Photopolymerizable & Photocleavable Resin and Low Stress

Composite Compositions

Field of the Invention

[0001] This invention relates to photopolymerizable & photocleavable resin monomers and resin composite compositions, which feature by its unique balanced overall performance including very low polymerization shrinkage and very low shrinkage stress as well. The photoreactive moiety incorporated into such new resin's main frame enable to make the resin and/or the cured resin networks that are based upon such resin photocleavable. Thus the polymerization rate of free radical reaction for (meth)acrylate- based resin systems should be substantially reduced since it alter the network formation process and consequently allow the shrinkage stress getting relief significantly. In addition, it is expected that radically polymerizable resin systems containing such P&P resin would find wide range application in microelectronic, special coating and restorative dentistry where the dimensional stability and contraction stress within cured materials are critical to the total performance.

Background of the Invention

[0002] Highly cross-linked polymers have been studied widely as matrices for composites, foamed structures, structural adhesives, insulators for electronic packaging, etc. The densely cross-linked structures are the basis of superior mechanical properties such as high modulus, high fracture strength, and solvent resistance. However, these materials are irreversibly damaged by high stresses due to the formation and propagation of cracks. Polymerization stress is originated from polymerization shrinkage in combination with the limited chain mobility. Which eventually leads to contraction stress concentration and gradually such a trapped stress would released and caused microscopically the damage in certain weak zone like interfacial areas. Macroscopically it was reflected as debonding, cracking, et al. Similarly, The origin of contraction stress

in current adhesive restorations is also attributed to the restrained shrinkage while a resin composite is curing, which is also highly dependent on the configuration of the restoration. Furthermore, non-homogeneous deformations during functional loading can damage the interface as well as the coherence of the material. Various approaches have been exploring by limiting the overall stress generation either from the restorative materials, or by minimizing a direct stress concentration at the restored interface. It included, for example, new resin, new resin chemistry, new filler, new curing process, new bonding agent, and even new procedure.

[0003] There have been tremendous attention paid on new resin matrix development that could offer low polymerization shrinkage and shrinkage stress. For example, various structure and geometry derivatives of (meth)acrylate-based resin systems; non- (meth)acrylates resin systems, non-radical-based resin system. In addition, for light curable, low shrink dental composites, not only new resin systems and new photoinitiators, new filler and filter's surface modification have also been extensively explored, such as filler with various particle size and size distribution, from nanometer to micrometer, different shape, irregular as milled or spherical as-made. It can also be different in composition like inorganic, organic, hybrid. Although an incremental improvement has been achieved with each approach and/or their mutual contribution, polymerization stress is still the biggest challenge in cured network systems.

[0004] This invention is related to a new kind of resin composition. However, unlike conventional resin system, a new concept is involved in designing such a new resin composition, which would render the polymerization stress in post-gel stage to a subsequent, selective network cleavage in order to have the stress partially released. As mentioned above, all of previous arts towards low shrink and low stress are based on the limitation on the shrink and stress formation in general. However, the shrinkage and stress development in cured network system should have two different stages: a pre-gel phase and a post-gel phase. Actually, most efforts of current arts are focussed on the pre- gel stage and some of them were proved effective. Unfortunately, these approaches become ineffective in terms to control the stress development in post-gel stage, where the

shrinkage is not as much as in the pre-gel stage but the stress turns to much more sensitive to any polymerization extend. It is the immobility nature of the increasing crosslink density within the curing system that leads to the increasing stress concentration within the curing system, period. Even worse, the problem does not stop here and the trapped stress would eventually get relief from slow relaxation, which can create additional damage on a restored system. Therefore, our approach is based on such a concept that in the post-gel stage if some of "closed net" of any cross-linked system can be selectively broken to promote an extended stress relief period, the total stress concentration would be substantially reduced. To fulfil such a task, a photopolymerizable and photocleavable resin is proposed and a general molecular constitution is designed. It was expected that such a resin monomer can be polymerized like any other resin monomer but its mainframe is able to be triggered to break upon additional light source such as near UV is blended. This is a typical photocleavable process, but it is its capability to be photopolymerized and embedded into a cross-linked system make it unique. In addition, it also makes possible to avoid regenerating any leachable species through such secondary breakage.

