Kinetically controlled cyclization in step-growth polymerization of AB2 macromonomer: Role of molar mass of macromonomer
Cyclization has been widely reported to compete with interchain growth and lead to limited degree of polymerization in step-growth polymerization of AB 2 monomers . However, little attention has been paid to kinetical comparison between interchain growth and cyclization in preparation of long-subchain hyperbranched polymers (LHPs) from AB 2 macromonomers in semi-dilute solution. This work aims to elucidate how cyclization influences the polymerization of AB 2 macromonomers via kinetical competition with interchain growth and how molar mass of macromonomer affects this competition. The corresponding kinetics in semi-dilute solution is theoretically derived in consideration of cyclization. For a given mass concentration, interchain growth rate ( r g ), cyclization rate ( r c ) and their ratio ( r g / r c ) highly depend on the molar mass of macromonomer (M), following the scale rule of r g ∝ M − 2 , r c ∝ M − 2.5 and r g / r c ∝ M 0.5 , respectively. It indicates that macromonomers with lower molar mass tend to generate products with lower degree of polymerization in terms of macromonomer ( DP macromonomer ) and higher self-cyclized macromonomer ratio (CMMR) at a faster rate, while macromonomers with higher molar mass act oppositely. These proposed theories are further verified via the experimental investigation about the step-growth of AB 2 polystyrene (PSt) macromonomers with different molar masses. The structural analyses of the resultant products from PSt macromonomers with a low molar mass of 2.5 k indicate a very low DP macromonomer with a nearly complete conversion of focal A groups, which implies predominant position of cyclized compositions in resultant products, not only cyclized macromonomers (CMMs) but also cyclized long-subchain hyperbranched polymers (CLHPs). More importantly, experimental results show that conversion rate and CMMR increase but DP macromonomer decreases sharply as molar mass of macromonomer decreases, which quite agrees with the theoretical inference.
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