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What are polymers? For one thing, they are complex and giant molecules and are different from low molecular weight compounds like, say, common salt. To contrast the difference, the molecular weight of common salt is only 58.5, while that of a polymer can be as high as several hundred thousand, even more than thousand thousands. These big molecules or ¡®macro-molecules¡¯ are made up of much smaller molecules, can be of one or more chemical compounds. To illustrate, imagine that a set of rings has the same size and is made of the same material. When these things are interlinked, the chain formed can be considered as representing a polymer from molecules of the same compound. Alternatively, individual rings could be of different sizes and materials, and interlinked to represent a polymer from molecules of different compounds.

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butadiene + butadiene + ??? + butadiene--¡úpolybutadiene (4 000 time)

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One can thus see how a substance (monomer) with as small a molecule weight as 54 grow to become a giant molecule (polymer) of (54¡Á4 000¡Ö)200 000 molecular weight.

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It is essentially the ¡®giantness¡¯ of the size of the polymer molecule that makes its behavior different from that of a commonly known chemical compound such as benzene. Solid benzene, for instance, melts to become liquid benzene at 5.5¡æ and , on further heating, boils into gaseous benzene. As against this well-defined behavior of a simple chemical compound, a polymer like polyethylene does not melt sharply at one particular temperature into clean liquid. Instead, it becomes increasingly softer and, ultimately, turns into a very viscous, tacky molten mass. Further heating of this hot, viscous, molten polymer does convert it into various gases but it is no longer polyethylene. (Fig. 1.1) .

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Another striking difference with respect to the behavior of a polymer and that of a low molecular weight compound concerns the dissolution process. Let us take, for example, sodium chloride and add it slowly to s fixed quantity of water. The salt, which represents a low molecular weight compound, dissolves in water up to s point (called saturation point) but, thereafter, any further quantity added does not go into solution but settles at the bottom and just remains there as solid. The viscosity of the saturated salt solution is not very much different from that of water. But if we take a polymer instead, say, polyvinyl alcohol, and add it to a fixed quantity of water, the polymer does not go into solution immediately. The globules of polyvinyl alcohol first absorb water, swell and get distorted in shape and after a long time go into solution. Also, we can add a very large quantity of the polymer to the same quantity of water without the saturation point ever being reached. As more and more quantity of polymer is added to water, the time taken for the dissolution of the polymer obviously increases and the mix ultimately assumes a soft, dough-like consistency. Another peculiarity is that, in water, polyvinyl alcohol never retains its original powdery nature as the excess sodium chloride does in a saturated salt solution. In conclusion, we can say that (1) the long time taken by polyvinyl alcohol for dissolution, (2) the absence of a

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saturation point, and (3) the increase in the viscosity are all characteristics of a typical polymer being dissolved in a solvent and these characteristics are attributed mainly to the large molecular size of the polymer. The behavior of a low molecular weight compound and that of a polymer on dissolution are illustrated in Fig.1.2.

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¡ª¡ªGowariker VR, Viswanathan N V, Sreedhar J. Polymer Science. New York: John Wiley & Sons, 1986.6 UNIT 2 Chain Polymerization µÚ¶þµ¥Ôª Á´Ê½¾ÛºÏ·´Ó¦

Many olefinic and vinyl unsaturated compounds are able to form chain-0like macromolecules through elimination of the double bond, a phenomenon first recognized by Staudinger. Diolefins polymerize in the same manner, however, only one of the two double bonds is eliminated. Such reactions occur through the initial addition of a monomer molecule to an initiator radical or an initiator ion, by which the active state is transferred from the initiator to the added monomer.

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In the same way by means of a chain reaction, one monomer molecule after the other is added (2000~20000 monomers per second) until the active state is terminated through a different type of reaction. The polymerization is a chain reaction in two ways: because of the reaction kinetic and because as a reaction product one obtains a chain molecule. The length of the chain molecule is proportional to the kinetic chain length.