[0005] Photocleavage is nothing new in solid synthesis of peptides, from which new peptides was directed on certain template in designed sequence, then it was cleaved from its template via a subsequent light exposure. There is no chemical contamination with such a process. On the other hand, photoacid and photobase could be viewed as extended applications for photocleavage. Acidic or basic component is temporally latent to avoid any unwanted interaction with others in the system and they can be released on demand such as light exposure to trigger the regeneration of the acid or base, which then act as normal acidic or basic catalyst for next step reactions. Recently, thermally removable or photo-chemically reversible materials are developed in order to make polymer or polymeric network depolymerizable or degradable for applications such as easily removing of fill-in polymer in MEMS, thermally labile adhesives, thermaspray coatings and removable encapsulation et al. Most recently, photoclevable dendrimers are explored in order to improve the efficiency for drug delivery. Based on our knowledge, there is no prior art involved photocleavale segment in cured network for contract stress control.

However, all of those related arts could be used as a practical base to justify this investigation.

^< V ^α

Photopolymanzabon

Scheme Illustration for P&P Resin and the Cured Networks Therefrom

APPROACH:

[0006] Theoretically speaking, if any somewhat environmentally sensitive moiety, such as a thermally cleavable or photo-labile linkage were incorporated into polymerizable resin monomers, such resin or its resulting polymeric material would become command- responsible, more specifically enable them thermo-cleavable or photo-cleavable. The chemistry of some classical photo-initiators could be adopted as the base for designing such photopolymerizable and photocleavable resin monomers, because such an initiator was explored as polymerizable photoinitiator or macroinitiator. However, none of them was really incorporated into polymer chain or polymeric network to make the polymeric chain or network breakable one way or another.

[0007] It is the another objective of this investigation to develop a new resin system for next generation low shrink and low stress restorative materials by incorporating a photocleavable or thermally liable moiety as part of a photopolymerizable resin monomer. It was expected with such an unusual approach it would enable a conventional polymerized network should be selectively cleavaged, thus to disperse the stress from postpolymerization and furthermore to result in a self stress-relief, ultimately to minimize the overall stress concentration.

[0008] In order to make a polymerized network cleavable-on-command by light or photocleavable, a light responsible moiety should be stable towards standard light exposure process such as visible light curing until additional exposure to specific light with distinguished energy level. In particular, such energy source can be anything other than the standard visible blue light. Near UV light would be one of typical examples among the many possible choices. Furthermore, it was expected that compounds derived from ortho-nitrobenzyl segment or from α-hydroxyalkylphenone should be ideal candidates for this new class resin monomers that be photopolymerized by visible light and be triggered to be breakable by extra UV light if needed.

Scheme I: Typical Polymerizable and Photocleavable Resin Monomer based on α-hydroxyalkylphenone

[0009] Its feasibility of this approach allows a rapid exploration on its versatility for a new class of resin. Accordingly, a variety of such polymerizable and photocleavable resin monomers was successfully prepared with wide range of constructions as illustrated in Scheme II.

X

Scheme II: General Reaction Pathway towards P&P Resin Monomers

[0010] Thus, one of such α-hydroxyalkylphenone is 4-(2-hydroxyethoxy)-phenyl-2- hydroxy-2-methyl-2-ρropanone, HP, a classic α-hydroxyacetophenone photoinitiator. Its dual hydroxyl groups allow constructing a wide range of polymerizable monomers via various simple reactions as illustrated in Scheme π. For example HP can be incorporated with methacylate, acrylated or vinyl ether et al, via one or two-step reaction. Various photopolymerizable HP derivatives have been prepared via ester-ester, ester-carbonate, carbonate-carbonate, and urethane-urethane linkage. From them, the urethane-urethane linkage offers the best robust process: easy control, non-solvent process.

• [0011] In addition, such new resin monomer was formulated with other conventional resin monomers like BisGMA, TEGDMA, UDMA or experimental resin monomer like macrocyclic resin in a variety ratio in order to have overall performance got balanced for the resulting composites. As showed in the following examples, remarkable low shrinkage, low stress and excellent mechanical property plus the good handling characteristics were demonstrated by those composites based on such new class P&P resin monomers.

