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专业: 牙科材料学

[交流] 英文翻译已有1人参与

A group of monomers which has found commercial
use axe the urethane dimethacrylates. The first type of urethane dimethacrylate to appear was synthesized
from hydroxyalkyl methacrylates and diisocyanates
(91), These monomers have molecular
weights nearly equal to that of BisGMA, but are
less viscous. The most commonly used monomer
of this type, l,6-bis(methacrylyloxy-2-etlioxycarbonylamino)-
2,4,4-trimethylhexan (UEDMA
= UDMA), is the reaction product of 2-
hydroxyethyl methacrylate and 2,,4,4-trimethylhexamethylenediisocyanate
(Fig, 11), This monomer
has been used alone (e,g, Isocap®, Vivadent;
Isopast®, Vivadent; Isomolar®, Vivadent) or in
combination with other monomers, e,g.,, BisGMA
and TEGDMA (e,g, Heliomolar®, Vivadent; Estic
Microfill Composite®,, Kulzer; Estilux MicroflU®,
Kulzer; Durafill Light-curing Composite®, Kulzer)
As mentioned above, urethanes have also been
synthesized by the reaction of the secondary
hydroxyl groups of BisGMA with isocyanates to
create less hydrophilic monomer systems. Thus,
Nuva-Fil® (L,D. Caulk) as well as Ful-Fil® and
Prisma-Fil® (L.D. Caulk) were based on oligomers
synthesized from BisGMA and hexamethylene
diisocyanate (Fig. 12) (32, 34). Another urethane
dimethacrylate system was used in Fotofil®
(Johnson & Johnson/ICI), the very first proprietary
resin composite to be visible light activated (32,, 92,
93). As may be seen from Fig. 13, this monomer
system contains di(urethanophenyl) methane
groups, which are susceptible to photo-oxidation
with formation of quinoid groups accompanied by
intense yellowing (1). In a subsequent resin composite,
Occlusin® (ICI), the sensitive and rigid di(urethanophenyl) methane group was substituted by a hexamethylene diurethane group,, which is
more flexible because of reduced barriers to free
rotation about the bonds (Fig. 14) (34). The
increased flexibility has been suggested to explain the higher degree of conversion obtainable with
Occlusin compared with Ful-Fil (34),
The advantages of UEDMA have been reported
to be the lower viscosity (approximately 11,000
mPa-s at 23°C; ASMUSSEN, personal communication)
and a greater flexibihty of the urethane linkage,
which may improve toughness (94). However,
a general difference between resin composites based
on UEDMA or other urethane dimethacrylates and
conventional BisGMA-based resin composites is
very difficult to deduce from existing in vitro and
in vivo studies (95-103). One reason for this difBculty
is that actual differeBces caused by dissimilar
monomer systems are camouflaged by the fact that
the materials differ in many other aspects, e.g., type
and amount of filler, type and amount of initiators,
and quality of silanization of the filler particles, the
effect of which may be more determinative for the
properties. This, however, does not imply that
the choice of monomer system is insignificant. The
results of in vitro studies, in which all other
components than the monomer system were kept constant, indicate that resin composites based on
UEDMA will have improved mechanical properties
compared with resin composites based on BisGMA
(13, 14, 104). Also, there are indications that higher
degrees of conversion are obtainable with urethane
dimethacrylates as compared with BisGMA:
TEGDMA mixtures (16, 94). All other things being
equal, this would result in improved biocompatibility
of the resin composite. Fewer pendant methacrylate
groups and less residua! monomer may be
oxidized to formaldehyde or methacrylic acid.
Furthermore, by use of urethane dimethacrylates,
the risk of bisphenol A formation is nonexistent.
Dimethacrylates with aromatic groups have been
reported to produce rigid polymers, while dimethacrylates
with aliphatic groups produce flexible polymers
(81, 105). It has been theorized that dimethacrylates with "hard" segments (aromatic
groups) and "soft" segments (ahphatic groups) in
the same molecule will restilt in polymers with
increased toughness (106). For the purpose of
designing stich a polymer, a dimethacrylate with a
polyurethane chain was synthesized from HEMA
and a poljoirethane diisocyanate, imprecisely
described as a "polyester urethane with propylene
glycol and hexanediol" (106). The dimethacrylate,
(PUDMA; Fig. 15), incorporated two
phenylmethyl groups as hard segments and propylene
glycol or polymethylene as a soft segment.
When heat-cured, this homopolymer exhibited less
water sorption, lower volume shrinkage, and
greater deflection than did polymers of BisGMA
or of TEGDMA, while maintaining acceptable
flexural strength and modulus of elasticity. The
high toughness of PUDMA was explained by the
favorable combination of the rigidity of the aromatic
ring and the flexibility of the long chain.
However, it seems that the idea of combining hard and soft segments in one dimethacrylate was implemented
already in the Fotofil® and Occlusin®
tirethanes (Fig. 13）

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