StaudingerµÚÒ»¸ö·¢ÏÖÒ»ÀýÏÖÏó£¬Ðí¶àÏ©ÌþºÍ²»±¥ºÍÏ©Ìþͨ¹ý´ò¿ªË«¼ü¿ÉÒÔÐγÉÁ´Ê½´ó·Ö×Ó¡£¶þÏ©ÌþÒÔͬÑùµÄ·½Ê½¾ÛºÏ£¬È»¶ø£¬½öÏÞÓÚÁ½¸öË«¼üÖеÄÒ»¸ö¡£ÕâÀà·´Ó¦ÊÇͨ¹ýµ¥Ìå·Ö×ÓÊ×Ïȼӳɵ½Òý·¢¼Á×ÔÓÉ»ù»òÒý·¢¼ÁÀë×ÓÉ϶ø½øÐеģ¬¿¿ÕâЩ·´Ó¦»îÐÔÖÐÐÄÓÉÒý·¢¼Á×ªÒÆµ½±»¼Ó³ÉµÄµ¥ÌåÉÏ¡£ÒÔͬÑùµÄ·½Ê½£¬½èÖúÓÚÁ´Ê½·´Ó¦£¬µ¥Ìå·Ö×ÓÒ»¸ö½ÓÒ»¸öµØ±»¼Ó³É£¨Ã¿Ãë2000¡«20000¸öµ¥Ì壩ֱµ½»îÐÔÖÐÐÄͨ¹ý²»Í¬µÄ·´Ó¦ÀàÐͶøÖÕÖ¹¡£¾ÛºÏ·´Ó¦ÊÇÁ´Ê½·´Ó¦µÄÔ­ÒòÓÐÁ½ÖÖ£ºÒòΪ·´Ó¦¶¯Á¦Ñ§ºÍÒòΪ×÷Ϊ·´Ó¦²úÎïËüÊÇÒ»ÖÖÁ´Ê½·Ö×Ó¡£Á´·Ö×ӵij¤¶ÈÓ붯Á¦Ñ§Á´³¤³ÉÕý±È¡£

One can summarize the process as follow (R. is equal to the initiator radical): Á´Ê½·´Ó¦¿ÉÒÔ¸ÅÀ¨ÎªÒÔϹý³Ì£¨R¡¤Ï൱ÓëÒý·¢¼Á×ÔÓÉ»ù£©£ºÂÔ One thus obtains polyvinylchloride from vinylchloride, or polystyrene from styrene, or polyethylene from ethylene, etc. Òò¶øÍ¨¹ýÉÏÊö¹ý³ÌÓÉÂÈÒÒÏ©µÃµ½¾ÛÂÈÒÒÏ©£¬»òÓɱ½ÒÒÏ©»ñµÃ¾Û±½ÒÒÏ©£¬»òÒÒÏ©»ñµÃ¾ÛÒÒÏ©£¬µÈµÈ¡£ The length of the chain molecules, measured by means of the degree of polymerization, can be varied over a large range through selection of suitable reaction conditions. Usually, with commercially prepared and utilized polymers, the degree of polymerization lies in the range of 1000 to 5000, but in many cases it can be below 500 and over 10000. This should not be interpreted to mean that all molecules of a certain polymeric material consist of 500, or 1000, or 5000 monomer units. In almost all cases, the polymeric material consists of a mixture of polymer molecules of different degrees of polymerization.

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Polymerization, a chain reaction, occurs according to the same mechanism as the well-known chlorine-hydrogen reaction and the decomposition of phosegene.

¾ÛºÏ·´Ó¦£¬Á´Ê½·´Ó¦£¬ÒÀÕÕÓëÖÚËùÖÜÖªµÄÂÈ£¨Æø£©-Çâ£¨Æø£©·´Ó¦ºÍ¹âÆøµÄ·Ö½â»úÀí½øÐС£ The initiation reaction, which is the activation process of the double bond, can be brought about by heating, irradiation, ultrasonics, or initiators. The

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