[0012] As showed in the example, a typical urethane-based P&P resin can be easily prepared via a simple bulk process. Depending the nature of isocyanate, one or two-step reaction will be involved to prepare such a P&P resin. HEM is the simplest polymerizable isocyanate. Unfortunately, its toxicity limits its application as biomedical materials. Here we have successfully developed a simple two-step process to make such a phopolymerizable and photocleavable resin: in step I, HP was capped with diisocyanate to form new diisocyanate with well-controlled sequence, which was then further reacted with any (meth)acrylate containing a hydroxyl group as step π reaction. This procedure would not only allow carrying the reaction in one reactor, but also allow using that easily available dissocyanate instead of EEM.

[0013] Suitable diisocyanates include the alklene diisocyanates wherein the alkylene group ranges from 2 to about 18 carbon atoms and arylene and substituted arylene di-and polyisocyanates. Thus, exemplary disicyanates and polyisocyanates include, for example:

Alklene diisocyanates ethylene diisocyanate propylene diisocyanate tetramethylene diisocyanate pentamethylene diisocyanate hexamethylene diisocyanate(HDI) hexamethylene diisocyanate biuret hexamethylene diisocyanate trimer(isocyanurate) octamethylene diisocyanate decamethylene diisocyanate undecamethylene diisocyanate dodecamethylene diisocyanate isophorone diisocyanate (IPDI) hydrogenated diphenyl methane diisocyanate (Hi ₂ MDI)

Arylene diisocyanate xylylene- 1 ,4-diisocyanate(p-XDI) xylylene- 1 ,3-diisocyanate(m-XDI) m-pheylene diisocyanate p-pheylene diisocyanate toluene-2,6-diisocyanate(2,6-TDI) toluene-2,4-diisocyanate(2,4-TDI) mesitylene diisocyanate durylene diisocyanate benzidene diisocyanate

1 -methyl phenylene-2,4-diisocyanate naphthylene- 1 ,4-diisocyanate

1,2,4-benzene triisocyanate

4,4'-diisocyanato diphenyl methane(MDI)

3,3'-dimethyl-4,4'-diisocyanato diphenyl methane

4,4'-diphenyl propane diisocyanate dianisidine diisocyanate m-tetramethylenexylene diisocyanate(TMXDI)

In addition, suitable (meth)acrylate containing a hydroxyl group include 2-hydroxyethyl (meth)acrylate 2-hydroxypropyl (meth)acrylate 3-(acryloxyl)-2-hydroxypropyl (meth)acrylate diethylene glycol monohydroxyl (meth)acrylate triethylene glycol monohydroxyl (meth)acrylate tetraethylene glycol monohydroxyl (meth)acrylate polyethylene glycol monohydroxyl (meth)acrylate

[0014] Surprisingly, it has now been found that the resinous dental compositions based on according to the invention for polymeric networks after curing thereof and even without addition of any filler, which has remarkably low polymerization stress even prior

to a photocleavage (Figure 1). It is believed somehow the HP moiety incoporated in such novel P&P resin had involved in the radical reaction thus to slow polymerization rate during conventional photopolymerization while it was exposed to visible light, even though no cleavage occurred at this point. The relatively slow polymerization rate (Figure 2) for those P&P resin-based materials allow a substantial relaxation than those rapid cured systems.

