Kevlar fibres. For example, the formation of polyethene (also called polyethylene) involves thousands of ethene molecules bonded together to form a straight (or branched) chain of repeating -CH2-CH2- units (with a -CH3 at each terminal):
Polymers are often named in terms of the monomer from which they are made. Because it is synthesized from ethene in a process during which all the double bonds in the vinyl monomers are lost, polyethene has the unsaturated structure:
If it were named according to its final structure, it would have the alkane designation "polyethane".
Because synthetic polymer formation is governed by random assembly from the constituent monomers, polymer chains within a solution or substance are generally not of equal length. This is unlike basic, smaller molecules in which every atom is stoichiometrically accounted for, and each molecule has a set molecular mass. An ensemble of differing chain lengths, often obeying a normal (Gaussian) distribution, occurs because polymer chains terminate during polymerization after random amounts of chain lengthening (propagation).
蛋白質是胺基酸的聚合物。 Typically, hundreds of the (nominally) twenty different amino acid monomers make up a protein chain, and the sequence of monomers determines its shape and biological function. (There are also shorter oligopeptides which function as hormones.) But there are active regions, surrounded by, as is believed now (Aug 2003), structural regions, whose sole role is to expose the active regions. (There may be more than one on a given protein.) So the exact sequence of amino acids in certain parts of the chains can vary from species to species, and even given mutations within a species, so long as the active sites are properly accessible. Also, whereas the formation of polyethylene occurs spontaneously under the right conditions, the synthesis of biopolymers such as proteins and nucleic acids requires the help of enzyme catalysts, substances that facilitate and accelerate reactions. Unlike synthetic polymers, these biopolymers have exact sequences and lengths. (This does not include the carbohydrates.) Since the 1950s, catalysts have also revolutionised the development of synthetic polymers. By allowing more careful control over polymerization reactions, polymers with new properties, such as the ability to emit coloured light, have been manufactured.
[编辑] 聚合物的物理性质
聚合物的物理性质包括聚合度，分子量分布，结晶度和相转变等，详见高分子物理学。
[编辑] Branching
See also: Branching (chemistry)
During the propagation of polymer chains, branching can occur. In free-radical polymerization, this occurs when a chain curls back and bonds to an earlier part of the chain. When this curl breaks, it leaves small chains sprouting from the main carbon backbone. Branched carbon chains cannot line up as close to each other as unbranched chains can. This causes less contact between atoms of different chains, and fewer opportunities for induced or permanent dipoles to occur. A low density results from the chains being further apart. Lower melting points and tensile strengths are evident, because the intermolecular bonds are weaker and require less energy to break.
Besides branching, polymers can have other topologies: linear, network (cross-linked 3D structure), IPN (integrated polymer network), comb, or star as well as dendrimer and hyperbranched structures.
[编辑] Stereoregularity
Stereoregularity or tacticity describes the isomeric arrangement of functional groups on the backbone of carbon chains. Isotactic chains are defined as having substituent groups aligned in one direction. This enables them to line up close to each other, creating crystalline areas and resulting in highly rigid polymers.
In contrast, atactic chains have randomly aligned substituent groups. The chains do not fit together well and the intermolecular forces are low. This leads to a low density and tensile strength, but a high degree of flexibility.
Syndiotactic substituent groups alternate regularly in opposite directions. Because of this regularity, syndiotactic chains can position themselves close to each other, though not as close as isotactic polymers. Syndiotactic polymers have better impact strength than isotactic polymers because of the higher flexibility resulting from their weaker intermolecular forces.
[编辑] Constitution of polymers
[编辑] Copolymers
Copolymerization with two or more different monomers results in chains with varied properties. There are twenty amino acid monomers whose sequence results in different shapes and functions of protein chains. Copolymerising ethene with small amounts of 1-hexene (or 4-methyl-1-pentene) is one way to form linear low-density polyethene (LLDPE). (See polyethylene.) The C4 branches resulting from the hexene lower the density and prevent large crystalline regions from forming within the polymer, as they do in HDPE. This means that LLDPE can withstand strong tearing forces while maintaining flexibility.
