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由于纳米材料的小尺寸效应,在异相催化剂中,超小的催化剂颗粒往往具有很好的催化性能,多种多样的合成稳定的小纳米颗粒的方法如百花齐放地报道出来.在这些合成方法中,为了防止小颗粒的长大,往往需要稳定剂,常用的稳定剂如功能性纳米材料,树枝状分子等.但是,由于其稳定性较差,制备超小的纳米颗粒往往非常困难,表面吸附的稳定剂也会影响其催化活性.Suzuki反应在现代精细化工合成中具有非常重要的地位.合成容易回收分离的且足以催化氯苯的异相Pd催化剂,将是一个重大的突破.我们使用单电子转移活性自由基聚合(SET-LRP)方法,合成了夹心型的PEG-PNIPEM聚合物,PNIPAM聚合在PEG-1的两端,两端都连接了25个NIPAM分子.通过氢核磁共振(1H NMR),凝胶渗透色谱(GPC)和傅里叶变化红外光谱(FTIR),我们证明了PNIPAM已经成功地接枝在PEG的两端,PEG-PNIPAM的数均分子量Mn,GPC为7841.通过简单的负载流程,我们将Pd纳米颗粒成功的地负载在PEG-PNIPAM共聚物上,得到Pd/PEG-PNIPAM催化剂,Pd的负载量为4.4 wt％.在透射照片中,PEG-PNIPAM看起来像一个薄薄的片层.Pd的团簇颗粒很小,最大约2 nm.我们测量了制得的催化剂Pd/PEG-PNIPAM水溶液随着温度变化的光透过率曲线,最后确定该材料的LCST为41℃.我们认为在温度高于LCST进行反应时,催化剂的载体由亲水变成亲油,这样亲油的反应物分子将容易向催化剂载体扩散并富集.催化剂会形成一个亲油的微环境富集反应物,并形成反应微环境将大大提高催化速率.该催化剂在Suzuki反应中表现出了极好的催化能力.使用该催化剂催化Suzuki反应,我们发现苯硼酸和碘苯在80℃反应时10s内即可完全转化,TOF为4.3× 104 h-1.对于异相Pd催化剂而言,达到这个TOF是非常难得的,可与活性很高的均相催化剂比拟.室温下催化速率明显减慢,但也仍在1 min内转化完全,TOF为7.2×103h-1.当加大反应物的量,反应物/催化剂的比从120增加到1600,反应在3 min内达到100％转化,TOF为3.2× 104 h-1.即使使用苯硼酸和氯苯进行反应,也在5 min内达到了65％的转化.异相Pd催化剂催化氯苯的Suzuki反应是很难进行的,表明该纳米材料具有极佳的催化活性.然而,在催化苯硼酸与碘苯的连续3次反应中,催化剂的活性明显降低,直至失活.使用后的催化剂颗粒长大至几百纳米,许多小颗粒被包裹于其中.Pd颗粒有所长大,PEG-PNIPAM相互缠绕发生团聚,这也就是催化剂失活的原因.由于Pd/PEG-PNIPAM复合物可以通过其温度响应性回收,我们认为其在高效催化方面具有很好的应用前景.

Ultra-small metal nanoclusters have high surface energy and abundant active sites,and therefore their catalytic activities are usually significantly higher than those of larger nanoparticles.A temperature-responsive copolymer,namely poly(ethylene glycol)-co-poly(N-isopropylacrylamide) (PEG-PNIPAM) was synthesized as the first step,and then ultra-small Pd clusters stabilized within PEG-PNIPAM copolymer micelles were formed by direct reduction.Pd nanoclusters of size less than 2 nm showed outstanding catalytic activity in the Suzuki coupling reaction.The reaction between iodobenzene and phenylboronic acid was completed in as little as 10 s (turnover frequency =4.3 ×104 h-1).A yield of 64％ was achieved in 5 min in the reaction between chlorobenzene and phenylboronic acid.The catalyst showed significant deactivation during three consecutive runs.However,this composite catalyst consisting of Pd/PEG-PNIPAM can be easily recycled based on the reversible phase transition of temperature-responsive PEG-PNIPAM.This catalyst therefore has good potential for practical applications.