Table Ia: Polymerization Shrinkage and Stress for Various Activated Resin Mix

Shrinkage (%) Stress (MPa) by Helium Pycnometer by Tensometer

Denfortex Resin 10.2 4.1

TPH Resin/999446 6.8 4.5

TPH Resin/999447 7.3 4.3

Harpoon Resin/xj5-12 5.5 3.1

Harpoon Resin/xj5-26 5.8 3.2

P&P Resin/LB5-158-l 5.2 1.4

P&P Resin/LB5- 167-3 6.2 1.5

P&P Resin/LB5- 167-4 6.9 1.5

P&P Resin/LB6-54-l 5.6 0.6

P&P Resin/LB6-54-2 5.5 0.7

P&P Resin/LB6-58-4 6.2 1.2

P&P Resin/LB6-73-l 6.3 1.7

P&P Resin/LB6-73-2 5.9 1.6

P&P Resin/LB5-114 6.0 1.4

Table Ib: Polymerization Shrinkage and Stress for Various P&P/TPH Resin Mixtures

TPH Resin, PiSP Resin, Viscosity @20C, Stress Shrinkage

999446/030804 LB6-114

% % Pa.s MPa %

P&P Resin/LB6-114 0 100 102 1.41 5.96

XJ5-208-1 10 90 74 1.92 6.17

XJ5-208-2 20 80 61 2.17 6.37

XJ5-208-3 30 70 47 2.51 6.54

XJ5-208-4 40 60 39 2.75 6.70

XJ5-208-5 50 50 32 2.81 6.76

XJ5-208-6 60 40 27 3.22 6.90

XJ5-208-7 70 30 24 3.69 7.00

XJ5-208-8 80 20 21 3.99 7.10

TPH Resin/999446 0 100 15 4.73 7.35

Table II: Polymerization Shrinkage, Stress and Microstrain for Various Composites

Shrinkage (%) Microstrain (ue) Srrew (MPa) by Helium Pycnometer by Strain Gage by Tensometer

TPH/A2 3.10 1600 2.9

EsthetX/A2 2.92 1995 2.5

SureFil/A 2.09 1840 2.7

Supreme/A2B 2.65 1720 2.8

Supreme/YT 2.39 2005 3.3

Haφθon/A2 1.34 1000 1.7

P&P/LB6-55 1.01 N/A 0.8

P&P/LB6-56 1.07 N/A 0.8

P&P/LB6-69 1.40 N/A 1.4

P&P/LB6-74 1.68 N/A 1.3

P&P/LB6-75 1.57 N/A 1.4

P&P/LB6-115 1.82 N/A 1.5

Table Hl: General Physical Property for Activated Neat P&P Resin

100% P&P Resin (LB6-72) 100% P&P Resin (EBR6983)

0.15% CQ 0.15% CQ

0.20% EDAB 0.20% EDAB

0.02% BHT 0.02% BHT

Lot# LB6-73-2 LB6-1 14

Viscosity at 20 ⁰ C 1050 1020 poise

Uncured density 1.1164 1.1 162 g/cm ³

Cured density 1.1865 1.1867 g/cm ³

Shrinkage @ 24hrs 5.91 5.96

%

Stress @ 60min

1.6 1.4 MPa

δH, in N2 113 110 w/o UV filter

to 20 15 seconds

tmax 38 31 seconds

δH, in N2 110 107 w/ UV filter

t _o , seconds 23 17

t _ma x, seconds 46 36

Table IV: Physical and Mechanical Properties for Typical P&P Resin-based Composites

Pastes LB6-75 LB6-115

Lab Batch of P&P P&P Resin (EBR6983)

Resin Composition LB6-73-2 LB6-114

(18%) (18.5%)

Filler Composition LB6-91-3 LB6-91 -3

(82%) (81.5%)

PZN Enthalpy (VisAJV) (Vis/UV) δH (JVg) 20/ 21/ in N2

Induction Time to 22/ 14/ seconds

Peak Time ϊmax 58/ 44/ seconds

Uncured density g/cm ³ 2.1553 2.1411

Cured density g/cm ³ 2.1898 2.1808

Shrinkage (%) by pycnometer 1.6 1.8

@ 24hrs

Shrinkage Stress

MPa 1.4 1.6

Flexural Strength

MPa 137+/-4 128+/-9

Flexural Modulus

MPa 10800+/-440 9963+/- 136

Compressive Strength

MPa 344+M 1 316+/-24

Compressive Modulus

MPa 8080+/-530 7920+/-214

[0015] The present invention will now be described in detail with reference to the following examples that by no means limit the scope of the invention.

The following abbreviations are used in the example.

BisGMA: 2,2-bis(4-(3-methacryloyloxy-2-hydroxypropoxy)-phenyl)propan e

HP: 4-(2-hydroxyethoxy)-phenyl-2-hydroxy-2-methyl-2-propanone

HEMA: 2-hydroxyethyl methacrylate

HEPA: 2-hydroxypropyl methacrylate

TEGDMA: triethylene glycol dimethacrylate

UDMA: di(methacryloxyethyl)trimethyl- 1 ,6-hexaethylenediurethane

HEMASA: mono-2-(methacryl-oxy)ethyl-succinate

TMDI: 2,2,4(2,4,4)-trimethyl- 1 ,6-hexanediisocyanate

HDL hexamethylene diisocyanate

IEM: methacryloxyethyl isocyanate

ICEM: 1 -methacryloxyethyl-2,4,4(2,2,4)-trimethyl-6-hexaneisocyanate

CDI: l,r-carbonyldiimidazole

BHT: butylhydroxytoluene

DBTDL:dibutyltin dilaurate

TCDCA: tricyclo[5.2.1.0 ² ' ⁶ ] decane-dimethanol diacrylate

Example 1 Preparation of the adduct of IEM-HP-IEM (XJ5-129).

