Thiocarbonylthio compounds (S=C(Z)S-R) in free radical polymerization with reversible addition-fragmentation chain transfer (RAFT polymerization). Effect of the activating group Z
Thiocarbonylthio compounds (S=C(Z)S-R) in free radical polymerization with reversible addition-fragmentation chain transfer (RAFT polymerization). Effect of the activating group Z
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DOI：
10.1021/ma020883
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发表时间：
2003-04-08
期刊：
影响因子：
5.5
通讯作者：
Thang, SH
中科院分区：
文献类型：
--
作者：
Chiefari, J;Mayadunne, RTA;Thang, SH
在合适的加成-断裂链转移剂[S=C(Z)S-R]（RAFT剂）存在下进行的自由基聚合具有活性聚合的特征（即，聚合物产物可以重新活化以进行扩链和/或嵌段合成，分子量由RAFT剂浓度和转化率预先确定，可以实现窄多分散性）。对两个系列的RAFT试剂[S=C(Z)S-CH2Ph和S=C(Z)S-C(Me)(2)CN]进行苯乙烯聚合(110℃，热引发)。链转移系数在 Z 为 Ph > SCH2Ph 类似于 SMe 类似于 Me 类似于 N-吡咯远大于 OC6F5 > N-内酰胺 > OC6H5 > O(烷基) 远大于 N(烷基)(2) 的系列中降低（该系列中只有前五个在间歇聚合中提供窄多分散性聚苯乙烯 (< 1.2)）。更一般地，链转移系数按以下顺序降低：二硫代苯甲酸盐>三硫代碳酸盐二硫代链烷酸盐>二硫代碳酸盐(黄原酸盐)>二硫代氨基甲酸盐。然而，Z上的吸电子取代基可以增强RAFT试剂的活性，从而改变上述顺序。因此，使氧或氮孤对电子不易与C S 离域的取代基可以分别显着增强黄原酸盐或二硫代氨基甲酸盐的有效性。 RAFT 剂相对有效性的趋势根据 Z 与 C S 双键的相互作用来合理化，以激活或失活该基团以进行自由基加成。 RAFT 试剂的分子轨道计算和估计的 LUMO 能量可以以定性方式用于预测 Z 取代基对 RAFT 试剂活性的影响。
Free-radical polymerization in the presence of suitable addition-fragmentation chain transfer agents [S=C(Z)S-R] (RAFT agents) possess the characteristics of a living polymerization (i.e., polymer products can be reactivated for chain extension and/or block synthesis, molecular weights are predetermined by RAFT agent concentration and conversion, narrow polydispersities are possible). Styrene polymerizations (110 degreesC, thermal initiation) were performed for two series of RAFT agents [S=C(Z)S-CH2Ph and S=C(Z)S-C(Me)(2)CN]. The chain transfer coefficients decrease in the series where Z is Ph > SCH2Ph similar to SMe similar to Me similar to N-pyrrolo much greater than OC6F5 > N-lactam > OC6H5 > O(alkyl) much greater than N(alkyl)(2) (only the first five in this series provide narrow polydispersity polystyrene (< 1.2) in batch polymerization). More generally, chain transfer coefficients decrease in the series dithiobenzoates > trithiocarbonates dithioalkanoates > dithiocarbonates (xanthates) > dithiocarbamates. However, electron-withdrawing substituents on Z can enhance the activity of RAFT agents to modify the above order. Thus, substituents that render the oxygen or nitrogen lone pair less available for delocalization with the C S can substantially enhance the effectiveness of xanthates or dithiocarbamates, respectively. The trend in relative effectiveness of the RAFT agents is rationalized in terms of interaction of Z with the C S double bond to activate or deactivate that group toward free radical addition. Molecular orbital calculations and the estimated LUMO energies of the RAFT agents can be used in a qualitative manner to predict the effect of the Z substituent on the activity of RAFT agents.