A block copolymer is formed when the reaction is carried out in a stepwise manner, leading to a structure with long sequences or blocks of one monomer alternating with long sequences of the other. There are also graft copolymers, in which entire chains of one kind (e.g., polystyrene) are made to grow out of the sides of chains of another kind (e.g., polybutadiene), resulting in a product that is less brittle and more impact-resistant. Thus, block and graft copolymers can combine the useful properties of both constituents and often behave as quasi-two-phase systems.
The following is an example of step-growth polymerization, or condensation polymerization, in which a molecule of water is given off and nylon is formed. The properties of the nylon are determined by the R and R' groups in the monomers used.
The first commercially successful, completely synthetic polymer was nylon 6,6, with alkane chains R = 4C (adipic acid) and R' = 6C (hexamethylene diamine). Including the two carboxyl carbons, each monomer donates 6 carbons; hence the name. In naming nylons, the number of carbons from the diamine is given first and the number from the diacid second. Kevlar is an aromatic nylon in which both R and R' are benzene rings.
Copolymers illustrate the point that the repeating unit in a polymer, such as a nylon, polyester or polyurethane, is often made up of two (or more) monomers.
[编辑] Chemical properties of polymers
[编辑] Intermolecular forces
The attractive forces between polymer chains play a large part in determining a polymer's properties. Because polymer chains are so long, these interchain forces are amplified far beyond the attractions between conventional molecules. Also, longer chains are more amorphous (randomly oriented). Polymers can be visualised as tangled spaghetti chains - pulling any one spaghetti strand out is a lot harder the more tangled the chains are. These stronger forces typically result in high tensile strength and melting points.
The intermolecular forces in polymers are determined by dipoles in the monomer units. Polymers containing amide groups can form hydrogen bonds between adjacent chains; the positive hydrogen atoms in N-H groups of one chain are strongly attracted to the oxygen atoms in C=O groups on another. These strong hydrogen bonds result in, for example, the high tensile strength and melting point of kevlar. Polyesters have dipole-dipole bonding between the oxygen atoms in C=O groups and the hydrogen atoms in H-C groups. Dipole bonding is not as strong as hydrogen bonding, so ethene's melting point and strength are lower than Kevlar's, but polyesters have greater flexibility.
Ethene, however, has no permanent dipole. The attractive forces between polyethene chains arise from weak van der Waals forces. Molecules can be thought of as being surrounded by a cloud of negative electrons. As two polymer chains approach, their electron clouds repel one another. This has the effect of lowering the electron density on one side of a polymer chain, creating a slight positive dipole on this side. This charge is enough to actually attract the second polymer chain. Van der Waals forces are quite weak, however, so polyethene melts at low temperatures.
[编辑] Polymer characterization
The characterization of a polymer requires several parameters which need to be specified. This is because a polymer actually consists of a statistical distribution of chains of varying lengths, and each chain consists of monomer residues which affect its properties.
A variety of lab techniques are used to determine the properties of polymers. Techniques such as wide angle X-ray scattering, small angle X-ray scattering, and small angle neutron scattering are used to determine the crystalline structure of polymers. Gel permeation chromatography is used to determine the number average molecular weight, weight average molecular weight, and polydispersity. FTIR is used to determine composition. Thermal properties such as the glass transition temperature and melting point can be determined by differential scanning calorimetry and dynamic mechanical analysis. Pyrolysis followed by analysis of the fragments is one more technique for determining the possible structure of the polymer.
Polymer known as polymer substrate is used for everyday banknotes in Australia, Romania, Papua New Guinea, Samoa, Zambia, Vietnam, New Zealand and a few others, and the material is also used in commemorative notes in some other countries. The process of polymer substrate creation was developed by the Australia CSIRO.
[编辑] See also
- 高分子化學
- 聚合
- Biopolymer
- Condensation polymer
- Addition polymer
- Synthetic polymer
- Glass transition temperature
- 高分子物理學
- Important publications in polymer chemistry
