Lee Chang-Uk, Smart Thomas P, Guo Li, Epps Thomas H, Zhang Donghui
Department of Chemistry and Macromolecular Studies Group, Louisiana State University, Baton Rouge, LA 70803.
Macromolecules. 2011 Nov 29;44(24):9574-9585. doi: 10.1021/ma2020936.
N-Heterocyclic carbene (NHC)-mediated ring-opening polymerization of N-decylN-carboxylanhydride monomer (De-NCA) has been shown to occur in a controlled manner, yielding cyclic poly(N-decyl-glycine)s (c-PNDGs) with polymer molecular weights (MW) between 4.8 and 31 kg·mol(-1) and narrow molecular weight distributions (PDI < 1.15). The reaction exhibits pseudo-first order kinetics with respect to monomer concentration. The polymer MW increases linearly with conversion, consistent with a living polymerization. ESI MS and SEC analysesconfirm the cyclic architectures of the forming polymers. DSC and WAXS studies reveal that the c-PNDG homopolymers are highly crystalline with two prominent first order transitions at 72-79°C (T(m,1)) and 166-177°C (T(m,2)), which have been attributed to the side chain and main chain melting respectively. A series of amphiphilic cyclic diblock copolypeptoids [i.e.,poly(N-methyl-glycine)-b-poly(N-decyl-glycine) (c-PNMG-b-PNDG)] with variable molecular weight and composition was synthesized by sequential NHC-mediated polymerization of the corresponding N-methyl N-carboxyanhydride (Me-NCA) and De-NCA monomers. (1)H NMR analysis reveals that adjusting the initial monomer to NHC molar ratio can readily control the block copolymer chain length and composition. Time-lapsed light scattering and cryogenic transmission electron microscopy (cryo-TEM) analysis of c-PNDG-b-PNMG samples revealed that the amphiphilic cyclic block copolypeptoids self-assemble into spherical micelles that reorganize into micron-long cylindrical micelles with uniform diameter in room temperature methanol over the course of several days. An identical morphological transition has also been noted for the linear analogs, which occurs more rapidly than for the cyclic copolypeptoids. We tentatively attribute this difference to the different crystallization kinetics of the solvophobic block (i.e., PNDG) in the cyclic and linear block copolypeptoids.
N-杂环卡宾（NHC）介导的N-癸基N-羧基酐单体（De-NCA）的开环聚合反应已被证明是以可控方式进行的，生成了聚合物分子量（MW）在4.8至31 kg·mol⁻¹之间且分子量分布窄（PDI < 1.15）的环状聚（N-癸基甘氨酸）（c-PNDGs）。该反应对于单体浓度呈现准一级动力学。聚合物分子量随转化率呈线性增加，这与活性聚合一致。电喷雾质谱（ESI MS）和尺寸排阻色谱（SEC）分析证实了所形成聚合物的环状结构。差示扫描量热法（DSC）和广角X射线散射（WAXS）研究表明，c-PNDG均聚物具有高度结晶性，在72 - 79°C（T(m,1)）和166 - 177°C（T(m,2)）有两个明显的一级转变，分别归因于侧链和主链的熔化。通过相应的N-甲基N-羧基酐（Me-NCA）和De-NCA单体的顺序NHC介导聚合反应，合成了一系列具有可变分子量和组成的两亲性环状二嵌段共聚多肽[即聚（N-甲基甘氨酸）-b-聚（N-癸基甘氨酸）（c-PNMG-b-PNDG）]。¹H NMR分析表明，调节初始单体与NHC的摩尔比可以很容易地控制嵌段共聚物的链长和组成。对c-PNDG-b-PNMG样品进行的时间分辨光散射和低温透射电子显微镜（cryo-TEM）分析表明，两亲性环状嵌段共聚多肽在室温甲醇中会自组装成球形胶束，并在几天内重新组织成直径均匀的微米长圆柱形胶束。对于线性类似物也观察到了相同的形态转变，但其发生速度比环状共聚多肽更快。我们初步将这种差异归因于环状和线性嵌段共聚多肽中疏溶剂嵌段（即PNDG）不同的结晶动力学。
