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Chap._300020 - Syndiospecific Displacement(Monomer-Monomer...

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Unformatted text preview: Syndiospecific Displacement: (Monomer-Monomer Interaction and Secondary Insertion) Stereospeciflc polymerizations with ' é é syndiotactic placement occur in the : relatively few instances when the initiator : [mun CH2=CHCH5 x—i-uu-H-fi can coordinate with the monomer in both of , /1 4 cu, the two possible configurations (that is, R : " x and S). In such cases the driving force for , l syndiotactic placement is similar to thatj previously described for low«temperature - polymerizations, the repulsive interactionsj pr, between substituent groups on adjacent - H£$fi D x monomer units. Coordination accentuates: / f”: these interactions in the same manner as , 5 X CH1 1/ | an =cacn _ , a ’ fHCHI-CH m lowenng the reaction temperature would. C X_V—CHCH2CH-r\f\ --————— x— X/xl 3": / The use of homogeneous (soluble) initiators : allow coordination of the monomer in either : of two possible modes. In summary, syndiotactic Ziegler-Nada: ' x . H, initiator exaggerate the inherent tendencyj toward syndictactic placement by /| " accentuating the, methyl-methyl replusion: x . . . . , x mteractrons between the propagating cham end and incoming monomer ' Properties SOME CHARACTERISTICS or POLYPROINLENE 7 In addition to all the properties shown I M in HDPE, the most noticeable Polypropylene ' attribute Of iSOtaCtic PP is the M increased softing point and . . - " . -‘ Ahctic . . r . Chali‘c‘cnsuc Imamc‘u sy'm‘olm'c ' ' consequent higher mammum serv1ce _ , - , - ' - ., __ _ (0‘ temperature. However, due to high : l‘CArC‘llt'l‘iilr"?:m(;§:-i ._ . _ 13:2? . :33? ' ' 0' Tg (~ O 0C), the impact strength of PP rs _“ . , V .Y-‘ kP/ ‘ 1 370450 - falls marketedly as the temperature is “rim“ Slrcng‘h ( . cm i “ - - ' reduced whereas PE shows little ' change. In PP, each alternate carbon atom in the polymer chain bears a teriary hydrogen atom which is relatively - labiie. Thus, compared to HDPE, PP i is more prone to attack by oxidation , agents. Of particular significance is the susceptibility of the polymer to oxidation by air at elevated temperatures, and antioxidants are generally added. :szxzxz xxxzzxu: emetic fi Ra RE Hui \ \ azaxzxxx xzxxmxm: WKZIZIII zzwxzznz ...
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