[0016] A 250ml three-necked flask equipped with a mechanical agitator, dry air inlet and water-cooling condenser, which was immersed in an oil-bath, was charged 25.5 grams of IEM, and 0.15gram of DBTDL and 0.1 gram of BHT. Then it was charged with 14.7 grams of grounded HP. The reaction was kept at 25 ⁰ C with the oiL-bath temperature, then the temperature was slowly raised to 30-35 ⁰ C for overnight reaction. The conversion of HP reached up to 95%. Additional diluent (33 grams of TCDCA) was used to adjust viscosity.

Example 2 Preparation of the adduct of HEMA-TMDI-HP-TMDI-HEMA (XJ5-140)

[0017] A 500ml three-necked flask equipped with a powder addition funnel, mechanical agitator, dry air inlet and water-cooling condenser, which was immersed in an oil-bath, was charged 71.5grams of TMDI, and 0.21 gram of DBTDL. Then 22.4 grams of grounded HP were slowly in portion added into the flask in a period of 5-6hrs. In such a manner to avoid rapid reaction heat increase, which might jeopardize the desired sequence of TMDI-HP-TMDI. The fully conversion of HP as fully capped with TMDI can be easily determined by IH NMR. Then 0.15gram of BHT was charged into the system. With continuous purge of dry air into the reaction system, 71.6 grams of HEMA was added into the flask through a dropping funnel during a period of 2hrs. An effective agitation is critical during the initial stage of HEMA addition in order to minimize the reaction rate so as to avoid overheat in the system, which can cause premature polymerization or gelation. The reaction temperature has to be controlled below 60 ⁰ C, best for below 45 ⁰ C. After HEMA addition, allow for additional lhr reaction at 35-40 ⁰ C. Then additional diluent such as TEGDMA was charged into system and mixed for a couple of hours: Again IH NMR was used to determine the HEMA/NCO conversion. A typical two-step reaction would be completed withinl4hrs with yield of 97-99%.

Example 3 Preparation of the adduct of HEMA-HDI-HP-HDI-HEMA (XJ5-136)

[0018] A 250ml three-necked flask equipped with a powder addition funnel, mechanical agitator, dry air inlet and water-cooling condenser, which was immersed in an oil-bath, was charged 40.8grams of HDI, and 0.17gram of DBTDL and 0.15 gram of BHT. Then 22.4 grams of grounded HP were slowly in portion added into the flask in a period of 3- 4hrs. The reaction temperature was remained at 25 ⁰ C through the step. In such a manner to avoid rapid reaction heat increase, which might jeopardize the desired sequence of HDI-HP-HDI. The reaction was kept running overnight and the resulting resin turns

opaque as evident of partially crystallized. The fully conversion of HP as fully capped with TMDI can be easily determined by IH NMR. With continuous purge of dry air into the reaction system, 27.0 grams of HEMA was slowly added into the flask. The reaction temperature was raised to 35-4O ⁰ C. After additional 6hrs reaction at 35-40oC, 70 grams of diluent, was mixed with the resulting resin for a couple of hours prior to discharged.

