甲基丙烯酸羟丙酯催化链转移聚合及其催化剂链转移常数的变化

THE EVOLUTION OF THE CHAIN TRANSFER CONSTANT DURING THE CATALYTIC CHAIN TRANSFER POLYMERIZATION OF 2-HYDROXYPROPYL METHACRYLATE

采用COBF[bis(aqua)bis((difluoroboryl)dimethylglyoximato)cobalt(II)为催化剂,AIBN为引发剂,60℃下进行甲基丙烯酸β-羟丙酯的催化链转移自由基本体聚合,得到了端基含有双键的低分子量聚合物.分别用Mayo方程法和链长分布(CLD)方程法测算反应过程中催化剂的链转移常数,发现随着反应的进行,催化剂的链转移能力逐渐下降,表观链转移常数Csapp从反应初期的2000左右下降到600左右.这主要是由于反应初期形成了部分比较稳定的碳钴键,导致Csapp在一开始迅速下降,然后趋于缓慢;反应至中后期,由于粘度效应,表观链转移常数进一步降低到300以下.研究进一步发现,由CLD方程法所得的表观链转移常数值普遍低于由Mayo方程法所得的值,且高转化率时误差更大.这是因为GPC测得的是累积产物的分子量分布,对于中、高转化率情况,有必要将其转化为瞬时产物的值.由于累积产物的数均聚合度转化为瞬时产物的数均聚合度相对容易,因而Mayo方程法较适合于测算中、高转化率时的表观链转移常数.

Using cobaloxime boron fluoride （COBF） and AIBN as catalyst and initiator respectively, the catalytic chain transfer bulk polymerization of 2-hydroxyporpyl methacrylate （HPMA） was carried out at 60℃ and the low molecular weight polymer with a terminal double bond was prepared. The evolution of the apparent chain transfer constant （ Cs^app ） of COBF during the polymerization was determined by the Mayo equation and the CLD （chain length distribution） equation respectively. At the beginning of the reaction, the chain transfer constant showed values of up to about 2000, but with the increasing of the monomer conversion, the chain transfer constant decreases and keeps at a value of about 600 for sometime and then decreased again. The most likely cause for the catalyst deactivation is the stable cobalt-carbon bond formed at the first period of the reaction and then reached equilibrium point. When the polymerization proceeds up to middle and high conversion, the chain transfer polymerization is further manipulated by diffusions of polymeric propagating chain and catalyst molecules, and the apparent chain transfer constant fell to value of below 300. It has been found also, that the data determined by the CLD equation are usually lower than those by the Mayo equation, especially at high conversion stage. It is because that both original equations are available for the instantaneous produced polymer or the case of very lower conversion stage. However, the GPC measurements give molecular weight distributions of the cumulated products rather than the instantaneous produced polymer. The number-average polymerization degrees for the instantaneous produced polymers were obtained from the GPC data, and used in the Mayo equation method together with the modification of the monomer concentration. It was believed that the apparent chain transfer constants from the modified Mayo equation method are more available for the middle and high conversion cases.