2.6.1 Chemical Irregularities
This kind of sequence defect occurs in the statistical copolymers, where the species
of monomers can crystallize. On the backbone of polyethylene chains, the short
branches can be regarded as the non-crystallizable comonomers. In high-density
polyethylene (HDPE), the branching probability is about 3 branches/1,000 back-
bone carbon atoms, and its crystallinity can reach levels as high as 90 %; while in
low-density polyethylene (LDPE), the branching probability is about 30 branches/
1,000 backbone carbon atoms, and its crystallinity reaches only 50 %. The most
common industry product is actually linear low-density polyethylene (LLDPE), and
its branching probability is determined by the copolymerization process of CH 2 ¼
CH 2 and CH 2 ¼CHR (R means side alkane groups for short branches).
Assuming an ideal free-radical copolymerization reaction, the reactivity ratiosr 1
andr 2 are fixed by the single-site Ziegler-Natta catalyst, as shown in Fig.2.11. The
product ofr 1 r 2 determines the sequence characters of statistical copolymers,
with the values between zero for alternating copolymers and infinity for diblock
copolymers. Different feeding styles such as the continuous loop reaction or the
intermittent batch reaction also make different characteristic distributions of chain
sequences. The continuous reaction keeps the feed compositionFconstant, and
consequently the comonomer distributions are homogeneous among copolymers.
The products can be referred tohomogeneous copolymers. The intermittent reaction
shifts the feed composition upon the proceeding of batch reaction, so the comono-
mer distributions are heterogeneous among copolymers. The products can be called
heterogeneous copolymers. In extreme cases, the heterogeneous copolymers can be
regarded as a kind of binary blends.
2.6.2 Geometrical Irregularities
There may exist asymmetry in the structure of repeating units like –(CH 2 -CHR)-,
where R 6 ¼H. Therefore, along a polymer chain, the repeating units can have head-
Fig. 2.11 Illustration of free-radical copolymerization with the statistical propagation of two
monomers along the chain. The sequences are thus determined by the reactivity ratios (r 1 ,r 2 ) and
the feed composition (F)
30 2 Structure–Property Relationships