CH2 CH in .NET

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CH2 P(C6H5)2
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Rh[P(C6H5)3]2 Cl II
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(C6H5)3P
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3. Compounds n-C4H9I and n-C18H37I react at the same rate with pyridine, but n-C4H9I is almost fourfold more reactive than n-C18H37I toward poly(4-vinylpyridine) [Grieg and Sherrington, 1978].
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9-1h
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Polymer reactions involving the conversion of uncharged functional groups to charged groups often exhibit a decrease in reactivity with conversion. Thus the quaternization of poly(4-vinylpyridine) becomes progressively slower with increasing conversion, since reaction proceeds with the buildup and concentration of charge on the polymer molecule [Noah et al., 1974; Sawage and Loucheux, 1975]. An unreactive pyridine group is less reactive by a factor of 3 when neighboring pyridine groups have reacted. This autoretardation behavior is not observed for the small molecule homolog, such as 4-ethylpyridine, since unreacted molecules are separated from reacted 4-ethylpyridine by solvent.
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9-2
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Br CH2
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REACTIONS OF POLYMERS
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The same phenomenon is responsible for the decrease in the ionization constant of a polymeric acid such as poly(acrylic acid) with increasing degree of ionization [Kawaguchi et al., 1990, 1992; Morawetz and Wang, 1987]. This effect is somewhat moderated for acrylic acid ethylene copolymers, since charge density on the ionized polymer is decreased. The basicity
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CH2 CH CH2 CH COOH COOH CH2CH CH2 CH COO COOH + H+
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9-3
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of a polymeric base such as poly(4-vinylpyridine) or poly(vinyl amine) decreases with increasing protonation of nitrogen [Lewis et al., 1984; Satoh et al., 1989]. These autoretardation effects are partially alleviated by the addition of salts that shield the charge on the polymer. Charge on a polymer molecule can also affect reactivity by altering the concentration of the small molecule reactant within the polymer domains. The reaction of a charged polymer with a charged reactant results in acceleration for oppositely charged species and retardation when the charges are the same. For example, the rate constant for the KOH saponi cation of poly(methyl methacrylate) decreases by about an order of magnitude as the reaction proceeds [Plate, 1976]. Partially reacted poly(methyl methacrylate) (IV) repells hydroxide ion, while
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CH3 CH2 C COO IV CH3 CH2 C COOCH3
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unreacted polymer does not. The concentration of OH within the polymer domains decreases progressively with increasing conversion. Acceleration of reactivity is observed for the SN2 reaction of poly(4-vinylpyridine) and a-bromoacetate ion when the polymer is partially protonated [Ladenheim et al., 1959]. The positive charge on the polymer attracts and concentrates a-bromoacetate ion within the polymer domains.
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9-4
CH2COO
Polymer charge can also affect polymer reactivity toward neutral reagents. The nucleophilic reactivity of the carboxylate anions of a partially ionized poly(methacrylic acid) toward a-bromoacetamide decreases with increasing charge on the polymer when the reaction is carried out in water [Ladenheim and Morawetz, 1959]. Water is preferentially concentrated within the polymer coils with the partial exclusion of the neutral reagent, resulting in a decreased reaction rate. Similar retardation is observed when partially protonated poly(4vinylpyridine) acts as the catalyst in hydrolysis of 2,4-dinitrophenyl acetate [Letsinger and Savereide, 1962]. These retardation effects are decreased or eliminated when reaction is carried out in an organic solvent where there is less tendency for preferential concentration of the solvent within the polymer domains. Related phenomena include the effect of a polymer on the reaction between two small reactants. Acceleration occurs when the polymer attracts both reactants (both reactants
PRINCIPLES OF POLYMER REACTIVITY
have the same charge and both are opposite in sign to the charge on the polymer backbone). An example is the redox reaction performed in the presence of poly(sodium vinyl sulfonate).
Co(NH3)5Cl 2+ + Hg 2+ + H2O Co(NH3)5H2O 3+ + HgCl +
9-5
The rate increases with polyanion concentration, reaches a maximum, and then decreases with further increase in polyanion concentration [Morawetz and Vogel, 1969]. Retardation occurs for reactions between ions of opposite charge because the polymer attracts one ion but repels the other. 9-1i Neighboring-Group Effects
The reactivity of a functional group on a polymer is sometimes affected directly by an adjacent neighboring group. (The effects described in the beginning of Sec. 9-1h are indirect effects of neighboring groups.) Thus, the saponi cation of poly(methyl methacrylate) and related polymers proceeds with autoacceleration when carried out with weak bases such as pyridine or low concentrations of strong bases [Gaetjens and Morawetz, 1961; Kheradmand et al., 1988]. (Saponi cations at high OH concentrations involve retardation due to hydroxide carboxyl anion repulsions as described in Sec. 9-1h). After the initial formation of some carboxylate anions, subsequent hydrolysis of ester groups occurs not directly by hydroxide ion but by neighboring carboxylate anions. The reaction proceeds by the sequence in Eq. 9-6 with the intermediate formation of a cyclic acid anhydride. Direct rate enhancement by a neighboring group, referred to as anchimeric assistance, occurs primarily when the cyclic anhydride intermediate is the favored 5- or 6-membered ring. Such effects are also encountered in low-molecular-weight bifunctional compounds, as in succinic esters.