mass. In contrast, LDPE is a branched homopolymer and is, therefore, distributed
regarding molar mass and molecular topology (branching). The branching in LDPE
is called ‘long chain branching’ (LCB) while in LLDPE the branching is called
‘short chain branching’ (SCB). SCB stems from the fact that ethylene is
copolymerized with other α-olefins, resulting in the formation of branches with
typically one (propene) to six (octene) carbon atoms. Accordingly, LLDPE exhibits
molar mass and chemical composition distributions. The incorporation of short
chain branches reduces the crystallinity of the polyolefin and, therefore, the density
of the material.
When a Ziegler (multiple-site) catalyst is used, the incorporation of monomer is
not uniform and an average chemical composition will not reflect the molar masschemical composition interdependence. To address this interdependence, the chemical composition as a function of molar mass and/or the molar mass as a function of
chemical composition must be investigated. A schematic representation of the
molecular population in LLDPE is presented in Fig. 1.1 [7].
The MMD curve in Fig. 1.1a shows that the comonomer content, given as the
number of CH 3 endgroups per 1,000 carbons (CH 3 /1,000C), decreases with increasing molar mass. This indicates that the shorter polymer chains are more branched
than the longer polymer chains. The CCD curve in Fig. 1.1b shows bimodality in
chemical composition. Similar to Fig. 1.1a, less branching at higher molar masses is
found.
The development of single-site (metallocene) catalysts resulted in better defined
microstructures with uniform comonomer incorporation and narrow MMDs [8, 9].
Different from ethylene polymerization, the homopolymerization of propylene
produces polymers that may exhibit different microstructures in addition to the
MMD. Propylene is a chiral monomer and can, thus, form meso and racemo diads
along the polymer chain; see Fig. 1.2a. If the polymer chain consists only of meso
diads then it is called isotactic polypropylene (iPP); if it consists only of racemo
diads then it is called syndiotactic polypropylene (sPP); see Fig. 1.2b.
In addition to the above described types of microstructures, there may be
undesired stereo errors in the growing polymer chain that result from missinsertions
such as head-to-head or tail-to-tail instead of head-to-tail. Such missinsertions can
be identified by NMR spectroscopy.
When propylene is copolymerized with other α-olefins, copolymers are formed
that, in addition to MMD and microstructure variations, will exhibit a CCD. It has
been shown that the insertion of ethylene units into a growing PP chain will disrupt
the chain order and crystallinity will decrease. This is graphically presented in
Fig. 1.3, where the decrease in crystallinity as a function of ethylene incorporation
is shown [7].
1.1 Molecular Heterogeneity of Polyolefins
3
regarding molar mass and molecular topology (branching). The branching in LDPE
is called ‘long chain branching’ (LCB) while in LLDPE the branching is called
‘short chain branching’ (SCB). SCB stems from the fact that ethylene is
copolymerized with other α-olefins, resulting in the formation of branches with
typically one (propene) to six (octene) carbon atoms. Accordingly, LLDPE exhibits
molar mass and chemical composition distributions. The incorporation of short
chain branches reduces the crystallinity of the polyolefin and, therefore, the density
of the material.
When a Ziegler (multiple-site) catalyst is used, the incorporation of monomer is
not uniform and an average chemical composition will not reflect the molar masschemical composition interdependence. To address this interdependence, the chemical composition as a function of molar mass and/or the molar mass as a function of
chemical composition must be investigated. A schematic representation of the
molecular population in LLDPE is presented in Fig. 1.1 [7].
The MMD curve in Fig. 1.1a shows that the comonomer content, given as the
number of CH 3 endgroups per 1,000 carbons (CH 3 /1,000C), decreases with increasing molar mass. This indicates that the shorter polymer chains are more branched
than the longer polymer chains. The CCD curve in Fig. 1.1b shows bimodality in
chemical composition. Similar to Fig. 1.1a, less branching at higher molar masses is
found.
The development of single-site (metallocene) catalysts resulted in better defined
microstructures with uniform comonomer incorporation and narrow MMDs [8, 9].
Different from ethylene polymerization, the homopolymerization of propylene
produces polymers that may exhibit different microstructures in addition to the
MMD. Propylene is a chiral monomer and can, thus, form meso and racemo diads
along the polymer chain; see Fig. 1.2a. If the polymer chain consists only of meso
diads then it is called isotactic polypropylene (iPP); if it consists only of racemo
diads then it is called syndiotactic polypropylene (sPP); see Fig. 1.2b.
In addition to the above described types of microstructures, there may be
undesired stereo errors in the growing polymer chain that result from missinsertions
such as head-to-head or tail-to-tail instead of head-to-tail. Such missinsertions can
be identified by NMR spectroscopy.
When propylene is copolymerized with other α-olefins, copolymers are formed
that, in addition to MMD and microstructure variations, will exhibit a CCD. It has
been shown that the insertion of ethylene units into a growing PP chain will disrupt
the chain order and crystallinity will decrease. This is graphically presented in
Fig. 1.3, where the decrease in crystallinity as a function of ethylene incorporation
is shown [7].
1.1 Molecular Heterogeneity of Polyolefins
3