Example 4 Preparation of the adduct of HEMA-TMDI-HP-TMDI-HEMA (LB6-73)

[0019] A 1000ml jacked, cylinder resin kettle equipped with a powder addition funnel, mechanical agitator, dry air inlet and water-cooling condenser, through which 35oC of heated water was circulated during the reaction, was charged 96.6 grams of TMDI, and 0.25gram of DBTDL. Then 35.5 grams of grounded HP were slowly in portion added into the flask in a period of 6hrs. The fully conversion of HP as fully capped with TMDI can be easily determined by IH NMR. Then 0.20gram of BHT was charged into the system. With continuous purge of dry air into the reaction system, 86.2 grams of HEMA was added into the flask through a dropping funnel during a period of 2hrs. An effective agitation is critical during the initial stage of HEMA addition in order to minimize the reaction rate so as to avoid overheat in the system, which can cause premature polymerization or gelation. The reaction temperature has to be controlled below 60oC, best for below 45oC. After HEMA addition, allow for additional lhr reaction at 35-40oC. Then additional diluent such as 30-40 grams of TEGDMA was charged into system and mixed for a couple of hours. Again IH NMR was used to determine the HEMA/NCO conversion. A typical two-step reaction would be completed withinl4hrs with yield of 97-99%.

Example 5 through 14

[0020] Per similar reaction procedure as described in example 4, Table I listed in details on other reaction runs at 35 ⁰ C based on various compositions.

Table I: Reaction Runs at 35 ⁰ C with Different Compositions

Resins TMDI HP DBTOL HEMA TEGDMA Re. Time Yield Viscosity Viscosity

Comps 8 8 8 8 8 hr % @2(fC @5€PC poise poise

LB6-76 101.5 35.3 0.25 86.2 40 13 97 780 25

LB6-78 91.9 35.1 0.25 86.2 40 12 97 340 20

LB6-80 96.6 37.4 0.25 86.2 40 12 97 520 20

LB6-81 96.7 33.3 0.25 86.2 40 12 98 460 20

LN6-82 96.7 35.2 0.25 905 40 13 99 320 15

LB6-83 96.7 35.2 0.25 814 40 12 98 710 25

LB6-84 96.7 35.2 0.25 86.2 42 12 99 420 15

LB6-85 96.7 35.2 0.25 86.2 38 5 13 98 530 15

LB6-86 96.7 35.2 0.22 ^"* 86.2 40 13 98 480 25

LB6-87 96.7 35.6 0.20 86.2 30 14 98 950 35

Example 15

Preparation of the adduct of HPN-HDI-HPN (XJ5-160)

[0021] A 500ml three-necked flask equipped with a mechanical agitator, dry air inlet and water-cooling condenser, which was immersed in an oil-bath, was charged 9.7 grams of HDI, and 0.1 lgram of DBTDL and 19.3 grams of HPN at room temperature. Then the reaction temperature was raised to 35oC and it was kept running overnight.

Example 16 Preparation of the adduct of HP-TMDI-HP (XJ5-169)

[0022] A 250ml three-necked flask equipped with a mechanical agitator, dry air inlet and water-cooling condenser, which was immersed in an oil-bath, was charged 250ml dry methylene dichloride, 22.45 grams of HP, and 0.14gram of DBTDL at room temperature.

HP is insoluble in the solvent. Then 10.55 grams of grounded TMDI was added into the flask dropwisely within lhr, then the reaction was kept running overnight. Then 50 grams of TEGDMA was mixed with the resulting viscose liquid.

Example 17 Preparation of the adduct of HPN-TMDI-HEMA (XJ5-157)

[0023] A 250ml three-necked flask equipped with a mechanical agitator, dry air inlet and water-cooling condenser, which was immersed in an oil-bath, was charged 22.63 grams of TMDI, and 0.16gram of DBTDL at room temperature. Then 16.4 grams of HPN in a dropping addition funnel was slowly added into the flask within 1.5hrs. After additional lhr reaction, 18.8 grams of HEMA was charged slowly into the flask via the dropping addition funnel during a period of 2hrs. The reaction was kept running overnight.

Comparative Example 1 Preparation of the adduct of ICEM-HP-ICEM (XJ5-146)

[0024] A 500ml three-necked flask equipped with a mechanical agitator, dry air inlet and water-cooling condenser, which was immersed in an oil-bath, was charged 56.3grams of ICEM, and 20.0grams of TEGDMA. Then 0.1 lgram of DBTDL and 0.05gram of BHT. The bath temperature was set at 25oC. Then it was added all of 11.2 grams of grounded HP. The reaction occurred slowly as evident by the slow temperature increase. Heated up to 35oC and kept for additional 222hrs prior to reach its maximum conversion, 96%, as determined by same IHNMR method as used above. Obviously as showed in Examples 3 through 14, the two-step reaction process is more efficient that one-step process as described in this control example.