To obtain MJLCPs that form smectic phases, side chains with special interactions can
be used. Microphase separation between incompatible segments in side chains favors
smectic structures. And jacketed polymers containing side-chain functional groups with
other non-covalent interactions like hydrogen bonding (Cheng et al. 2011a) and ionic
interaction (Cheng et al. 2011b) can also show smectic phases.
In addition to polyacrylate/polymethacrylate and polystyrene main chains that are
usually used, polysiloxane backbone has also been employed to prepare MJLCPs
through hydrosilylation of polymethylhydrosiloxane with styrenic compounds
(Zhang et al. 2010). Other than flexible main chains, more rigid backbones such as
polynorbornene (Yang et al. 2013; Zhu et al. 2014b), polyacetylene (Chen et al.
2006a; Peng et al. 2010; Yu et al. 2013), and polythiophenes (Yang et al. 2009) have
also been successfully introduced into MJLCP systems.
Synthesis
MJLCPs can be synthesized by radical polymerizations, including many controlled/
living radical polymerization methods, which do not require highly pure solvents
and reagents. Recently, another living polymerization method, ring-opening metathesis polymerization (ROMP), has been applied in the synthesis of MJLCPs with a
polynorbornene backbone. ROMP was first used by Pugh et al. in the synthesis of
polynorbornene-based side-on SCLCPs, resulting in polymers with controlled MWs
and quite narrow MWDs (Pugh and Schrock 1992). MJLCPs can also be obtained by
polymer reactions from prepolymers such as poly(methylsiloxane)s, similar to the
case of the synthesis of some conventional SCLCPs. In addition, supramolecular
approaches with non-covalent interactions such as hydrogen bonding and ionic
interaction can be utilized to prepare MJLCPs.
Conventional Free Radical Polymerization
Because the monomers of many MJLCPs are vinyl ones, conventional free radical
polymerization is frequently used. The first MJLCPs reported were obtained by this
method (Zhou et al. 1987, 1989). One example is shown in Scheme 2. Similarly, many
other series of MJLCPs have also been prepared via free radical polymerizations.
MJLCPs with quite bulky side groups can still be obtained. For example, polymers with dendrons (Chart 2) (Jin et al. 2010), Tp moieties (PPnV, Chart 3) (Zhu
et al. 2012; Zhu et al. 2014a), and polyhedral oligomeric silsesquioxane (POSS)
units (PnPOSS, Chart 4) (Zhu et al. 2015) in the two ends of the side chains have
been synthesized. The polymerization is more difficult compared to other monomers
with less bulky side groups, which leads to limited DPs of the resulting polymers.
For the MJLCPs with dendritic side groups, the second-generation polymer PCbzG2
has a lower DP than that of the first-generation one PCbzG1 even though the same
conditions were used in the polymerization of the two polymers with different
dendron generations. The conversion in the polymerization of the second-generation
42
Z. Shen
be used. Microphase separation between incompatible segments in side chains favors
smectic structures. And jacketed polymers containing side-chain functional groups with
other non-covalent interactions like hydrogen bonding (Cheng et al. 2011a) and ionic
interaction (Cheng et al. 2011b) can also show smectic phases.
In addition to polyacrylate/polymethacrylate and polystyrene main chains that are
usually used, polysiloxane backbone has also been employed to prepare MJLCPs
through hydrosilylation of polymethylhydrosiloxane with styrenic compounds
(Zhang et al. 2010). Other than flexible main chains, more rigid backbones such as
polynorbornene (Yang et al. 2013; Zhu et al. 2014b), polyacetylene (Chen et al.
2006a; Peng et al. 2010; Yu et al. 2013), and polythiophenes (Yang et al. 2009) have
also been successfully introduced into MJLCP systems.
Synthesis
MJLCPs can be synthesized by radical polymerizations, including many controlled/
living radical polymerization methods, which do not require highly pure solvents
and reagents. Recently, another living polymerization method, ring-opening metathesis polymerization (ROMP), has been applied in the synthesis of MJLCPs with a
polynorbornene backbone. ROMP was first used by Pugh et al. in the synthesis of
polynorbornene-based side-on SCLCPs, resulting in polymers with controlled MWs
and quite narrow MWDs (Pugh and Schrock 1992). MJLCPs can also be obtained by
polymer reactions from prepolymers such as poly(methylsiloxane)s, similar to the
case of the synthesis of some conventional SCLCPs. In addition, supramolecular
approaches with non-covalent interactions such as hydrogen bonding and ionic
interaction can be utilized to prepare MJLCPs.
Conventional Free Radical Polymerization
Because the monomers of many MJLCPs are vinyl ones, conventional free radical
polymerization is frequently used. The first MJLCPs reported were obtained by this
method (Zhou et al. 1987, 1989). One example is shown in Scheme 2. Similarly, many
other series of MJLCPs have also been prepared via free radical polymerizations.
MJLCPs with quite bulky side groups can still be obtained. For example, polymers with dendrons (Chart 2) (Jin et al. 2010), Tp moieties (PPnV, Chart 3) (Zhu
et al. 2012; Zhu et al. 2014a), and polyhedral oligomeric silsesquioxane (POSS)
units (PnPOSS, Chart 4) (Zhu et al. 2015) in the two ends of the side chains have
been synthesized. The polymerization is more difficult compared to other monomers
with less bulky side groups, which leads to limited DPs of the resulting polymers.
For the MJLCPs with dendritic side groups, the second-generation polymer PCbzG2
has a lower DP than that of the first-generation one PCbzG1 even though the same
conditions were used in the polymerization of the two polymers with different
dendron generations. The conversion in the polymerization of the second-generation
42
Z. Shen