Environmental Engineering Reference
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Block copolymers can also be produced. In one method, a ring-opening polymerization
mechanism is employed using either coordination insertion catalysis or anionic catalysis
[124]. Monomer addition sequence is important in this method, those that produce primary
hydroxyl groups, such as lactones [113, 125-129], trimethylene carbonate (TMC) [122, 128],
and 1,5- dioxepan-2-one (DXO) [130] must generally be polymerized first. Exceptions to the
order of addition involve both glycolide [131] and D,L-3-methyl glycolide (MG) [109], which
can be polymerized sequentially before or after PLA to form block copolymers because they
possess the same type of propagating chain end as lactide. Ring-opening polymerization from
polymeric initiators, a second method to produce block copolymers, employs polymeric
initiators that possess hydroxyl groups on their chain ends. These groups initiate
polymerization with the same catalysts used in polymerization of linear PLA. Effective
polymeric initiators include poly(ethylene glycol) [124, 132-140], poly(ethylene glycol-co-
propylene glycol) [141], poly(propylene glycol) [105], poly(tetrahydrofuran) [133, 142],
poly(dimethyl siloxane) [133, 143-145], hydroxyl functionalized polyethylene [146], and
hydroxy functionalized poly(isoprene) [147, 148]. Dendritic molecules with a single hydroxyl
group have also been used to produce linear/dendritic di-block copolymers [149].
Several methods for synthesis of highly branched polymers through the ring opening of
lactones have likewise been developed. Star formations are produced either by divergence,
where polymer chains grow from a central core, or through convergence in which a
termination reaction couples together individual chains [150-152], glycerol [133, 152-155].
Comb-shaped copolymers consisting of linear backbones with pendent linear chains may also
be produced [156-162]. Lactides have also been polymerized into hyperbranched structures,
in which many branch points lead to other branch points [155, 163-166]. Star-shaped
copolymers are generally produced in a similar manner to star-shaped homopolymers, with
hydroxyl groups on a multifunctional initiator used to initiate polymerization [167].
3.5. Fundamental Chain Properties
The characterization of PLA depends on the accurate knowledge of its fundamental
properties. These include such parameters as Mark-Houwink constants that relate intrinsic
viscosity to molecular weights, theta-condition front factors (K) used to calculate single chain
properties, and characteristic ratios (C) that give an indication of the bonding structure of
polymers. Unfortunately, each of these is reported inconsistently in the literature [5, 6, 150,
168- 174].
A recent study [175] has addressed these inconsistencies experimentally. PLA
homopolymers and copolymers spanning wide ranges of molecular weight and stereoisomer
proportion were prepared by ring-opening polymerizations of L-and D-lactides using tin
octanoate as the catalyst. Samples were then characterized by means of dilute-solution
viscometry in three different solvents; size-exclusion chromatography; static multiangle light
scattering; variable-angle spectroscopic ellipsometry; and melt rheology. Data provided by
these experiments include values of characteristic ratios as well as Schulz-Blaschke and
MarkHouwink constants, all of which show consistently that polylactides are typical linear
flexible polymers, in excellent agreement with recently published theoretical simulation
results [176]
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